METHOD AND APPARATUS FOR TRANSFERRING DATA BETWEEN TWO DATA PROCESSING EQUIPMENTS EACH DRIVEN BY AN INDEPENDENT CLOCK

Abstract: A method and apparatus for transferring data words from a transmitter register (REG1) to a receiver register (REG2) between transmitting and receiving equipment which are respectively driven by a first (CL1) and a second (CL2) clock. A state signal (WR1) which can assume two alternative definite values is applied from the transmitter side to the receiver side, whereafter the state of the signal is changed from the transmitter side. The state signal is processed logically to obtain the start of a write signal (WR2) which achieves writing into the receiver register (REG2) and the write signal is interrupted with the aid of the receiver side clock signal (CL2).

Description: METHOD AND APPARATUS FOR TRANSFERRING DATA BETWEEN TWO DATA PROCESSING EQUIPMENTS EACH DRIVEN BY AN INDEPENDENT CLOCK TECHNICAL HELD The invention relates to a method of transferring data words in parallel form from a transmitter register to a receiver register between transmitting equipment having a first independent clock and receiving equipment having a second independent clock. The invention also relates to an apparatus for carrying out the method. BACKGROUND ART In both large and small digital systems there is a need of transferring information between units which are driven by independent clocks. This information may signify such as an order to step an error counter, a request for entry into a memory or to transfer 8-bit parallel data to peripheral equipment driven by a clock independent of the transmitting unit. The task of the receiving unit can be such as to convert data obtained in parallel form to series data for linking in as PCM data in a PCM system. For example, in a practical case equipment having a 5 MHz clock on the transmitter side is co-operating with equipment having a 2 MHz clock on the receiver side. Difficulties will occur at the interface between the transmitting and receiving equipment, particularly when both equipments are driven by synchronous logic, i.e. when all state changes in the memory units included in counter, register, flip-flops etc take place with the equipment's own system clock. The simplest method for transferring in the cases discussed is that the data information on the receiver side is fed into a register with the aid of a write pulse coming from the transmitter side. This pulse must be adapted to the clock frequency on the receiver side such that it is just as long as the receiver side clock period. This signifies that the clock frequencies cannot be changed independently of each other. Lowering the reception side clock frequency, i.e. extending the clock period to twice its length, for example, results in the need of changing the write pulse from the transmitter side in a corresponding manner, in turn resulting in the necessity of altering the transmitter side components. DISCLOSURE OF INVENTION The disadvantage just mentioned is eliminated by the invention, it not being necessary to alter the transmission side clock frequency if an alteration of the receiver side clock frequency becomes necessary, since the write pulse is generated on the receiver side and its length is determined by the receiver side clock period. The invention is characterized by the disclosures in the claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in detail below with the aid of an embodiment and with reference to the accompanying drawing, on which Figure 1 schematically illustrates two coacting data processing units, the invention being applied at the interfaces of these units, Figure 2 illustrates two units having their own clocks, the units co-acting with each other across an interface in a conventional manner, Figure 3 is a pulse diagram of the function of the apparatus according to Figure 2, Figure 4 illustrates an apparatus operating according to the inventive principle and Figure 5 illustrates the pulse diagram of the function of the device according to Figure 4. BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 is a block diagram of a system where the Invention is applied. A computer DA is in communication with remaining unillustrated means and a first interface unit El via a bus system BU. The interface unit El is in communication via an 8-wire line with a second interface unit E2 on the receiver side for transferring parallel data words. The receiver unit E2 sends data words in series form to a PCM system. The interface units El and E2 are each driven by their respective clock signals CLl and CL2. Figure 2 illustrates the interface units El and E2 in the form of a block diagram. A register REG 1 on the transmitter side is built up from 8 flip-flops which can be fed with an 8-bit information word via the inputs IN 1-8 when a feed signal LO1 simultaneously occurs with the receding flank of the clock signal CLl on the inputs of the register flip-flops. A control logic COL driven by the clock pulses CLl has the task of a) generating the feed signal LOl for writing the information word into the register REGl, b) generating a write signal WRl, which is fed via the interface to the receiver side for writing into a register REG2 the data word transferred in parallel on the lines D1-D8. The binary values from REG 2 are transmitted to a logic AL which converts parallel data to series data for further transmission to the PCM system. The conditions are illustrated by the pulse diagrams 3a-3h, where Figure 3a illustrates the clock pulses CLl on the transmitter side and 3b illustrates the feed pulse LOl. When the feed pulse LOl occurs simultaneously as the receding flank of the clock pulse CLl, the binary values occuring on the inputs IN1-8 on the register REGl are written into the register flip-flops. These values then occur on the lines Dl-8 so that the previous value on the flip-flop outputs is changing according to Figure 3c. The write signal WRl in Figure 3d simulta¬ neously occurs on the output of the control logic COL and is led to the receiver side E2. The receiver register REG2 obtains the write signal WRl (Figure 3d) from the transmitter side as well as the clock signal CL2 (Figure 3e) from the receiver side and the lenght of the write signal WRl is selected such that the receding flank of the clock signal will occur before the write signal WRl ceases. In this way, when the receding flank of the clock signal CL2 occurs the binary values from REGl are transferred to REG2 via the lines Dl-8, and these values will occur on the REG2 outputs Ut 1-8 for further passage to the logic AL. This arrangement allows there to be no alteration of the clock pulse CL2. If it should be necessary to change the clock period for some reason, e.g. to double the length as illustrated in Figure 3, the transmission cannot be carried out, since the write pulse WRl would cease before the receding flank in the twice as long clock pulse occurs (Figure 3g). No change of state on the outputs Dl-8 would be received (Figure 3h) by REG2 to occur on the outputs U1-U8 of the latter on occurrence of the receding flank of the clock signal CL2. The only possibility is thus to alter the length of the write pulse WRl, which cannot be done without necessary structural alterations to the transmitter side. The object of the invention is to enable alteration of the receiver side clock period without any structural alteration on the transmitter side. Figure 4 illustrates the inventive solution, the apparatus shown containing, in the same way as the apparatus according to Figure 2, a reigster REGl on the transmitter side El and a register REG2 on the receiver side E2, the registers being in communication with each other via an 8-wire line and are controlled by their individual clock signals CLl and CL2. The write signal which will occur on the receiver side will be dependent on the write signal on the transmitter side and also on the receiver side clock signal. The outputs D 1-8 of REGl are actuated in the same way as for the apparatus according to Figure 2, and their binary values are altered by the feed signal LOl occuring simultaneously with the receding flank (Figures 5a,5b) of the clock signal CLl. A state signal WRl occurs on the Q output of a first J-K flip-flop Fl (Figure 5c). The condition for transmitting to REG2 is that both a write signal and the receding flank of the clock signal CL2 occur simultaneously. In accordance with the invention, the length of this write signal WR2 is independent of the transmission side clock signal CLl, and is solely determined by the receiver side clock with the aid of a logical circuit. This logical circuit comprises in accordance with the embodi¬ ment a D flip-flop D2, an EXCLUSIVE-OR circuit X2 and a JK-flip-flop F2. When no transmission takes place the outputs of all these flip-flops have identical values, which is the one of two possible values. When there has been a change in state on the Q output of the Fl flip-flop, i.e. it has assumed a second alternative value, this signifies that the state signal WRl is fed to the D flip- flop D2, which obtains the same state on its Q output (Figure 5f) on the occurence of the receding flank of the clock pulse CL2. As soon as this change in state has occured, the output signal WR2 (Figure 5h) of the EXCLUSIVE-OR circuit X2 is changed when the input signals to the circuit X2 are instantly different (figures 5f, 5g). For the next subsequent receding flank of the clock signal CL2 the binary values are transmitted on the lines Dl-8 to REG2 and occur on the outputs UT-8 of REG2 and the value on the Q output of the circuit F2 is changed, whereby the signals on the X2 circuit inputs once again become the same, and consequently the signal WR2 ceases from the circuit X2. In this way the write signal WR2 has been interrupted and the state of the outputs on the circuits Fl, D2, X2 and F2 are once again identical. This means that REG2 is ready for reception of the next information word, which is started by a new change of state on the Q output of the circuit Fl. If it is desired to alter the receiver side clock period, e.g. to make an extension thereof, no structural alteration to the sender side is required, and the write pulse WR2 will be automatically adjusted to the period length of the clock signal CL2. This is illustrated in the right hand part of Figure 5 where the clock pulse has been extended. As will be seen, the period length of the signals CL2 will in this case regulate the length of the write pulse WR2 in a manner such that the write pulse WR2 opens the flip-flops of the register REG2 for reception of binary information via the wires D1-D8 when the receding flank of the clock pulse CL2 occurs. In this way there is complete independence of the relationship between the period lengths of the two clocks, and only the period time of the clock signal CL2 will determine the minimum time necessary for transferring information from the transmitter side to the receiver side.

