DATA PROCESSING CIRCUITS AND INTERFACES

Abstract: An integrated circuit contains (102) a microprocessor core (100), program memory (104) and separate data storage (106, 108), together with analogue and digital signal processing circuitry (110). The ALU (302) is 16 bits wide, but a 32-bit shift unit (312) is provided, using a pair of 16-bit registers. The processor has a fixed length instruction format, with an instruction set including multiply and divide operations which use the shift unit over several cycles. No interrupts are provided. External pins of the integrated circuit allow for single stepping and other debug operations, and a serial interface (SIF) which allows external communication of test data or working data as necessary. The serial interface has four wires (SER<u></u>IN, SER<u></u>OUT, SER-CLK, SER<u></u>LOADB), allowing handshaking with a master apparatus, and allowing direct access to the memory space (104-110) of the processor core (100), without specific program control. Within each processor cycle, the processor circuitry is divided into plural stages, and latches are interposed between the stages to minimise power consumption.

Description: DATA PROCESSING CIRCUITS AND INTERFACES The present invention relates primarily to single-chip data processing devices, but also to microprocessors and to digital circuits generally, and to interface circuits. In the present day, many products incorporate microprocessor based data processing circuits, for example to process signals, to control internal operation and/or to provide communications with users and external devices. To provide compact and economical solutions, particularly in mass-market portable products, it is known to include microprocessor functionality together with program and data storage and other specialised circuitry, in a custom "chip" also known as an application-specific integrated circuit (ASIC). Field Programmable Gate Arrays (FPGA) such as those made by Xilinx™, Actel™ and Altera™ may also be used to implement such solutions. However, for various reasons, the integrated microprocessor functionality conventionally available to an ASIC designer tends to be the same as that which would be provided by a microprocessor designed for use as a separate chip. The present inventors have recognised that this results in inefficient use of space and power in the ASIC solution, and in fact renders many potential applications of ASIC technology impractical and/or uneconomic. Various aspects of the invention are defined in the appended claims, while the applicant reserves the right to claim any further aspects of the invention that may be disclosed herein. In accordance with certain aspects of the present invention, microprocessor architectures are proposed which overcome the above drawbacks, being optimised for integration within an ASIC by providing a combination of functional features and architectural features unlike any conventional microprocessor. For example, in a conventional general purpose microprocessor, the arithmetic and logic unit (ALU) has a certain data width (eight bits, sixteen bits etc.), and provides operations of arithmetic addition or subtraction, logical AND, OR combinations and left and right bit shifts, all on data of this basic width. One aspect of the invention disclosed herein is to provide a separate shifting unit, wider than the ALU width, which allows multiplication and division of two numbers, each as wide as the ALU itself, in a circuit of relatively small size. The shifter will typically be associated with one double-width register of the processor. Other aspects of the invention relate to the provision of special functional features and interfaces within the chip and/or between the chip and the external environment. While these other aspects can be employed advantageously in the novel processor architecture proposed herein, it will be apparent that these specific techniques are in fact applicable and advantageous in a wide range of different microprocessor architectures, or even in the field of sequential digital circuitry generally irrespective of whether it is program- controlled or not. As one particular such feature, the invention in another aspect provides, in a program controlled processor, a mechanism whereby response to external stimuli is provided automatically, but only at times known in advance to the programmer. Examples of such stimuli include requests for communication from external devices, and entry of "sleeping" state for power conservation. In the present embodiments, special instructions are defined whereby the programmer can define fixed periods in which external communication may take place, and fixed points for entry into the sleeping state. The various aspects of the invention will become apparent from the following description of specific embodiments. These are presented by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows the basic arrangement of an integrated circuit including a processor embodying the invention; Fig. 2 shows the programmer's model and instruction format of the processor of the Fig. 1 circuit; Fig. 3 shows the data architecture of the processor; Figs. 4A and 4B illustrate the execution of multiply and divide operations in the processor of Fig. 3; Figs. 5A to 5G show waveforms illustrating various functional features of the circuit of Fig. 1; Fig. 6 is a schematic diagram of a serial interface (SIF) similar to that of the circuit of Fig. 1; Fig. 7 shows in more detail a shift register of the serial interface of Fig. 6; Fig. 8 is a flowchart showing operation of a master apparatus (EXT) in relation to the serial interface of Figs. 6 and 7; Fig. 9 is a flow-chart showing operation of a slave apparatus (ASIC) in relation to the serial interface; Fig. 10 shows circuitry for monitoring external signals during a sleeping state of the processor of Figs. 1 to 6; Fig. 11 is a schematic diagram of the electronics for a domestic gas meter, including the novel processor; and Fig. 12 shows a modification of the serial interface of Figs. 6 and 7, allowing multiple slave devices. The description of the embodiments which follows includes the following sections: - Overview of the Specific Embodiment including a novel processor core Basic Arrangement of the processor and associated hardware integrated within an ASIC Programmer's Model for the processor - Instruction Set for the processor Data Architecture of the processor Pin Description together with detailed operation Construction and Operation of the Serial Interface (SIF) - Power Saving Features Application Example - Domestic Gas Meter Further Notes and alternative embodiments. OVERVIEW OF THE SPECIFIC EMBODIMENT Single chip solutions are ideal for many products, and can comprise mixed mode CMOS circuitry such that analogue and digital signal processing can be performed on-chip. For some applications, however, there is a further need for more logic processing, for functions such as user interfaces, controlling E 2 PROMs, networks and protocol conversions. Conventionally this requires a traditional microprocessor or microcontroller, which increases product size for two reasons. Firstly, the circuit board of a product has two chips instead of one, and secondly, the stand-by modes of conventional low-power processors still consume substantial power, so that the product requires a larger battery. The larger battery and bigger circuit board make the product quite expensive, while the need for communication between chips makes the system more sensitive to electromagnetic interference. There are many other disadvantages of a multiple chip solution. There is clearly a desire for single chip solutions in which the processor, program storage and RAM are embedded in a single ASIC, to facilitate products such as: portable wireless products, instrumentation, utilities metering systems, low data rate radio systems, medical diagnostics, safety critical and verifiable systems, pagers, and certain sections of mobile telephones. Certain conventional designs can be reduced to a single ASIC by embedding a processor or a memory corresponding to the conventional external processor, if the ASIC can be made large enough. However, this goes only a small way to addressing the problems identified above. The processor which is the subject of the present disclosure is a custom microprocessor which has been developed for use in ASIC designs. The requirements and trade-offs when designing for an ASIC are quite different from discrete designs, so the novel processor differs in a number of important aspects from a conventional microprocessor. As a result, ASIC designs that use the novel processor are lower cost, lower power and lower risk than those using existing processor designs. Some key features of the processor are as follows. A very low gate count of, for example 3000 gates can be achieved, which compares with 8000 gates for an Intel 8051 microprocessor core. To efficiently support signal processing applications, the novel processor has powerful signed arithmetic functions including hardware multiply (16 bits by 16 bits), divide (32 bits by 16 bits) and normalisation, which are conventionally only found in much larger processor designs. In particular, a shift unit separate from the conventional ALU provides double- width shift operations, in cooperation with a pair of registers. Power consumption is minimised in three ways. Firstly the architecture and instruction set have been chosen for power efficiency - especially the low power implementation of signal processing functions. Secondly, the detailed design minimises clatter and unnecessary transitions. Thirdly, when idle, the processor can enter a sleep mode, where it uses very little power. Furthermore, by use of static circuits, the circuit can stop the processor clock completely without loss of data, giving virtually zero power consumption. Special asynchronous circuitry monitors external wake-up signals with minimum power dissipation. These features complement the inherently lower power achievable with single chip designs to make possible new applications. A built-in serial interface (the SIF) allows external access to the address space and processor registers. It is used to provide efficient IC manufacturing test (allowing testing of the processor, ROM, RAM and memory mapped peripherals) , prototype device proving, and production board level test. It can also be used for ASIC to microprocessor communication in systems that use a tied external microprocessor, eliminating the need for separate interfaces and on-board circuitry for communications and testability functions. The processor uses separate program and data spaces (a Harvard architecture), and the program instruction word can thus be wider or narrower than the data word. This is an example of where the on-chip nature of the design favours a different solution to the traditional microprocessor (von Neumann architecture) because the need for separate address busses on-chip does not cause any increase in the number of connections (pins) off- chip. On both address busses, the timing allows either synchronous or asynchronous devices. This is important as synchronous devices are often smaller and lower power than their asynchronous counterparts. For verification and also to reduce gate count, the processor does not support interrupts. Instead, efficient polling instructions and the use of sleep/wake-up allow efficient multi-event responses. The processor has a RISC instruction set. Instructions are single word and mostly execute in a single cycle. Particular operations taking several cycles to execute are the Multiply (16 cycles), Divide (16 cycles) and Shift (variable) operations, which use the special shift unit mentioned above. There are four addressing modes: Immediate, Direct and Indexed by registers X and Y. The processor is suitable for programming in a high level language such as 'C, or in assembler. The instruction set and addressing modes allow efficient, readable assembler programs to be easily written. The registers and addressing modes also allow effective compilation of high level languages such as C. However, it may be that, for efficiency and verification reasons, programs are best written in assembler, although development effort may be higher. BASIC ARRANGEMENT Fig. 1 shows the basic arrangement of the novel processor core and associated hardware included in a single integrated circuit. The "core" of the processor is shown at 100, while the boundary of the integrated circuit is shown at 102. Various "pins" of the processor are defined, for communication with other functional units on the chip, and with the external world. Also included on chip is program memory 104, fixed data memory (ROM) 106, data RAM 108 and memory-mapped input/output circuitry 110. The program space address bus is 16 bits wide and labelled PR0G_ADDR, while the data space address bus (also 16 bits) is labelled ADDR. "Pins" of the processor core which are connected to physical pins at the chip boundary 102 are related to the debug functions (pins STOPB, RUN_STEP) and to the serial interface (SER_IN, SER_OUT, SER_CLK, and SER_L0ADB). The person skilled in the art will appreciate that, in addition to the elements explicitly shown in the Basic Arrangement of Fig. 1, other analogue and/or digital signal processing functions will typically be provided within the ASIC itself, according to the particular application. Where