Abstract:
A controller for interfacing bus-coupled peripheral devices with a microcomputer is described. The controller provides generalized compensation for variations in peripheral device access recovery time and for differences in access recovery times among peripheral devices, central processing units and input/output buses.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to microcomputer systems incorporating commercially available peripheral device controllers. In particular, this invention relates to microcomputers in which a peripheral device controller is used for controlling data flow and address and data timing between peripheral devices, such as keyboard, printers and disk drives, and the microprocessor via the main input/output (I/0) bus of the microcomputer system. 
     While present-day microcomputer manufacturers have control over the design and configuration of the systems they produce, they typically must anticipate the parameters necessary for compatibility of their system with new ancillary devices, including add-on peripherals, accessories and memory options produced by other manufacturers. The performance and interface characteristics of microprocessors and memory devices often vary substantially from one release of the same device to the next; similarly, such characteristics of peripherals, accessories and memory options will also vary substantially among the manufacturers of these devices. 
     The performance characteristics of peripheral devices are often designed for less than optimum performance, i.e., &#34;detuned&#34; to accommodate variations in microcomputer system designs. Microprocessor chip and memory devices, however, are not usually so detuned. Therefore, the manufacturer of high performance microcomputers must allow for different, even inferior, performance characteristics of peripheral, accessory devices and some memory options in order to produce a system which is compatible with the maximum number of devices attachable to the system. In addition, the microcomputer manufacturer must anticipate upgrades and changes of microprocessor chip sets and memory devices. If the microcomputer manufacturer does not so anticipate such upgrades, it will limit the marketability of the system to less than the total market available for his product. 
     A complete microcomputer, which is often intended for desktop applications, includes subsystems such as a central processing unit (hereafter referred to as the &#34;CPU&#34;, &#34;processor&#34; or &#34;microprocessor&#34;), a math &#34;coprocessor&#34;, DMA capabilities, memory, an input/output (I/O) bus, miscellaneous system ports, and separate interface logic for peripheral devices such as video, keyboard, floppy disks, serial and parallel ports for printers, scsi devices, a mouse pointing device and the like. 
     The microcomputer functions by manipulating address, data, and control signals among the subsystems within the system. The control data flow into and out of peripheral devices is provided by various controllers which usually controls the data flow and timing between the processor, main system memory, and the main I/O bus. 
     Most keyboard controllers incorporated into state-of-the-art microcomputer systems require compensation for access recovery time. The requirement for such compensation arises because keyboard controllers incorporate technology in which the response time for processing signals representing access commands is slower than present I/O bus technology is designed to accommodate. For example, the typical minimum recovery time between any two operations performed by state-of-the-art bus technology is 100 ns, but the recovery time required by present keyboard controllers is 1000 ns. Thus, the I/O bus is ready for the next operation long before the keyboard controller is ready to process it. 
     In the past, computer programs supplied with keyboard controllers provided such access recovery time compensation as part of the program code. However, such a software-based accommodation for access recovery time is generally regarded as inefficient and may not operate at all in a microcomputer system having a cached microprocessor. 
     Peripheral device control in a microcomputer system according to the present invention comprises an asynchronous controller for interfacing peripheral devices with present state-of-the-art I/O bus technology, such as the Micro Channel Architecture (MCA) manufactured by IBM Corporation, and state-of-the-art microprocessors, such as the 80386 microprocessor, manufactured by Intel Corporation. Accordingly, many special functions previously supported by separate peripheral device controllers are consolidated into one controller to reduce system size and cost. 
     One embodiment of the peripheral device controller of the present invention comprises programmable registers and counters that receive and transmit data over the MCA. The functional sections include, interrupt controllers, counter timers, parallel printer port, address decode, micro channel command processing, misc. system registers, pos registers, clock generation circuits, and assembly of information for transmission on the system data bus. 
     During initialization and normal operation, the microprocessor reads from and writes to any one of several I/O port addresses provided by the device of the present invention. Addressable functions include interrupt control, programmable counters, option and configuration (POS), refresh rate counter, parallel port control and address decoding for several I/O ports. 