Claims: C L A I M S 1. A method of transferring data words in parallel form from a transmitter register (REGl) to a receiver register (REG2) between transmitting and receiving equipment, respectively driven by a first (CLl) and a second (CL2) independent clock, by activating the receiver register with a write signal, characterized in that a state signal (WRl) which can assume two alternative definite values is applied from the transmitter side to the receiver side, the state of the applied signal being changed from the sender side on starting the transmission, the state signal being processed logically together with the receiver side clock signal, to a) obtain the start of the write signal (WR2) in response to a flank of the receiver clock signal and b) to interrupt the write signal with the aid of a flank of the receiver side clock signal (CL2) 2. Apparatus for transferring data words in parallel form from a transmitter register (REGl) to a receiver register (REG2) between transmitting and receiving equipment which are respectively driven by a first (CLl) and a second (CL2) independent clock, by activating the receiver register with a write signal according to claim 1, characterized in that the transmitter side includes a first logical circuit (Fl) which transmits a state signal (WRl) on starting the transmission to the receiver side, the signal (WRl) being enabled to assume two alternative definite values, the receiver side including a second logical circuit (D2, X2, F2) actuated by the alteration of the state signal (WRl) and by the clock signal in the receiver, so that in response to a flank of this clock signal a write signal (WR2) is started, this signal being fed to the receiver register (REG2) for activating it on the occurence of one flank of the second clock signal (CL2), the second logical circuit (D2, X2, F2) then interrupting the write signal after a following flank of the clock signal has occured.

Inventors: HOEGBERG BERTIL GUNNAR, HOEGBERG, BERTIL, GUNNAR
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1987
Main classification: H04L25
IPC codes: H04L7, H04L25