high speed signal processing functions are to be performed by the ASIC, it will typically be desirable to have these performed by dedicated circuitry rather than by the programmed processor core described above, and to have less computationally intensive, but more logically complex parts of the required functionality implemented by the processor under program control. Therefore, for example, if a number of external analogue signals are to be sensed with a high bandwidth, and combined in accordance with predetermined repetitive algorithms to obtain a meaningful measurement, the A-D conversion and these high-speed processing functions can be performed by dedicated circuitry, and the measured value supplied periodically to the processor core via the memory mapped input/output circuitry. This principle is applied in the gas flow meter disclosed below as an example application, with reference to Fig. 11. PROGRAMMER'S MODEL Fig. 2 shows the logical arrangement of internal registers and memory space of the processor of Fig. 1, commonly known as the programmer's model. Registers AH, AL, X and Y are provided for transient storage of data and address values within the processor core, together with a register for the program counter PC, and for various flags (C, S, N, Z) generated by the arithmetic and logic circuits of the processor. The program storage space (memory 104) comprises 64K (65536 decimal) locations, each storing an 18 bit instruction word. The instruction words are of fixed format, and the format is illustrated at the foot of Fig. 2. Bits numbered 17 (MSB) down to 8 of the instruction word (hereinafter INSTRN[ 17 :8] ) contain an address value or data for the instruction. Bits INSTRN[7:4] contain an operation code (opcode) for the operation to be performed. Bits INSTRN[3:2] can be used to extend the operation opcode, and, in particular, are used by certain instructions to identify a particular storage register from among those shown in the programmer's model. Finally, bits INSTRN[1:0] identify an addressing mode for the current instruction. In the data space (formed by memories 106, 108, 110) there are 64K (65536 decimal) locations, each storing a 16 bit data word. 512 locations at the beginning of the data space (0000 to 01FF), and 512 locations at the end of the data space (FE00 to FFFF) can be addressed directly with the 10 address bits of each instruction word. The remaining 63K locations can be addressed with indirect (for example indexed) addressing modes. The instruction set implemented in this example processor will now be illustrated in the format of assembler language mnemonics, which will be readily understood by those familiar with the design or use of microprocessors. INSTRUCTION SET ASSEMBLER OPERATION FLAGS SET NOP None (NB instruction all - zeroes) SLEEP Enter sleep mode - PRINT None (debug request for - simulator) ASSEMBLER OPERATION FLAGS SET BCS branch addr If C = 1 PC - branch addr - ADDRESSING MODES Address mode for instructions with "data" Addr mode Assembler "data" source/destination format Immediate #12 data = addr16 Direct @12 data = @addrl6 Indexed X @(12,X) data = @(X+addrl6) Indexed Y @(12,Y) data = @(Y+addrl6) Address mode for instructions with "branch_addr" Addr mode Assembler "branch addr" new PC value format PC relative $+12 branch_addr = PC + addr16 Direct @12 branch_addr = @addr16 X relative 12,X branch_addr = X+addrl6 Indexed Y §(12,Y) branch_addr = @(Y+addrl6) In the above definition of the instruction set, and as mentioned above in relation to Fig. 2, the bits INSTRN[3:2] of the instruction word are used to identify registers in instructions having the operand "reg" only. Also: the '@' indicates "contents of"; 'addrlδ' is the 10 bit address value INSTRN[17:8] sign-extended to 16 bits; '12' is simply an example data value; while 'PC indicates the address of the current instruction. DATA ARCHITECTURE Fig. 3 shows in block form the data architecture of the processor core. The operation of all elements in a given cycle is controlled in accordance with one program instruction by a control and decode section 300. The principal elements of the data architecture are the arithmetic and logic unit (ALU) 302; the A register 304 (32 bits comprising AH and AL); X register 306 and Y register 308 (16 bits each); program counter (PC) register 310 (11 bits); shift/load logic 312; a PC incrementer 314, comprising a 16 bit half adder; an address sign extender 316; an address adder 318; various multiplexers 320 to 332; a flags and condition logic 334; and a tri-state buffer 335. Communication with the program memory is via the program address bus PR0G_ADDR and the instruction bus INSTRN. Communication with the data memory (including ROM, RAM and memory mapped I/O circuits) is via the data address output ADDR and the bi-directional data bus DATA. Also provided is the serial interface register having an address part 336 (bits SIF_ADDR [15:0]), an address space determining part 338, a read/write control part 340, and a data part 342 of eighteen bits. Also associated with the serial interface is a tri-state buffer 346. The operation of the processor in execution of instructions will now be described. Instructions are read from the INSTRN bus, where they have been latched after read-out from the program ROM 104. The lowest eight bits of the latched instruction I[7:0] determine the operation to be performed and are passed to the control and decode section 300 which sets up the data architecture multiplexers 320 to 332 and controls the sequence of instruction execution. The top 10 bits of the latched instruction I[17:8] specify an address or immediate value for the instruction. As well as instruction execution, the data architecture provides the pathways for SIF operations. The architecture will now be described as it applies in detail to each class of instruction and SIF operation. ALU Basic Functions The ALU 302 has two inputs A and B of 16 bits each, and a 16 bit output Σ. A carry bit input CO is provided, and flag outputs Cn, S (sign), N (negative) and Z (zero). The basic functions implemented by the ALU under control of the control and decode section 300 are listed within the block 302 in Fig. 3. The options in each cycle are: (1) to perform a logical AND of the A and B inputs; (2) to add the negative of A to B while subtracting the input carry bit CO; (3) to pass B directly to Σ; (4) to subtract B from A with subtraction of the carry bit CO, outputting Cn = 1 in the case of a "borrow"; (5) to output the logical exclusive -OR (XOR) of A and B; (6) to output A + B + CO (add with carry); and (7) to output the logical OR of A and B. The majority of instructions are two operand instructions, taking one operand from a register, the other from memory or as an immediate, and returning the result to the same register. The ALU A input receives the register input and the ALU B input the memory/immediate value. The ALU performs the required logical or arithmetic operation and presents the result on the Σ output from where it can be written back into the same register. For the AL/AH registers 304, the additional shift/load block 312 is inserted between Σ db the registers. This allows shift operations to be performed on the combined register pair and is also an integral part of the scheme by which multiply and divide are performed in this embodiment. No Operand Instructions: NOP, BRK, PRINT, SIF NOP, BRK, PRINT and SLEEP do not involve the data architecture at all. SIF executes as a SIF cycle if a SIF request is pending (described more fully below), otherwise it behaves as a no-op. Data Address Modes Memory/immediate values are generated and applied to the ALU B input, using the multiplexers 320, 322, 324, 328 and neighbouring components. There are four data address modes: Immediate, Direct, Indexed X and Indexed Y. Immediate takes a value directly from the instruction (I[15:0]). The other modes use the instruction value to select a value from memory. The top 10 bits of the latched instruction are sign extended at 316 to 16 bits. To this is added (318) either zero (Immediate or Direct), X (Indexed X) or Y (Indexed Y) as selected by multiplexer 324. The output of adder 318 is fed to the ADDR bus via multiplexer 326. For immediate values multiplexer 328 counts the output of adder 318 directly to the ALU B input. For other modes, multiplexer 328 selects the DATA input, containing the value read from memory. Single Memory/Immediate Operand Instructions: LD. TST The appropriate memory/immediate value is generated on the ALU B path as described above. This is passed unchanged through the ALU 302 by setting it to 'B'. In the case of LD, the result Σ is written to the appropriate register. In both cases, the N and Z flag values from the ALU are loaded into their flag bits. Store Instruction: ST (and PRINT. This is the only instruction that writes to memory. The register whose value is to be stored is selected by multiplexer 320 on the ALU A path. This is then routed via multiplexer 322 and tri-state buffer 335 to the DATA bus. The store address is formed in exactly the same way as for a normal memory/immediate operand instruction. The storing of data prior directly, rather than via the ALU avoids the need for special timing for this instruction. The ST instruction sets the flags. For the valid addressing modes, multiplexer 328 is always set to 1 (DATA), so the stored value is also presented on the ALU B path. By selecting ALU operation 'B', the ALU generates N and Z flag values which can be loaded into their flag bits. The opcode for ST with the Immediate addressing mode does not make sense and is used for the PRINT instruction. This has special meaning to a simulator and gate level debugger for use in developing the ASIC, but is executed by the processor as a no-op. Dual Operand Instructions: ADD, ADDC, SUB, SUBC, NADD. CMP, OR, AND, XOR These instructions operate between a register and a memory/immediate value, returning the result to the same register. The register value is presented on ALU input A via multiplexer 320. The memory/immediate value is presented on ALU B input as above. The result Σ from the ALU is fed to the appropriate register. For these instructions multiplexer 330 and the shift/load unit 312 are both set so that Σ is propagated unchanged to X, AH and AL as well as Y. The appropriate register only is clocked. Arithmetic instructions set all four FLAG bits out of the ALU. Logical instructions set only the N and Z bits. Branch Instructions: BRA, BLT, BPL, BNE, BEQ, BCC, BCS Normally multiplexer 332 is set to 0 so that at the end of each instruction, PC is clocked and the program counter increments. PROG_ADDR, the address in program space, is equal to the PC value. When a branch instruction is executed, if BRANCH_TRUE is high (output by the condition logic 334), multiplexer 332 is switched to 0 so as to load a new PC value from ALU output. Otherwise, the PC increments, as normal. BRANCH_TRUE checks the branch condition by reference to the appropriate flag bits, using a multiplexer which is hard wired to the appropriate instruction code bits. The branch address (new PC value) can be specified using one of four addressing modes. These are similar to the data addressing modes and the same hardware to generate them. The four modes are Direct, PC relative, X relative and Indexed Y. Multiplexer 324 is set to add the correct register/value (inputs 0 to 3 respectively) to the sign extended value specified in the instruction. Multiplexer 328 selects its input 0 for immediate modes PC relative and X relative, and selects 1 for Direct and Indexed Y. Note that the PC value used for PC relative addressing is that of the branch instruction itself, as the processor does not pre-increment the program counter. One use of the Relative X mode is to return from subroutines (as the return address is stored in the X register) . The new PC value is presented on the ALU B path. The ALU is set to 'B' and the value routed through to Σ where it can be loaded into PC. Branch to Subroutine Instruction: BSR BSR is identical to the unconditional branch BRA, except that the return address is stored in the X register (return address is the current PC + 1). This is achieved by the 1 input of the multiplexer 320 on the input to the X register which allows the incremented PC to be loaded into X whilst the main ALU pathway is used to transfer the new PC value of the branch address. Multiplexer 320 is used only for this instruction. Shift Instructions: SAL, SCL, SAR, SCR The shift instructions operate on the combined AL/AH register 'A' and are performed entirely locally to the shift/load logic 312 - the ALU is not involved. This allows the shift operations to be performed over the full 32-bit A register, while the ALU retains is compact, 16- bit size. Shifts are performed by successive one bit shifting in this embodiment requiring simply 32 three-way multiplexers, to give left shift, no shift or right shift per bit. Six different operations are possible, the appropriate one being selected by the control unit 300 in accordance with the current instruction: (1,2) load AH or AL directly with no shift; (3) shift AH and AL left; (4) load AH while shifting left; (5) shift AH and AL right; and (6) load AH while shifting right. The number of bit positions to shift is specified as a memory/immediate value to the instruction. This is read off from the ALU B path by the control architecture