     In addition, the controller of the present invention incorporates compensation for keyboard access recovery time as a generalized hardware function. Thus selectable recovery time compensation for other peripheral devices as may be required in addition to the keyboard is provided. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a microcomputer peripheral device controller constructed according to the principles of the present invention. 
     FIG. 2A is a timing diagram for control signals received and produced by a prior art microcomputer peripheral device controller. 
     FIG. 2B a timing diagram for control signals received and produced by the microcomputer peripheral device controller of FIG. 1. 
     FIGS. 3A and 3B are circuit diagrams of components of the command processing section of the microcomputer peripheral device controller of FIG. 1 for producing the control signals of FIG. 2B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Peripheral device controller 10, constructed according to the present invention as shown in FIG. 1, comprises address decoding section 101, command processing section 102, timer section 103, interrupt control section 104, parallel printer port 106, bus interface 107 and register section 108. When communicating with an 80386 microprocessor via the MCA, peripheral device controller 10 is treated as an 8-bit I/O device. 
     Address decoding section 101 provides address information that the registers comprising register section 108 require for proper access. An address decode map is provided in Table 1. 
     Device controllers, such as a keyboard controller, may form part of address decoding section 101. However, the address decoding function of section 101 also may be performed by a decoder separate from the controller of the present invention and thus forms no part of the present invention. 
     Command processing section 102 interfaces the MCA with local or system board peripheral controllers for peripherals such as floppy disk drives and the keyboard. Command processing section 102 incorporates logic, described elsewhere in this specification, which provides access recovery time for the keyboard and for other peripherals as required. 
     With continuing reference to FIG. 1, timer section 103 comprises three programmable timers. These timers are functionally compatible with and operate in similar to an 8254 programmable timer manufactured by Intel Corporation. Watchdog timer 103C is merely an 8-bit counter. 
     Interrupt control for the system is provided by master and slave interrupt control sections 104 and 105, respectively. An interrupt assignment map is given in Table 1. 
     
                       TABLE 1______________________________________SLAVE INTERRUPTS (priority #2)INTERRUPT #15:   Not assigned.INTERRUPT #14:   Fixed disk.INTERRUPT #13:   Math processor extention.INTERRUPT #12:   Mouse device.INTERRUPT #11:   Not assigned.INTERRUPT #10:   Not assigned.INTERRUPT #9:    Not assigned.INTERRUPT #8:    Real time clockMASTER INTERRUPTS:INTERRUPT #7:    Printer port.INTERRUPT #6:    Floppy disk.INTERRUPT #5:    Not assigned.INTERRUPT #4:    Serial port.INTERRUPT #3:    Serial port.INTERRUPT #2:    See interrupts 8:15.INTERRUPT #1:    Keyboard.INTERRUPT #0:    System timer tic.______________________________________ 
    
     Peripheral device controller 10 includes registers associated with a parallel printer port and provides the timing signals necessary to interface the MCA with peripheral port circuitry. Parallel printer port 106 is bi-directional to facilitate data transfers into and out of the microcomputer at a faster rate than provided by the serial port. 
     Bus interface 107 assembles bit information and transfers such information to the MCA via an 8-bit local peripheral bus. All of the bits in a byte of information may not be defined. In such instances, undefined bits are tri-stated, or simply not driven by this portion of peripheral device controller 10. Thus, bit information may be easily modified or enhanced external to peripheral device controller 10. 
     Finally, in addition to miscellaneous system registers, register section 108 includes configuration registers for software configuration of plug-in adapters as well as other peripheral device functions not provided on the internal system board of the microcomputer. A detailed bit map is provided in Table 2. 