and used to generate the appropriate number of cycles. Each cycle then shifts by one bit left or right. In the conditioning logic 334, the SHIFT_IN value is selected to allow the carry flag C to be shifted in for SCL/SCR shifts, zero to be shifted in for SAL, and the current sign bit (AH bit 15) to be shifted in (extended) for SAR.SHIFT_OUT, the bit shifted out of A is loaded into the carry flag C with each bit shifted. Multiply Instruction: MULT Multiply is performed by repeated shift and add and takes 16 cycles. The algorithm uses a combination of the ALU 302 and the shift/load logic 312, shown schematically in Fig. 4A for one cycle. The shift/load block 312 is multiplexer controlled by the instruction decoder and, during MULT, by the current lowest bit A[0] in the A register. One of the operands is the initial value of AL, the other is a memory/immediate value presented on ALU input B. The result is produced in AH/AL. The initial value of AH acts as an addend to the result, and AH should normally be cleared before the operation. As shown by the asterisk (*), the ALU 302 executes an addition for the first 15 cycles, but a subtraction in the last cycle. In pseudo code, the algorithm is as follows. A is the concatenation of AH and AL. {} indicates bit concatenation. Repeat 16 times If last time round loop then op = SUBTRACT else op = ADD If A[0] == 1 then A[31:15]:= A[31:15] op ALU_B —add (*) ALU_B into top 16 bits A[31:0]:= {ALU_S,A[31:1]} — shift right else A[31:0]:= {A[31] ,A[31:1] } —sign extend shift right end if end The ALU calculates A[31:15] "op" ALU_B on each cycle ("op" is either "plus" or "minus", set by the control architecture according to the cycle). A[0] is fed to the control architecture. This then sets the shift/load logic 312 accordingly, either to sign extend shift right (when A[0]=0) or to load AH and shift right shifting in ALU sign flag S (A[0]==1). Divide Instruction: DIV Divide is performed by repeated subtraction and takes 16 cycles. Again, the algorithm uses a combination of the ALU 302 and the shift/load logic 312, as shown schematically in Fig. 4B. The dividend is the initial value of AH,AL, the divisor is a memory/immediate value presented on ALU input B and the result and remainder are generated in AH,AL. The pseudo-code for DIV is: Repeat 16 times ALU_RESULT:= A[31:15] - ALU_B If ALU_CN == 1 then A[31:0]:= {A[30:0],0} —shift left, -0 else A[31:15] := ALU_RESULT —subtract ALU_B from top 16 bits A[31:15]:= {A[30:0],l} —shift left, -1 endif end The ALU calculates A[31:15] minus ALU_B on each cycle, and according to the value of ALU_CN, the control architecture sets the shift/load logic 312 either to shift left (when ALU_CN = 0) or to load AH and shift left (when ALU_CN = 1) . SIF cycles There are four cases of SIF cycles: memory read, memory write, register read and register write. These will be described more generally later, with reference to the generalised embodiment if Fig. 7. Briefly, and with reference to Fig. 3, SIF memory read cycles set multiplexer 326 to the SIF address and load the result directly and in parallel from the DATA bus into the SIF DATA register 342. SIF memory write cycles set multiplexer 326 to the SIF address and write directly from the SIF_DATA register 342 to the DATA bus via the SIF_WRITE tri-state buffer 346. SIF register read cycles use the DATA bus as an intermediate. The register to be read is selected using multiplexers 320 and 322. This can be AH, AL, X, Y, PC, flags or the current instruction. This value is enabled onto the DATA bus via tri-state buffer 335 and from there to the SIF_DATA register 342 as if it were a memory read. Note that the top 2 bits of the current instruction are fed directly to the SIF_DATA register and will be loaded for any SIF read operation. These bits are only of interest when reading from the current instruction, however. Since each instruction word is eighteen bits wide, the DATA bus alone can only be used to carry the lowest sixteen bits. SIF register write cycles again use the DATA bus as an intermediate. The value to be written is gated onto DATA via SIF_WRITE tri-state buffer 346 and from there through the ALU on the B path. ALU output Σ can then be loaded into the appropriate register. PIN DESCRIPTION The following provides a complete and detailed description of the function of each of the "pins" of the processor core. As shown in the basic arrangement of Fig. 1, some of these pins are connected to physical pins of the integrated circuit (ASIC), while most are connected only internally of the integrated circuit. In the following description of pin functions, the pin name and a pin type input, output or bi-directional (tri- state) is presented. The functions and operations of the pin are then described. Figs. 5A to 5G of the drawings are presented to illustrate the waveforms present on the pins, as described below. RST (input ) - Asynchronous reset. Resets processor to known state. Registers and flags 0 Sleep state Awake Run/Stop state Run SIF cycle None pending On releasing RST the processor will start executing code from address 0. The internal reset signal is held active until a falling clock edge after RST is released to ensure a clean restart. PCLK(input . - Processor clock. As shown in Fig. 5A, each processor cycle requires 4 PCLK clocks. Both edges of the clock are used. Cycles always start on a rising clock edge and comprise this and the following 7 clock edges. The edges are referred to as clock 0 through clock 7 and the parts of the cycle are numbered according to the clock they follow. PCLK may be stopped and restarted at will to switch the processor on and off. Note that output signals will freeze in whatever state they were in and that the processor will not be sensitive to any input signals except RST whilst the clock is stopped. The 4 clock sequencer is disabled when the processor is idle. The phase of cycles is therefore not fixed from reset for all time. Most processor instructions execute in a single cycle. WAKE UP (input) - Wakes the processor from sleep. As shown in Fig. 5B, when the processor executes the SLEEP instruction it goes into a sleep mode where activity is minimised and the power consumption is very low. WAKE_UP is then sampled on the rising edge of every PCLK and the processor wakes up and restarts with the instruction following SLEEP when a 1 is detected. In terms of the cycle, the PCLK rising edge where the 1 is detected is clock 0. Note that if WAKE_UP is held at 1, SLEEP behaves just like a NOP. STOPB OUT (output.. STOPB IN (input). RUN STEP (input. - Processor start/stop, breakpoint and single step debug facility. This is intended only for debugging and is not for use by the application. These signals must be brought out to outside pins STOPB and RUN_STEP via pads as shown. The description that follows describes the pin level behaviour. The processor is either running or stopped. This is a top level state above whether it is asleep or awake. STOPB is open-drain with an internal pullup and is normally high. As shown in Fig. 5C, when the processor executes the BRK instruction, it drives STOPB low and stops. This indicates to the outside world that the processor has hit a breakpoint. The processor can be manually stopped by pulling STOPB low externally during a rising PCLK edge. The processor immediately takes over holding STOPB low and will stop at the end of the current instruction. RUN_STEP is an input with a built-in pullup. As shown in Fig. 5D, when high, it forces the processor into the run state. RUN_STEP over-rides STOPB, so that if it is held high, the processor will run continually ignoring breakpoints and stop requests. In normal use, the pin should be allowed to pull itself into this condition. For debug, RUN_STEP should be normally low. Breakpoints are then enabled. Note that this means that strategic breakpoints can be left in the final code and enabled by control of RUN_STEP. This is a powerful tool for test and verification purposes . RUN_STEP is then taken high for one clock to restart the processor. As shown in Fig. 5E, single stepping requires control of both STOPB & RUN_STEP (illustrated for single cycle instruction) : (1) Take RUN_STEP high (2) Wait until STOPB rises (3) Take RUN_STEP low and drive STOPB low. (4) Wait > 1 clock (5) Release STOPB PROG ADDR (output). PROG CLK (output.. INSTRN (input. - Program space memory interface. Unclocked or clocked ROMs can be used. PC is the instruction address (16 bits). As shown in the Fig. 5F waveforms PR0G_CLK is the clock for clocked ROM (active rising edge) and INSTRN is the instruction data (18 bits). For multi-cycle instructions, PROG_CLK rises on the first cycle, stays high from intermediate cycles and falls on the last cycle. PC changes only on the last cycle. ADDR, R B (output) . LIM WRITEB (output) , DATA CLK (output), DATA (bidirectional) - Data space memory interface. Unclocked or clocked memory can be used. ADDR is the address (16 bits). R_WB is the read/write select line, LIM_WRITEB allows protection of parts of the memory space from SIF writes, DATA_CLK is a cycle strobe which also acts as a clock for clocked memory and DATA is the bidirectional data bus (16 bits). See the waveforms of the Fig. 5G. If an instruction does not require a memory cycle, DATA_CLK will stay low. For multi-cycle instructions, DATA_CLK will go high on the first cycle, stay high for intermediate cycles and go low on the last cycle. ADDR etc will stay valid from the first cycle to the last cycle. LIM_WRITEB is normally high. It goes low when a write to memory is requested from the SIF in normal operation (as opposed to in debug with the processor stopped). Including this signal in an address decode prevents user writes to the decoded device from the SIF. This is necessary for protecting some applications against accidental damage. Other devices may be written from the SIF, allowing for instance communication between the ASIC and a general purpose microcontroller. TEST OUT (output) - IC test output. An XOR of the ALU output bus which can be used to give visibility of processor operation for IC test. It is either brought out as a specific pin or included in an XOR tree which is externally visible. SER IN (input), SER OUT (output), SER CLK (input). SER LOADB OUT (output). SER LOADB IN (input) - Serial interface (SIF). These signals are brought to outside pins SER_IN, SER_OUT, SER_CLK and SER_LOADB via pads as shown. The description that follows describes the pin level behaviour. The SIF provides a method for transferring data serially into and out-of a device, by means of a shift register in the device. The present processor uses a SIF shift register length of 36 bits. The data input to the shift register is SER_IN and the data output SER_OUT. SER_CLK is the clock. Data is clocked into the shift register on the negative edge, and out on the positive edge. SER_CLK is completely asynchronous to the processor clock and can be faster or slower than it if required. Data is clocked in and out MSB first. SER__LOADB is used to co-ordinate transfers via the shift register. The serial interface allows (1) the program space to be read, (2) the data space to be read and written and (3) the processor registers to be read and written. In normal running, SIF transfers are only carried out when a SIF instruction is executed. When the processor is asleep or stopped, SIF transfers are carried out immediately, starting the cycle engine for one cycle to perform the transfer. In normal running, the registers and limited data space areas cannot be written. In stopped state the registers and the full data space be written to. SER_LOADB is an open-drain output with an internal pullup. To perform a cycle, the shift register is loaded and SER_LOADB pulled low for > 1 clock. The process detects the transfer request and holds SER_LOADB low until the cycle has been completed. This handshake system is robust and places no timing constraints on either side of the interface. SER_CLK must be low when SER_L0ADB is low to allow the shift register to be loaded by the processor. Fig. 6 shows the SIF shift register arrangement. The operation of the SIF will be described in more detail below. The SLEEP bit is read only. Undefined bits read as 0. Program space read operations, read from the PC address and increment PC. This allows sequential reads without specifically setting up an address. The definition allows just '1's to be shifted into SER_IN as the data is read out to repeat. NOTES Literals which are outside the range -512..+511 can be automatically implemented as direct by the assembler with the value stored in data ROM. From the user point of view it will seem that full range literals are available. Branch targets which are outside the PC relative range can be automatically implemented as direct by the assembler with the offset stored in data ROM. From the user point of view it will seem that branches to anywhere in address space are available. When executing a PC relative branch, PC points to the branch instruction itself. Subroutine calls use the X register for the return address: BSR fred Branch to subroutine with BSR fred Subroutine BRA 0,X Return with BRA If the subroutine itself calls a subroutine or otherwise uses X, it must be saved in memory and restored before exit. Conditional returns can be implemented by using BCC instead of BRA. Multiple exit points can be implemented by returning to 0,X or 1,X etc. Addition and subtraction operations work on signed or unsigned, integer or fractional values. ADD, ADDC and NADD treat C as a carry. SUB, SUBC and CMP treat C as a borrow. ADDC and SUBC facilitate straightforward multi-precision arithmetic. The S flag applies to operations on signed values and gives the true sign of the result, independent of overflow into the MSB. It is calculated as N Λ V and is especially useful for CMP where it indicates signed "less than" (N only indicates this over a limited operand range) . MULTiply is signed. Integer and fractional multiplies are implemented as follows ("&" is the assembler macro parameter substitution operator): IMULT data ; Multiply integers AL * data. Result in A LD AH,#0 MULT &data FMULT data ; Multiply fractionals AL * data. Result in A LD AH,#0 MULT fidata SAL #1 Integer multiply never overflows, even if AH is non-zero to add to the result. Fractional multiply only overflows when multiplying -1.0 by -1.0. AH adds to the result as a signed fractional divided by 2 15 , and cannot precipitate overflow. Unsigned integer multiply is implemented by a macro: UIMULT data, temp ; Multiply unsigned integers AL ; and data. Result in A temp is a workspace RAM > location that must be supplied LD AH,#zero ST AL,@&temp MULT &data TST Sdata BPL $+2 ADD AH,@&temp TST @&temp BPL $+2 ADD AH,&data However, if the code of UIMULT is used without zeroing AH it does not provide an unsigned multiply/accumulate, as AH is added as a signed value (sign extended). Divide is signed, positive only (MSB of A and data must be zero). Positive integer and fractional divides are implemented by macros in the assembler as follows: PIDIV data ; Divide +ve integers A ÷ data. ; Result in AL, remainder in ; AH[15:1], AH[0]=0 SAL #1 DIV sdata PFDIV data ; Divide +ve fractionals A ÷ data. ; Result in AL, rounding bit ; AH[15] DIV &data Note that A is a long integer or a double precision fractional in the above. IDIV and FDIV will overflow if AH > data. No indication of overflow is provided. To implement a full signed divide, the operands must be made positive before the divide and the result corrected afterwards. Shifts can be from 0 to 15 bits. A 0 bit shift leaves A unchanged and sets C as if a 1 bit shift had been performed. This can be used to get the top or bottom bit of A into C. Some other instructions common on other processors are implemented as macros: Branch instructions for the full range of signed and unsigned comparisons are implemented as macros: BHI branch_addr ; Branch higher (unsigned) BCS &skip BNE &branch_addr &skip: BHS branch_addr ; Branch higher or same (unsigned) BCC &branch_addr BLO branch_addr ; Branch lower (unsigned) BCS &branch_addr BLS branch_addr ; Branch lower or same (unsigned) BCS &branch_addr BEQ &branch_addr BGT branch_addr ; Branch less than (signed) BLT Scskip BNE &branch_addr &skip: BGE branch_addr ; Branch greater or equal (signed) BLT &skip BRA &branch_addr &skip: BLE branch_addr ; Branch less than or equal BLT &branch_addr BEQ &branch_addr The primitives BLT, BEQ and BNE complete the set. It is best to avoid using manual PC relative branch (e.g. BRA $+1). These will not work correctly in the multiple instruction macros above and in general make assumptions about instruction lengths which can cause problems when branching over macros. BRK, if it causes the processor to stop, leaves PC pointing at the BRK instruction. If it is over-ridden by the external signal RUN_STEP, BRK behaves as a NOP. PRINT is a debugging aid. It is a NOP as far as the processor is concerned but is detected by a gate level model debugger and a simulator and, if enabled, will print out the instruction's immediate value and specified register. The immediate value can be used to indicate the position in the program. The register value may or may not indicate some useful result. Multi-word variables can be stored in memory MS word first, as a convention. There is nothing in the instruction set to define an order. The following code fragment shows a multi-precision subtract: LD AL,@(varl+l) ; Subtract varl - var2 SUB AL,@(var2+l) LD AH,@varl ; Result in A. C,S,N correct for ; 32 bit SUBC AH,@var2 ; result. Z set on MS word only The Y register can be used as a software controlled stack pointer. The mnemonic "SP" is recognised in place of Y by the assembler. When used as a stack, Y points to the lowest used memory word in the stack. The following illustrates the use of Y for storing the return address and local variables for a subroutine: fred LABEL fred s STRUC ; Define local vars in struct varl DS 1 var2 DS 1 fred_s ENDS ST X,@(-1,SP) ; Store return address SUB Y,#(LENGTH fred s ; Allocate space + 1) • • • @( .varl,SP) ; Local variable address ADD Y,#LENGTH fred s ; Return space + BRA @(-l,SP) ; Return directly from stack Note the convention of storing the return address first on the stack. CONSTRUCTION AND OPERATION OF THE SERIAL INTERFACE (SIF) As described above with reference to Figs. 1 to 6, the serial interface (SIF) operates to allow external access to the memory spaces and registers of the processor via the external pins of the integrated circuit. Fig. 7 shows one practical implementation. The processor in Fig. 7 may be the same as that of Figs. 1 to 6, but could equally be of an entirely different design. The main part of the processor is illustrated at 500, and is shown schematically connected to the program memory 104, and the data memory and I/O - 106, 108, 110. Within the main part of the processor core, an instruction latch 502 receives program instruction words from the program memory 104, which are decoded by a control section 504 of the processor. The registers of the processor are shown at 506. The arithmetic and logic unit (ALU) and other functional units of the processor core are grouped schematically in a block 508. A data bus of the processor is shown at 510. Fig. 7 does not show in any detail the data paths and control lines between the various elements 502 to 510, which can readily be implemented using conventional techniques, for example to implement the functionality detailed above in relation to the instruction set, data architecture and pin description. The serial interface shift register is physically embodied at 512 and features the data field, address field, read/write field and address space field which are as shown in Fig. 6. In response to the serial interface clock signal SER_CLK, one bit at a time can be shifted from the shift register 512 out of the chip (SER_OUT) and/or into the chip (SER IN). An interface control block 514 is shown associated with the shift register 512 and the fourth control line of the interface (SER_LOADB), while in practice the control units 504 and 514 of the processor may be implemented as a single functional unit, or further sub-divided as desired. The address and read/write signals applied to the data memory 106, 108, 110 are primarily supplied (ADDR', R WB' ) by the normal functional elements 504, 508 of the processor core 500, for example as described in the Pin Description above. However, a multiplexer 516 is provided in these address and control lines, so that the address applied to the memory can instead be that which is contained within the interface register 512 (SIF ADDR). Similarly, the read/write control bit for the data memory can be derived from the relevant bit of the shift register 512 (SIF_R_WB) when the multiplexer 516 is controlled appropriately. In particular, a control line SIF_CYC is driven by the control and instruction decoding section 504 of the processor, so that the multiplexer is activated in this way during the cycle of execution of the special SIF instruction, assuming that the bit SIF_A_SPACE indicates that the data address space is to be accessed in a given SIF operation. Similarly, a multiplexer 518 and tri-state buffer 520 are driven to cause the data bits of the shift register 512 to be written from or written to the data bus 510, during a SIF write or SIF read operation, respectively. The main part of the processor core 500 ignores the data present on the data bus 510 during the execution cycle of a SIF instruction. For the case where access to the registers or program memory space of the processor is desired through the serial interface SIF, as indicated by the bit SIF_A_SPACE of the word loaded into the shift register 512, the multiplexer 518, a tri-state buffer 522 and a bi¬ directional selection circuit 524 provide access between the data bits of the shift register 512 and the internal registers 502, 506 of the processor core 500. The selection circuit 524 is controlled by the lower 4 bits SIF__ADDR[3:0] of the address field within the shift register 512, as detailed in Fig. 6. Therefore, during a SIF instruction execution cycle, any of the registers PC, F (flags), AH, AL, X or Y can be read or written, or the currently addressed location of the program memory 104 can be read via the instruction latch 502. As described already, the program counter value stored in register PC can be incremented automatically to allow sequential access to a range of locations in the program memory 104, when the debug mode is activated. In practice, the selection circuit 524 and multiplexer 518 may readily be combined with existing data path selection components of the processor core 500, to achieve a very compact circuit. The particular mode of handshaking between the chip (ASIC) and the external apparatus is explained in the Pin Description above, but will now be further illustrated with reference to Figs. 8 and 9. In the left hand flow chart of Fig. 8, beginning at 600, the actions of the external apparatus wishing to write a data value into a location within the storage space of the integrated circuit begin at 600. At 602, while the fourth wire SER_L0ADB remains high (passive state), the data line SER_IN and the serial clock line SER_CLK are used to load the various bits of the shift register 512 with the ASIC. A matching shift register will typically be provided within the external apparatus for this purpose. When the shift register on chip contains a complete SIF instruction word, the external apparatus (step 604) sets the fourth wire SER_LOADB to an active value (0) for a long enough time (one clock cycle or more) that the SIF control circuit 514 within the ASIC can recognise that an interface operation is desired. The external apparatus then releases the fourth wire at step 606, and then (step 608) watches the fourth wire to see whether it returns to the passive state (1) or whether it is being held active (0) by the ASIC. Only when the fourth wire returns to the passive state is the SIF write operation considered complete (610). Referring now to Fig. 9, the operations within the ASIC in relation to the serial interface begin at 700. At 702, the fourth wire SER LOADB of the interface is monitored by the SIF control unit 514 of the ASIC, until it is seen to enter the active state. At 704, the ASIC immediately actively holds the fourth wire in the active state. At 706, the command which has been loaded into the register 512 of the interface is processed during the next available SIF instruction cycle. When a write instruction is indicated by the bit SIF__R__WB in the register, this causes the DATA field to be written into the storage location of the ASIC determined by the fields SIF__A__SPACE and SIF ADDR. In the case of a read operation, the DATA bits of the register 512 are loaded with data read from that location. Only after the SIF operation has been processed within the ASIC does the control circuit 514 release the fourth wire of the interface (step 708). This is to inform the external apparatus that the write operation is completed, or that the information to be read can be clocked out of the serial interface now. It is not guaranteed, however, that the external apparatus will already have released the fourth wire (step 606), and a loop is implemented at step 710 to monitor the state of the fourth wire. Only when this wire is seen to go high again does control return to step 702. In this way, repeated SIF instructions are not implemented merely because the external apparatus is very slow to release the fourth wire. Returning to Fig. 8, the right hand flow chart beginning at step 620 illustrates the SIF read operation. Steps 622 to 628 are the same as corresponding steps 602 to 608 of the SIF write operation, except that at step 622, no data needs to be clocked into the shift register 512 of the ASIC, saving time. Also, of course, the bit SIF_R WB is set to indicate read instead of write. After it has been detected at step 628 that both the external apparatus and the ASIC have released the fourth wire (SER LOADB = 1) the data read from the desired address or register within the ASIC is present in the DATA bits of the shift register 512, and is clocked out of the shift register into the external apparatus in step 630. Double broken lines in Fig. 6 connect step 630 with step 602 or 622 to illustrate that, if desired, successive SIF operations can be pipelined by performing the step 630 of reading the shift register 512 simultaneously with the step 602 or 622 of loading the register 512, using the data input and output wires SER_IN and SER_0UT in parallel. POWER SAVING FEATURES In a static CMOS or similar circuit implementation, power dissipation arises only during signal transitions. In the processor described above overall power dissipation is greatly reduced, even compared