     
                                           TABLE 2__________________________________________________________________________REGISTER DEFINITIONS:This section contains the READ decode bit assignments.Reg.       Bits 7-00020       INTERRUPT CONTROLLER #10021       INTERRUPT CONTROLLER #10040       COUNTER-TIMER #00042       COUNTER-TIMER #20044       COUNTER-TIMER #30060       EXTERNAL DECODE &gt;KEYBOARD0061     BIT 7 BIT 6 BIT 5   BIT 4    mem par ck          io ch ck                cnt-tim 2 out                        ref det    BIT 3 BIT 2 BIT 1   BIT 0    io ck enan          m par enan                spkr ena                        c-t 2 gateReg.       Bits 7-00064       EXTERNAL DECODE &gt; KEYBOARD0071       EXTERNAL DECODE &gt; REAL TIME CLOCK/RAM0076       EXTERNAL DECODE &gt; RAM DATA    BIT 7 BIT 6 BIT 5   BIT 400910092     litea liteb         wd timeout0094     mb setupn   vga setupn0096     ch rst    BIT 3 BIT 2 BIT 1   BIT 00091                         cdsel fdbk0092     psswd lock  a20 gate                        alt pro rst00940096     ch setup en          ch su 2                ch su 1 ch su 0Reg.       Bits 7-000A0       INTERRUPT CONTROLLER #200A1       INTERRUPT CONTROLLER #20101       EXTERNAL DECODE &gt; ID CODE0102     BIT 7 BIT 6 BIT 5   BIT 4    pp mode          pp sel 1                pp sel 0                        pp ena    BIT 3 BIT 2 BIT 1   BIT 0    altserpn          sp ena                fd ena  mb enaReg.       Bits 7-00103       EXTERNAL DECODE &gt; RAM CONFIGURATION0278(0378/03bc)       PAR    PORT  DATA    BIT 7 BIT 6 BIT 5   BIT 40279(0379/03bc)    ppbusyn          ppackn                pppapend                        ppselin027a(037a/03be)      irqena  ppselout    BIT 3 BIT 2 BIT 1   BIT 0    pperm lirq7n    ppinitn          ppautofd                ppstrobeReg.       Bits 7-002F8(0318)-      EXTERNAL DECODE &gt; sp02FF(03ff)03FO-      EXTERNAL DECODE &gt; FD03F70020       INTERRUPT CONTROLLER #10021       INTERRUPT CONTROLLER #10040       COUNTER - TIMER #00042       COUNTER - TIMER #20043       COUNTER - TIMER CONTROL WORD0044       COUNTER - TIMER #30047       COUNTER - TIMER CONTROL WORD0060       EXTERNAL DECODE &gt; KEYBOARD0061     BIT 7 BIT 6 BIT 5   BIT 4    cir int 0    BIT 3 BIT 2 BIT 1   BIT 0    io ck enan          m par enan                spkr ena                        c-t 2 gateReg.       Bits 7-00064       EXTERNAL DECODE &gt; KEYBOARD0070       nmi enan EXTERNAL DECODE &gt; REAL TIME CLOCK/RAM0071       EXTERNAL DECODE &gt; REAL TIME CLOCK/RAM0074       EXTERNAL DECODE &gt; RAM ADDRESS(LOWER)0075       EXTERNAL DECODE &gt; RAM ADDRESS(UPPER)0076       EXTERNAL DECODE &gt; RAM DATA0091    BIT 7 BIT 6 BIT 5   BIT 40092     litea liteb0094     mb setupn   vga setupn0096     ch rst    BIT 3 BIT 2 BIT 1   BIT 00092     psswd lock  a20 gate                        alt pro rst00940096     ch setup en          ch su 2                ch su 1 ch su 0Reg.       Bits 7- 000A0       INTERRUPT CONTROLLER #200A2       INTERRUPT CONTROLLER #20101    BIT 7 BIT 6 BIT 5   BIT 40102     pp mode          pp sel 1                pp sel 0                        pp ena0103    BIT 3 BIT 2 BIT 1   BIT 00102     sp dec sel          sp ena                fd ena  mb ena0103                 fast ref                        ram enReg.       Bits 7-00278(0378/03bc)      PAR PORT DATA0279(0379/03bd)    BIT 7 BIT 6 BIT 5   BIT 4027a(037A/03be)          ppread                irqena  ppselout    BIT 3 BIT 2 BIT 1   BIT 0    ppinitn          ppautofd                ppstrobeReg.       Bits 7-002F8(03f8)-      EXTERNAL DECODE &gt; SP02FF(03ff)03FO-      EXTERNAL DECODE &gt; FD03F7__________________________________________________________________________ 
    
     Peripheral device controller 10 produces the control signals shown in FIG. 3B in response to MCA control signals 15 from the MCA and interrupt signals 17 from the microprocessor (not shown), employing address information decoded by address decoding section 101. Address decoding section 101 decodes address bits A0-A9 according to the register definitions given in Table 2. 