with conventional "low- power" designs, by interposing latches at various stages within long combinational logic data paths including for example the arithmetic and logic unit (ALU). Within each instruction cycle subdivided into a number of 'clock states' S0-S7, the latches are clocked only at a given state of the clock within the cycle, when all data and control lines which form inputs to the relevant stage have settled to a meaningful value. Transient states arising at the inputs of each stage before that time therefore cannot cause superfluous transitions to propagate far within the combinational circuitry of the processor throughout the instruction cycle. In the example processor of Figs. 1 to 6, for example, such latches are provided for examples on the current instruction (INSTRN), to isolate the decoder from ripple on the instruction bus. There are also latches on the ALU inputs A, B and CO, and also on the control lines from the instruction decoder 300. In addition to power saving in this way, these and further latches may be used to latch external inputs and to protect against "clock skew" at various points. Referring again to Fig. 3, it is also a power saving feature of the present processor design that most parts of the data architecture have dedicated data buses, rather than a shared data bus. The DATA bus which leaves the processor core and which is also connected to the SIF register is an exception, in that it is driven by logic with tri-state output buffers 345, 346 etc. However, as described above, the output Σ of the ALU 302 is connected by dedicated pathways to the registers 304 to 310, and the outputs of these registers are similarly connected to the inputs of the ALU by dedicated pathways and multiplexers 320 to 328. Compared with conventional processor designs in which all such elements are interconnected by means of shared data buses, the present processor design has eliminated many tri-state buffers that would otherwise be required at the outputs of the ALU and registers to drive a common data bus. Also, each dedicated data path has a lower capacitance than the conventional shared bus, with the end result that the power consumption of the processor core is lower. Fig. 10 shows schematically circuitry implemented for monitoring wake up signals during the SLEEP or STOPPED states of the processor. In these states, the main functional elements of the processor are not clocked by the usual high frequency clock signal (PCLK in this embodiment), to reduce power dissipation in those circuits. On the other hand, external inputs must be monitored to cause the clock signal to be applied once more to these parts. In the present embodiment, signals such as STOPB_IN, RUN_STEP and WAKE_UP must be monitored, and also the SER_LOADB_IN. Any of these may require the processor to perform at least one cycle of operation. In an alternative processor embodiment, external signals indicating interrupt requests may typically need similar monitoring. Fig. 10 shows the main high frequency clock signal PCLK input to the processor core. This is gated at 800 with a clock enable signal CENFF to generate an internal clock signal PCLKI. The main part of the processor circuitry is driven by PCLKI, so that when CENFF is low, the main processor circuitry is not running and power dissipation is reduced. A trigger circuit 812, however, is perpetually clocked by PCLK. An input on the trigger circuit 812 receives an asynchronous clock enable signal CEN and, at its output, generates the clock enable signal CENFF which is applied to the gate 800. Different external signal lines EXT1, EXT2 etc. are to be monitored, and each has its own flip flop 808, 810 etc. and XOR gate 804, 806 etc. for this purpose. The XOR gates 804, 806 produce individual clock enable signals CEN1, CEN2, from which an OR gate 902 generates the combined clock enable signal CEN. In operation, flip flop 808 has a D input monitoring the external signal EXT1, but is clocked only by the internal clock signal PCLKI (output of gate 800). Therefore, when the processor is stopped or sleeping, the Q output of flip flop 808 carries a signal EXT1FF which is a state of the EXT1 signal, memorised from the last time that the processor was running. At the same time, there is no power dissipation in the flip flop 808. The XOR gate 804 compares the actual input signal EXT1 with EXT1FF, and generates the individual wake up signal CEN=1 as soon as there is any change in the external signal EXT1 relative to EXT1FF. By the operation of OR gate 802, CEN goes high, and the trigger circuit 812 sets CENFF high, enabling the internal clock PCLKI for the entire processor. At this point, the present state of signal EXT1 becomes latched also as EXT1FF, so that the CEN1 signal itself disappears. A similar operation is provided in relation to input EXT2 by the XOR gate 806 and the flip flop 810. Any number of such inputs can be provided, with the same or different monitoring circuitry. Compared with a circuit in which, for example, each individual flip flop 808, 810 is clocked by the running clock PCLK to generate a synchronous clock enabled signal, the present arrangement achieves a reduced power dissipation in the sleeping or stopped state. Only the clock input of a single flip flop within the trigger circuit 812, in addition to the input of the gate 800, need to be continuously supplied with the running clock PCLK, no matter how many inputs are being monitored. To enter the sleep or stopped state, a clock disable signal CDIS from elsewhere in the control logic of the processor is applied to the trigger circuit 812, which sets CENFF low and so disables the internal clock PCLKI. APPLICATION EXAMPLE - DOMESTIC GAS METER WO-A1-95/04258 describes an ultrasonic domestic gas meter apparatus, which measures gas velocity by a "time of flight" principle. To be successful as a domestic gas meter, such a product must be designed for a world market and have different versions conforming to several national standards. It should be powered by batteries which will last more than ten years, even in a wide range of hot, cold, dry and damp environments. The principles of operation of the ultrasonic gas flow meter, and the measurement algorithms to be implemented are described in more detail in WO-A1-95/04258. Briefly, two ultrasonic transducers of the gas flow meter are used both to transmit and receive ultrasonic pulses. One transducer is installed upstream of the gas flow in the meter, the other downstream. In operation, the upstream transducer sensor sends an ultrasound pulse to the downstream transducer, and the time of flight for this pulse is measured with very high precision. Then the downstream transducer sends the pulse to the upstream transducer, and the time of flight is again measured. If the gas velocity is zero, both readings will be the same. However, as the gas flow increases, the time of flight downstream will become shorter than the time of flight upstream. From this information it is possible to calculate the gas velocity but only with sophisticated signal processing. Such readings are taken every few seconds, and the accumulated volume of gas which has flowed is updated. The signal processing electronics of the gas meter are therefore required to drive and receive signals from an ultrasonic transducer, to make very accurate time and voltage measurements, to perform sophisticated signal processing and yet be flexible enough to pass the requirements of several national standards. At the same time, the circuitry should run at a very low voltage, have a very low average current consumption, have a very good long term reliability and very low unit cost when manufactured in high volume. Nowadays an ASIC (custom IC chip) solution might naturally be adopted to implement a specialised instrument of this type, and CMOS ASIC processes in particular are known to provide many advantages such as low cost and low power consumption. The use of this technology enables the development for example of custom analog cells to drive and receive signals from the ultrasonic transducers. However, the extreme low power consumption desired in the this example product can only be obtained when the program controlled processor which implements the signal processing and control functions is also integrated, with its program store, on the same chip. Also, having the processor on-chip would reduce interference emissions and reduce susceptibility to interference from outside. Aside from the obvious benefits of compliance with EMC regulations with less shielding, higher emissions within the product would impair the accuracy of the measurement electronics. Unfortunately, as mentioned in the introductory part of this application, conventional processor designs tend to be too expensive in terms of chip area, and/or are not powerful enough per instruction for arithmetic-intensive applications. Large circuit size and high processor clock speed will only increase the power consumption. The problem of verification of the design also arises when all components and the control program are fixed in the chip hardware, and when the program implements real- time operations. It will not normally be possible with conventional designs to be sufficiently confident in the design and programming to commit both to hardware. Finally, when the design is completed, it must contain explicit provision for any product modifications that may be necessary in the future. The novel processor design described above largely overcomes these drawbacks, while the processor and serial interface can be provided as one or two cells in an ASIC design "library", to be incorporated in a wide range of designs. Fig. 11 shows schematically the arrangement of the electronics for such a gas meter in which a CMOS ASIC 1100 includes a data processor of the type shown in Fig. 1. The ASIC includes processor core 100, program memory 104, data ROM 106 and data RAM 108. ASIC 1100 also includes specialised digital circuitry 1102 for signal processing and control functions and also specialised analog circuitry 1104, including transducer drivers, switches and digital-to-analog converters. On the same printed circuit board 1105 are connections for ultrasonic transducers 1106 and 1108, a 3 volt Lithium battery 1110, a small liquid crystal display 1112, a conventional low cost microprocessor chip 1114 and some electrically erasable, programmable non-volatile memory (EEPROM) 1116. These last two components communicate with ASIC 1100 via the serial interface (SIF) 1118, which also provides a test port 1120 for the gas meter electronics. In performing signal processing, the powerful 16 bit arithmetic functions with the 32 bit accumulator register allow the algorithms to be implemented with a minimum of power consumption. Also, the digital circuitry 1102 provides integrated hardware for the high speed and repetitive processing of data from the analog circuity 1104. Reliable running at very low voltage, for example outside in very cold weather at the end of the life of the Lithium battery 1110, is more readily achieved with an integrated solution, since the entire ASIC design can be characterised for low voltages which would not be possible using various standard components. Average power consumption is reduced greatly by giving the ASIC 1100 control over the power supplies to the components 1112 to 1116. The processor core consumes very low power when it is in the sleeping state, and zero power when the clock signal PCLK is stopped. Simple timing circuitry on the ASIC can be provided to start the processor clock only intermittently, when measurements are required to be taken, and the processor 100 when running can take further control of the other circuity of the ASIC 1100 and the printed circuit board 1105, activating only those circuits which are required at a given time. The power consumption of the processor is very low even when it is running, due to various features mentioned above. Furthermore, because the arithmetic instructions are powerful for the size of the processor, the processor does not need to execute so many instructions for a given calculation. To maintain economies of scale while providing a domestic gas meter that can meet different national standards and allow several product variants, the ASIC 1100 (including its stored program) should be the same for all such variants. This would not be achieved readily in conventional microprocessor architecture, but in the present example the difference between the products is implemented in the low cost external microprocessor 1114 and the EEPROM 1118. This can be changed (re-programmed) at a much later stage in product development, as each new requirement comes to light, or even after installation in the field. The serial interface 1118 enables the external microprocessor 1114 to have direct read and write access to the memory space in the ASIC, so that parameters of the measuring process can be changed as necessary. It is an advantage of the serial interface design presented herein that, at the time of fixing the ASIC design and programming, it need not be determined to which memory locations the external microprocessor will need to have access. Therefore, there is great flexibility for unforeseen modifications in the product development. The novel processor and interface architectures allow "verifiable" design and programming, so that the risk of errors in the final ASIC can be reduced, despite the integration of processor and program ROM on-chip. Flexibility of the design for future product requirements is open, by means of the serial interface. With regard to cost, the processor core 100 can be very small (only 3000 gates), requiring only about 1mm 2 of silicon. The unit cost of such a silicon area, in volume manufacture, is about £0.15 at the present filing date, while it is not possible to buy a standard external 16 bit microprocessor for such a low price. Furthermore, by implementing so many components on the ASIC, the product has very few components in total, giving very low PCB, assembly and test costs. Reliability is also enhanced, because failures in electronics tend to happen because of broken or poor connections, and the number of connections in the design of Fig. 11 is very low. FURTHER NOTES Those skilled in the art will recognise that the detailed implementation of a microprocessor or other circuit embodying any aspect of this invention need not be limited to the examples given above. Of course the details of the instruction set can be changed to suit a given application, the widths of address and data busses, and the widths of various fields in the instruction word of the processor can be changed also. Even at a more general level, the scope of the present invention encompasses many individual functional features and many sub-combinations of those functional features, in addition to the complete combination of features provided in the specific embodiment. Whether a given functional feature or sub-combination is applicable in a processor having a different architecture, for example a processor with pipelined instruction decoding and execution, will be readily determined by the person skilled in the art, who will also be able to determine the adaptations or constraints imposed by the changed architecture. It will also be appreciated, that, whereas the program instructions and initial data for the processor operation are permanently fixed in ROM storage on-chip, embodiments are perfectly feasible for prototyping and/or final production in which the ROM is replaced by E 2 PR0M (electrically erasable programmable read only memory) or one-time-programmable ROM, where the processes used for manufacture (and the costs) will permit. All or part of the program store may in some cases need to be off-chip. If the pin count associated with the architecture is too high, it may be reduced for example by providing an 8-bit program ROM, and performing multiple accesses to build up each instruction word. Concerning the novel architecture for arithmetic operations, the provision of the 32-bit shift unit separately from the 16-bit ALU allows a combination of high signal processing performance and small circuit size. A conventional processor having shift functions in the ALU would typically provide a 32-bit wide ALU or forego the 16x16 multiply and divide instructions. Of course, the ALU of the present processor could be provided with 16-bit shift functions if desired. Also, although a single bit shift function has been provided in the above embodiment, requiring n cycles for an n-bit shift, there is also the possibility to include a "barrel shifter" to allow n-bit shifts in a single cycle. The choice of 1-bit or n-bit shift circuitry is a trade off between desired processing speed and circuit size. Concerning the serial interface (SIF) described above, it will be apparent that similar functions can be performed by other types of serial or parallel interface, and it will also be apparent that the SIF connections and protocols can be applied for communication between different apparatuses or devices which need not be ASICs or in any way similar to the above described processor. Other standard interfaces well known in the art are the I 2 C 2-wire interface of Philips Electronics NV, the SPI interface of Motorola Corp., and the SCI interface of Hitachi. The SIF described above can readily be interfaced, for example, to a Motorola processor (for example MC68HC11) having an SPI type interface. Another interface known in the art is the "Microwire" interface of National Semiconductor Corp, with particular application to E 2 PR0M components. The SIF described above has the advantage of a very simple and robust structure and protocol, with a single master, and can be used in many of the same applications as the known interfaces mentioned above. Although the Microwire interface defines a simple fixed master and slave relationship, the known interfaces like I 2 C and Microwire impose fixed word lengths and strict timing constraints on the slave device. The SIF as described herein allows variable word lengths, and each of master and slave can take as long as it needs to respond to the interface. This avoids the need to interrupt the flow of control in the ASIC at short notice, in response to unpredictable external stimuli, thereby simplifying the design and verification of new ASIC designs. In particular, the programmer of the present ASIC can keep track of real time supply by calculation from the clock speed and known instruction execution times. In a conventional processor design, where interrupts may occur in response to external stimuli, the current location in the program is no guide to elapsed real time, and other timer mechanisms, typically implemented by further interrupts, are required to implement real-time dependent operations. Also, since there is no speed constraint, the SIF can be used to read values from any of the memory-mapped I/O devices, which might require a lengthy wait for response from some off- chip peripheral (keyboard or sensor) . Fig. 12 shows how it is possible to enable the SIF architecture to allow a microcomputer to address plural slave ASICs, for example by providing a separate "chip select" line CS to each slave. The data and clock lines then become effectively a serial data bus. However, unlike conventional serial buses, each slave is free to define its own word length, according to need. Therefore, a slave need not provide a 36-bit shift register when only eight bits are ever needed by that slave, and the master need not waste time sending more bits than necessary to any slave. In Fig. 12, an external microprocessor 1200 is shown as master, while three ASIC devices 1202, 1204 and 1206 are shown as slaves. The serial databus comprises a common clock line SIF_CLK. The serial data line which inputs to the slaves is shown as SIF_M0SI ("master out, slave in") while the common data line on which data is output by the slaves is shown as SIF_MISO ("master in, slave out"). The bi¬ directional control line SIF_LOADB is shown connected in common to all devices, and corresponds to the control line SER_LOADB of the earlier described embodiments. The master 1200 also has individual chip select outputs SIF_CS0, SIF_CS1 and SIF_CS2, connected to chip select inputs of the slaves 1202, 1204 and 1206 respectively. In operation, when SIF_CS is low for a given slave ASIC, the SIF_MIS0 connection of that slave is put into its high impedance state using a tri-state buffer, and also the SIF_LOADB output of that slave is put in the high impedance state, using the tri-state buffer already provided (see Fig. 7). In the present embodiment, the chip select signal does not affect SIF_CLK or SIF_MOSI in any way. Therefore, it is still possible to clock new data into the shift register of the interface, even when the chip select is low. However, the data output of the slave SIF_MISO is disconnected from the register, so that no data will be clocked out. When the chip select is low, the slave will also ignore SIF_LOADB when it is pulled low by the master, and in particular will not latch SIF_LOADB low and will not queue up a SIF access operation for the next SIF instruction cycle. When SIF_CS is high for a given slave, however, than the SIF_MISO output is enabled, and the SIF_LOADB output line input/output is enabled in exactly the manner described above with respect to Fig. 7. Extension of the interface handshake mechanism to provide for plural masters is equally feasible, but will not be described herein. Another possible modification of the SIF described above concerns the SIF read operation. Since no address value needs to be present in the shift register bits SIF_ADDR[15:0] when the value is readout of the ASIC, it would be possible for example for every SIF read operation to provide access not only to the particular memory location requested, but also to supply a fixed set of status values such as the flags register or program counter PC. These values could be available at little extra cost, being loaded into the address field of the interface register at the same time as the data field is loaded, and need not be clocked out by the external device if they are not of interest. Another feature of the SIF type of interface is that the separate data input and output wires SER_IN and SER_0UT can be used simultaneously to read data from the ASIC and to load another word into the SIF shift register on the ASIC, to set up the next read or write operation. This potential for parallel operation of SER_IN and SER_0UT at each cycle of the clock SER_CLK is illustrated by the double broken lines in between the flowchart step 630 and the step 602 or 622 in Fig 8, and can be applied in other interfaces having a separate data wire for each direction. An interface of the SIF type can also be employed in an apparatus having memory and CPU (central processing unit) on different chips. The SIF shift register and the multiplexer which governs access to the address bus could then be provided by components internal or external to either chip. Concerning the SIF instruction in the processor instruction set described above, it will be apparent that the concept of releasing the memory address bus or register addressing circuitry for external access during a particular instruction (effectively an NOP instruction) is broadly applicable to a wide range of processor architectures. Such a concept can provide both testability and off-chip communications for normal operation in a single functional unit. Similarly, the scheme of hand-shaking provided by the SER_L0ADB line in the SIF interface circuitry can be applied in a wide range of interface types, and is particularly advantageous where the speed and timing capabilities of two devices wishing to communicate are either unknown or anyhow widely disparate. Also, rather than depending on the presence of stop state or a specific SIF instruction code within the stored program the processor sequencing circuitry can simply provide a regular time window, for example one cycle every 128 cycles, in which memory is addressed by the SIF and not by the processor itself. Alternatively, or in addition, the SIF instruction code may be replaced by a family of instruction codes, allowing different types of serial interface access. For example, a programmer may be happy to allow SIF read operations more often during the running of the program than SIF write operations. In such a case, separate instructions SIF_READ_ONLY (allowing the interface only to read from the memory space) and SIF_ALL (allowing both read and write operations) might be defined, for example. Then, if a SIF_WRITE instruction is received via the interface, this will only be processed during the next SIF_ALL cycle, irrespective of how many SIF_READ_ONLY cycles may have been executed in the meantime. Similarly, different instructions might be provided to allow access to different parts of the process of address space at different times. Of course, for each instruction of this type, it is still the case that the programmer determines only the timing of the memory access, while the specific memory access operation desired is defined by the external apparatus. The de-bug control circuitry, featuring the control lines STOPB and RUN STEP is similarly applicable in a wide range of processor architectures and applications. While this control mechanism is particularly useful in combination with the serial interface functions described, these features are also of use independently. The provision of a breakpoint instruction (BRK above) which is conditional on the existence of the de-bug mode is also advantageous in itself, particularly where the microprocessor control program is stored in ROM memory on chip. As described above, the breakpoint instruction BRK can be present in all prototype and final versions of the stored program, but will be effectively ignored by the processor during normal operation. The SLEEP operation, being defined in the instruction set, also gives the programmer control of the point in program execution at which the SLEEP rate will be entered. Even when the SLEEP state is to be commanded from outside the processor, using the WAKE_UP signal line, it will always be delayed until the processor is at a certain point in the program execution, which allows a more verifiable design. Similarly, the processor will always wake up at a known point in the program, which in this embodiment is the instruction after SLEEP. The above and other generalisations will be obvious to the skilled reader and are within the scope of the present invention. Although various specific aspects of the invention are defined above and in the attached claims, the applicant reserves the right to claim any novel feature or novel combination of features disclosed explicitly or implicitly herein.