     Address bits A10-A15, which are combined elsewhere in the microcomputer system, must be all zero for addressing registers in the preferred embodiment of the present invention. 
     Command processing section 102 processes MCA bus signals S0, S1, MEM and CMD. These signals are decoded, interpreted, latched and strobed to produce IORDN and IOWRN control signals. The output of command processing section 102 is coupled to timer section 103 and interrupt control sections 104 and 105. 
     Timer section 103 comprises audio timer 103A, system internal timer 103B and watchdog timer 103C. Control signals produced by this section are transmitted to master and slave interrupt sections 104 and 105, and to parallel printer port 106. Audio timer 103A comprises a counter similar in function to part no. 8254, manufactured by Intel Corporation. System timer 103B also comprises the same type of counter. Both timers 103A and 103B are 16-bit programmable counters. Finally, watchdog timer 103C comprises an 8-bit programmable counter which is a modified functional subset of timers 103A and 103B. 
     Master interrupt control section 104, the input of which is coupled directly to the microprocessor, comprises an interrupt controller which is functionally similar to part no. 8259, also manufactured by Intel Corporation. Slave interrupt control 105, coupled to master interrupt controller 104, also comprises an interrupt controller functionally similar to part no. 8259. 
     Parallel printer port 106 provides control signals to parallel printer devices and bidirectional data ports with such devices. Parallel printer port 106 includes a status port for receiving the status of the printing device attached thereto. Bit assignments for the port are given in Table 3. 
     
                                           TABLE 3__________________________________________________________________________PIN DEFINITION:__________________________________________________________________________4.1 - CPU AND BUS CONTROLLER INTERFACEALTRCN: Alternate Processor Reset.                    This output goes to the Bus                    Controller to direct that the                    CPU be reset without resetting                    the entire system. This signal                    reflects the status of bit 0 in                    the register at address 0092H.HOLDA: Hold Acknowledge. This input signal from the CPU                    indicates that the CPU has                    entered the hold state.INTR: Interrupt Request. This active high output signals                    the CPU that an interrupt is                    pending.NMIN: Non-Maskable Interrupt.                    This active low output                    indicates that a non-maskable                    interrupt is pending.RAMENA: RAM Enable.      This output signal indicates                    whether the system RAM should                    be enabled. The status of this                    line is indicated in bit 0 of                    the register at address 0103H.4.2 - MICROCHANNEL INTERFACEGATEC: Gate Command.     Input which is internally ANDed                    with CMD to generate IOWR. Can                    be used to delay the leading                    edge and/or hasten the trailing                    edge of the IOWR signal if                    required in the system design.CA9-CA0: Address Bus.    These input signals are the                    low-order bits of the address                    bus.CAUPPERN: Address Bus Upper Bits.                    This active low input signal                    indicates that the address bits                    CA15-CA10 are all 0, which is                    the range for addressing the                    registers in the Peripheral                    Controller.CCMDN: Channel Command.  This active low input signals                    that the address lines on the                    MicroChannel are valid and                    instructs the devices connected                    to the bus to respond                    accordingly.CD7-CD0: Channel Data Bus.                    These bi-directional lines are                    the MicroChannel data bus.CDRSTN: Channel Reset.   This active low output signal                    is used as a reset to the                    channel.CDSELFBK: Card Selected Feedback.                    This input signal indicates                    that a memory slave or I/O                    slave is present at the address                    output by the system CPU.CIOCHKN: I/O Channel Check.                    This active low output signal                    is used to indicate a serious                    system error condition.CMEM: Memory I/O.        This input signal indicates                    whether the current                    MicroChannel operation is a                    memory cycle (set to 1) or an                    I/O cycle (set to 0).CS1, CS0: These input signals indicate the status of the currentCMEM          CS1            CS0        OPERATION0             0  0          Interrupt Acknowledge0             0  1          I/O Write0             1  0          I/O Read0             1  1          Reserved1             0  0          Reserved1             0  0          Memory Write1             1  0          Memory Read1             1  1          ReservedPERRDY: Peripheral Controller Ready.                    This handshaking signal to the                    MicroChannel indicates that the                    Peripheral Controller has                    accepted the signals sent to                    it.CREFRESHN: Channel Refresh.                    