Claims: CLAIMS 1. A data processing apparatus including a processor constructed to operate under the control of a stored program comprising instructions selected from a predetermined instruction set, wherein an interface is provided to allow an external apparatus to signal a request for communication with the processor, and wherein means are provided to cause the processor to communicate in accordance with such requests only during pre- determined periods of time in the execution of the stored progra . 2. An apparatus according to claim 1 wherein each communication request is interpreted by the processor in accordance with a specific communication instruction loaded into an interface register by the external apparatus. 3. An apparatus according to claim 1 or 2, wherein said predetermined periods are defined by the inclusion in the stored program of a generic communication instruction (SIF) of the instruction set, the generic communication instruction having a fixed execution time. 4. An apparatus according to any of claims 1 to 3, wherein the processor and stored program are provided on a single integrated circuit. 5. An apparatus according to any of claims 1 to 4, wherein said interface comprises a serial interface. 6. An apparatus according to claim 5, wherein said serial interface includes separate data lines for input and output of data to the processor and wherein data can be shifted serially into and out of the processor in parallel via said data lines. 7. An apparatus according to claim 5 or 6, wherein said interface further includes a clock line for synchronising said data input and output asynchronously with respect to operation of said processor. 8. An apparatus according to any of claims 2 to 7, wherein the interface provides at least one control line which can be forced to an active state by either of the processor and the external apparatus, such that, in a request for communication by the external apparatus: the specific communication instruction can be loaded into the interface register of the processor under control of the external apparatus, while the control line remains in a passive state, the active state can then be imposed on the control line by the external apparatus, thereby signalling said communication request, the processor will detect the active level on the control line and itself maintain the active level until the communication request is dealt with during one of said predetermined periods of time, and the processor will release the control line when it is ready to receive a next communication request. 9. An apparatus as claimed in claim 8 wherein a further specific communication instruction will not be read internally from the interface register and processed until the control line has been released by both apparatuses for a predetermined time and then again forced to the active level by the external apparatus. 10. An apparatus according to claim 8 or 9, wherein, at least for a specific communication instruction which is recognised by the processor as requiring information in response, said information is loaded into the interface register of the processor during said predetermined time period, before the processor releases the control line, whereafter it can be read from the interface register by the external apparatus. 11. An apparatus according to any of claims 2 to 10, wherein the specific communication instruction is interpreted by hardware of the processor in accordance with pre-determined rules. 12. An apparatus according to any of claims 2 to 11, further comprising an addressable storage space, wherein at least a first type of specific communication instruction includes an address field interpreted by the processor as indicating a location in said storage space, and wherein the processor is responsive to the specific communication instruction during said predetermined time period to allow reading, writing or a selection of the two to be performed between the interface and the specified storage location. 13. An apparatus according to claim 12 wherein said storage space comprises the data space, register space, and/or program memory of the processor. 14. An apparatus according to claim 13, wherein a memory access interface of the processor includes means (LIM_WRITEB) for distinguishing between memory accesses performed under normal program control and those performed in response to the communication request. 15. An apparatus according to any of claims 12 to 14, wherein a read operation may be performed, wherein an address may be loaded into the address field portion of the interface register to initiate one read operation without loading the entire interface register. 16. An apparatus according to any of claims 12 to 15, wherein a read operation may be performed, wherein said interface register includes a data portion for data read from said storage location, separate from the address field portion, and wherein data may be read from the data portion of the interface register to complete one read operation without reading out the entire interface register. 17. An apparatus according to any of claims 12 to 16, wherein data may be read from the data portion of the interface register to complete one read operation while an address is being loaded into the address field portion of the interface register to initiate another read operation. 18. An apparatus according to any of claims 12 to 17, wherein a reading or writing operation is selected within the processor under control of a selection field in the specific communication instruction loaded into the interface register by the external apparatus. 19. An apparatus according to any of claims 1 to 18, wherein the processor is responsive to a predetermined external signals so as to enter a state in which program controlled operation is suspended, and wherein the processor is responsive to received communication requests at any time during said suspended state. 20. A data processing apparatus comprising synchronous or synchronous data processing circuitry and interface circuitry enabling communication between external circuitry and said data processing circuitry, the interface circuitry comprising at least one input line and an interface register which can be loaded with a communication word asynchronously with respect to operation of said data processing circuitry, wherein the data processing circuitry includes a plurality of addressable storage locations, and is responsive to each communication word received in said interface register to perform a memory access operation with an address specified in a predetermined address portion of said interface register. 21. An apparatus according to claim 20, wherein to perform a memory read operation said processing circuitry copies the content of the addressed storage location into a predetermined data portion of said interface register, different to said address portion, for asynchronous readout by said external circuitry. 22. An apparatus according to claim 21, wherein an address may be loaded into the address field portion of the interface register to initiate one read operation without loading the entire interface register. 23. An apparatus according to claim 21 or 22, wherein data may be read from the data portion of the interface register to complete one read operation without reading out the entire interface register. 24. An apparatus according to claim 21, 22 or 23, wherein data may be read from the data portion of the interface register to complete one read operation while an address is being loaded into the address field portion of the interface register to initiate another read operation. 25. An apparatus according to any of claims 20 to 24, wherein a reading or writing operation is selected within the processor under control of a selection field in the specific communication instruction loaded into the interface register by the external apparatus. 26. An apparatus according to any of claims 20 to 25, wherein a memory access interface of the processing circuitry includes means (LIM_WRITEB) for distinguishing between those memory accesses performed in accordance with a communication word and memory accesses performed otherwise under control of the processing circuitry. 27. An apparatus according to any of claims 20 to 26, wherein said interface register is a shift register loaded bit-serially under control of said external circuitry. 28. An apparatus according to claim 27, wherein said interface register includes a data portion separate to said address portion, and wherein said address portion is nearer than the data portion to the input end of the shift register. 29. An apparatus according to claim 28, wherein said shift register further includes a read/write selection portion nearer than the data portion to the input end of the shift register. 30. An apparatus according to any of claims 20 to 29, wherein the interface register contains more than thirty bits. 31. An apparatus according to any of claims 20 to 30, wherein said processing circuitry operates with predetermined timing under control of a stored program, and is adapted to perform memory access operations in response to communication words only during predetermined time periods. 32. An apparatus according to claim 31, wherein said time periods are determined by the inclusion of a generic communication instruction in the stored program. 33. A local communication system comprising at least one master apparatus and plural slave apparatuses connected by at least one common serial data writing line, each slave apparatus having an interface shift register for receiving a data word from the master apparatus via the common serial data line, wherein the interface shift registers of the plural slave apparatuses do not have equal bit lengths, and wherein the master apparatus is adapted to send correspondingly different length data words to the different slave apparatuses. 34. A system according to claim 31, the system further comprising a clock line, and at least one control line which can be forced to an active state by either of a selected slave apparatus and the master apparatus, such that, in an operation for the transfer of a data word from the master apparatus into the selected slave apparatus: said data word will be loaded into the interface shift register of the slave apparatus via the serial data writing line in accordance with a bit clock signal applied by the master apparatus to the clock line, while the control line remains in a passive state, the active state will then be imposed on the control line by the master apparatus, the selected slave apparatus will detect the active level on the control line and itself maintain the active level while reading the data word internally from the interface register, and the slave apparatus will release the control line when it is ready to receive a next data word. 35. A system as claimed in claim 34, wherein a further data word will not be read internally from the interface register and processed until the control line has been released by both apparatuses for a predetermined time and then again forced to the active level by the master apparatus. 36. A system according to claim 34 or 35, wherein, at least for a data word which is recognised by the slave apparatus as a command requiring information in response, said information is loaded into the interface register of the slave apparatus before the slave apparatus releases the control line, whereafter it can be read from the interface register via a serial data reading line. 37. A system according to claim 36, wherein said command is interpreted by hardware of the slave apparatus in accordance with pre-determined rules. 38. A system according to claim 36 or 37, wherein said data writing line and said data reading line are provided separately, so as to allow simultaneous input and output of data with respect to the shift register at each cycle of the serial clock signal. 39. A system according to any of claims 33 to 38, wherein each slave apparatus has a respective selection input for designating the selected slave apparatus. 40. An apparatus ("the slave apparatus") having an interface for receiving data from an external ("master") apparatus, the interface providing a serial data writing line, a clock line, and at least one control line which can be forced to an active state by either of the slave apparatus and the master apparatus, such that, in an operation for the transfer of a data word from the master apparatus into the slave apparatus: said data word can be loaded into an interface register of the slave apparatus via the serial data line in accordance with a bit clock signal applied by the master apparatus to the clock line, while the control line remains in a passive state, the active state can then be imposed on the control line by the master apparatus, the slave apparatus will detect the active level on the control line and itself maintain the active level while reading the data word internally from the interface register, and the slave apparatus will release the control line when it is ready to receive a next data word. 41. An apparatus as claimed in claim 40, wherein a further data word will not be read internally from the interface register and processed until the control line has been released by both apparatuses for a predetermined time and then again forced to the active level by the master apparatus. 42. An apparatus according to claim 40 or 41, wherein at least for a data word which is recognised by the slave apparatus as a command requiring information in response, said information is loaded into the interface register of the slave apparatus before the slave apparatus releases the control line, whereafter it can be read from the interface register via a serial data reading line. 43. An apparatus according to claim 42, wherein said command is interpreted by hardware of the slave apparatus in accordance with pre-determined rules. 44. An apparatus according to claim 42 or 43, wherein said data writing line and said data reading line are provided separately, so as to allow simultaneous input and output of data to the shift register at each cycle of the serial clock signal. 45. An apparatus according to any of claims 40 to 44, further including an addressable storage space, and wherein at least a first type of data word includes an address field interpreted by the slave apparatus as indicating a location in said storage space, and wherein the slave apparatus includes means automatically responsive to the reception of such a data word to allow reading, writing or a selection of the two to be performed between the interface register and said storage location. 46. An apparatus according to claim 45, wherein said storage space comprises data space, register space, and/or program memory of a program-controlled data processing apparatus. 47. An apparatus according to claim 46, wherein said read or write operation can be performed without interpretation by the processing apparatus, for example during execution of a generic interface instruction or cycle. 48. An apparatus according to claim 45, 46 or 47, wherein a read operation may be performed, wherein said data word, as interpreted by the slave apparatus, includes an address field specifying the storage location to be read, and wherein an address may be clocked into the address field portion of the interface register to initiate one read operation without clocking in an entire data word. 49. An apparatus according to claim 45, 46, 47 or 48 wherein a read operation may be performed, wherein said data word, as output by the slave apparatus, includes a field for data read from said storage location, and wherein said data may be clocked out of the data field portion of the interface register to complete one read operation without clocking out the entire data word. 50. An apparatus according to claims 48 and 49 combined. 51. An apparatus according to any of claims 45 to 50, wherein a reading or writing operation is selected within the slave apparatus under control of a selection field in the data word clocked into the interface register by the master apparatus. 52. An apparatus having an interface comprising at least one serial data writing line, a clock line and at least one control line, the apparatus having the technical features for operation as the master apparatus in connection with a slave apparatus according to any of claims 40 to 51. 53. A program controlled data processor comprising: an instruction decoding circuit (300) for implementing control within the processor in accordance with a stored program; an arithmetic unit (302) having input and output data paths of width n. bits; a register (A) of width greater than n. bits connectable under control of the instruction decoding circuit to at least one of the input and output paths of the arithmetic unit; and a shifting circuit (312) separate from the arithmetic unit for performing shift operations of said greater width using said register. 54. A processor according to claim 53, wherein said shifting circuit is physically connected between the output path of the arithmetic unit and data inputs of said register, with feedback also from data outputs of said register. 55. A processor according to claim 53 or 54, wherein said register of greater width comprises a pair of registers (AH,AL) each of width n., independently connectable to an input or output path of said arithmetic unit. 56. A processor according to any of claims 53 to 55, wherein said instruction decoding circuit is responsive to a predetermined multiplication instruction to control the arithmetic unit, the shifting circuit and the register so as to multiply two values of width n. bits, the result being obtained in the register. 57. A processor according to claim 56, wherein the multiplication operation is performed over plural operating cycles of the arithmetic unit and the shifting circuit. 58. A processor according to any of claims 53 to 57, wherein said instruction decoding circuit is responsive to a predetermined division instruction to control the arithmetic unit, the shifting circuit and the register so as to divide two values of width n. bits, the quotient and remainder being obtained in the register. 59. A processor according to claim 58, wherein the division operation is performed over plural operating cycles of the arithmetic unit and the shifting circuit. 60. A processor according to any of claims 53 to 59, wherein the shifting circuit can perform only single bit position shifting per operating cycle. 61. A processor according to any of claims 53 to 60, formed in an integrated circuit together with program storage and data storage space addressable independently via respective address busses of the processor, wherein the processor is arranged to execute fixed-length instructions that have been selected from a predetermined set of instructions and stored in the program storage space of the integrated circuit. 62. A processor according to any of claims 53 to 61, wherein the execution of each instruction takes a number of processor execution cycles, dependent on the type of instruction, and wherein a further instruction is not executable until such instructions are completely executed. 63. A processor according to any of claims 53 to 62, wherein, to the extent that it can be provided under program control, data communication externally of the processor is possible only via the data storage address space. 64. A processor according to any of claims 53 to 63, wherein dedicated unidirectional data paths are provided (i) between the register output and the input path of the arithmetic unit and (ii) between the arithmetic unit output and the register input. 65. A processor according to any of claims 53 to 64, without an interrupt routine execution handling facility. 66. A processor according to any of claims 53 to 65, further comprising an interface for receiving a communication request from external circuitry, the processor being responsive to the such a communication request only during predetermined time periods during normal program controlled operation. 67. A processor according to claim 66, wherein said predetermined periods are defined by inclusion of a predetermined communication instruction in the stored program. 68. A processor according to any of claims 53 to 67, wherein said stored program comprises instructions (SIF, BRK, SLEEP) selected from a predetermined instruction set, wherein the processor has at least one external control line, and wherein at least one instruction of the instruction set is decoded differently depending on a signal present on the external control line. 69. A processor according to any of claims 53 to 68, wherein the processor has a basic instruction cycle sub¬ divided into plural internal clock states, and wherein, for at least one combinational logic circuit having plural input lines and functioning under control of each stored program, means are provided to sample and latch input values for the combinational logic circuit only at a defined state or states within the operational cycle. 70. A processor according to claim 69, wherein said combinational logic circuit comprises the arithmetic unit of the processor. 71. A processor according to claim 70, wherein said combinational logic circuit further comprises the instruction decoding circuit of the processor. 72. A program controlled data processor comprising: an instruction decoding circuit (300) for implementing control within the processor in accordance with a stored program; an arithmetic unit (302) having input and output data paths of width n. bits; a register space (AH,AL,X,Y) of width n. bits connectable under control of the instruction decoding circuit to either the input or output paths of the arithmetic unit, wherein dedicated unidirectional data paths are provided (i) between the register space output(s) and the input path of the arithmetic unit and (ii) between the arithmetic unit output path and the input(s) of the register space. 73. A processor according to claim 72, wherein a further dedicated path is provided to one data register (X) of the processor from a program counter register (PC), said data path including an incrementer, so as to store a subroutine return address. 74. A data processor circuit adapted to operate under control of a program including a pre-stored linear sequence of instructions and a high frequency clock signal, the circuit comprising means for implementing a state of low power consumption in which execution of said program is suspended, and means for ending said suspended state so that execution of said program continues from the next instruction in the stored sequence without execution of instructions stored outside said sequence. 75. A processor according to claim 74, wherein said suspended state is entered in response in accordance with a signal (WAKE_UP,STOPB) applied to the processor , but only at a time in the instruction sequence defined by inclusion of a specific instruction, defined 76. A data processing apparatus comprising: a data processor circuit adapted to operate synchronously under control of a high frequency clock signal; - means for imposing a state of low power consumption in which execution of said program is suspended and said clock signal is isolated from the processor circuit, while the clock signal continues running; and - monitoring means responsive to any of a plurality of external signals for ending said suspended state by re-applying said clock signal to the processor, wherein said monitoring means comprises: plural individual monitoring circuits for detecting predetermined changes in respective ones of the external signals; and a common trigger circuit responsive to outputs of the individual monitoring circuits for re-applying said clock signal to the processor, and wherein the individual monitoring circuits, but not the common trigger circuit, are isolated from the running clock signal during said suspended state. 77. An apparatus according to claim 76, wherein at least on of said individual monitoring circuits comprises: means responsive to said clock signal prior to the suspended state for storing a value of the corresponding external signal; and asynchronous circuit means for, during the suspended state, comparing the external signal with the stored value. 78. A data processing apparatus constructed to operate sequentially under the control of a stored program comprising instructions selected from a predetermined instruction set, wherein the apparatus has at least one external control line (RUN_STEP), and wherein at least one instruction (BRK) of the instruction set is an instruction which will either halt the program-controlled processing operations or will effect no meaningful operation, depending on said signal. 79. An apparatus according to claim 78, comprising means (RUN_STOPB) for signalling externally that execution has been halted. 80. An apparatus according to any of claims 78 to 79, wherein said apparatus is integrated on a single integrated circuit together with said stored program in non-volatile memory. 81. A simulator or emulator for a program controlled processor having a predetermined instruction set, the instruction set including a plurality of specific debug instructions (PRINT) which, when included in a program for the processor, implement no operation in the processor itself but which cause the simulator or emulator to record information signalling execution of the respective instruction. 82. A simulator or emulator according to claim 81, wherein each such debug instruction specifies a data value to be recorded. 83. A simulator or emulator according to claim 82, wherein each such debug instruction further specifies a register location of the simulated or emulated processor whose current contents are also to be recorded. 84. A simulator or emulator according to any of claims 81 to 83, wherein an op-code part common to the specific debug instructions corresponds to a redundant addressing mode of another instruction in the instruction set. 85. A simulator or emulator according to any of claims 81 to 84, wherein the op-code part common to the specific debug instructions corresponds to an immediate addressing mode of a memory store instruction in the instruction set. 86. A processor or data processing apparatus having the features of any combination of the preceeding claims.

Inventors: BARLOW STEPHEN JOHN, COLLIER JAMES DIGBY YARLET, MORFEY ALISTAIR GUY, BARLOW, STEPHEN, JOHN, COLLIER, JAMES, DIGBY, YARLET, MORFEY, ALISTAIR, GUY
Applicants: BARLOW STEPHEN JOHN, CAMBRIDGE CONSULTANTS, COLLIER JAMES DIGBY YARLET, MORFEY ALISTAIR GUY, BARLOW, STEPHEN, JOHN, CAMBRIDGE CONSULTANTS LIMITED, COLLIER, JAMES, DIGBY, YARLET, MORFEY, ALISTAIR, GUY
Application year: 1995
Main classification: G06F9
IPC codes: G06F7, G06F9, G06F11