Indicates that a refresh                    operation is currently                    underway.4.3 - PERIPHERAL INTERFACECDSU2, CDSU1, CDSU0: Card Setup.                    These signals are used with a                    3:8 decoder to select one of                    eight card slots. The CDSUx                    lines are the encoded slot                    number. These lines are copies                    of the appropriate bits of the                    register at address 0096H.CDSUENA:                 This signal indicates the                    validity of the CDSUx card                    select lines. It would                    normally be used to enable the                    external card selection                    decoder.IORDN: I/O Read.         This active low output                    indicates to the peripheral                    devices that the current cycle                    is a read operation.IOWRN: I/O Write.        This active low output                    indicates to the peripheral                    devices that the current cycle                    is a write operation.IRQ0N, IRQ1N, IRQ3N-IRQ15N: Interrupt Inputs.                    These active low input signals                    are the external interrupts.PP7-PP0: Parallel Port Interface.                    These bi-directional lines                    connect to the parallel                    (printer) port data lines.PPACKN: Parallel Port Acknowledge.                    This active low input indicates                    that the printer has received                    the last character and is ready                    to receive another one. This                    line is read in the parallel                    port status register at bit 6.PPAFDN: Parallel Port Automatic Feed.                    This bi-directional signal                    indicates whether a line feed                    should follow after each                    carriage return (set to                    The status of this line is                    indicated in bit 1 of the                    parallel port control register.PPBUSY: Parallel Port Busy.                    This active low input indicates                    that the parallel port is busy.                    The status of this line is                    indicated in bit 7 of the                    parallel port status register.PPERN: Parallel Port Error.                    This active low input indicates                    that there is an error                    condition in the device on the                    parallel port. The status of                    this line is indicated in bit 3                    of the parallel port status                    register.PPINITN: Initialize Parallel Port.                    When this line is set to 0, the                    parallel port device is                    activated. The line is                    controlled by bit 2 of the                    parallel port control register.PPPAEND: End of Paper.   This input indicates that the                    printer on the parallel port is                    out of paper. The status of                    this line is indicated in bit 5                    of the parallel port status                    register.PPSELN: Parallel Port Selected.                    This active low input indicates                    that the device on the                    peripheral port acknowledges                    that it has been selected. The                    status of this line is                    indicated in bit 4 of the                    parallel port status register.PPSELON: Parallel Port Select.                    This active low output selects                    the parallel port device. The                    status of this line is                    indicated in bit 3 of the                    parallel port control register.PPSTBN: Parallel Port Strobe In.                    This input indicates the status                    of PPSTBOUT in bit 9 of the                    parallel port control register.PPSTBOUT: Parallel Port Strobe Out.                    This active high output clocks                    data to the parallel port.PARPEN: Parallel Port Enable.                    Controls the direction of the                    external parallel port                    transciever.SERPIRQ: Serial Port Interrupt Request.                    Programmable inside the chip                    for the interrupt number.SERPOUT2: Gate for the serial port interrupt.                    Companion signal for the                    interrupt request from the                    external USART. Both this                    signal and the interrupt                    request must be active to cause                    an interrupt.4.4 - OTHERAUDIO: Audio signal output.                    This is the output of the audio                    timer.CLK14M: 14 MHz clock.    This input signal drives the                    internal logic in the circuit.TIMCLK: 1.19 MHz Clock.  This output is derived from the                    14 MHz clock.CLKIM:                   1.19 MHz clock input used                    internally for the timer tick.SYSTIC: System Timer Tic.                    This output goes to interrupt                    zero.SYSRSTN: System Reset.   This input resets the chip.RD0101N: Read Port 0101H.                    Decode of the port address                    ANDed with the IORD signal.RD0103N: Read Port 0103H.                    Decode of the port address                    ANDed with the IORD signal.RD0076N: Read port 0076H.                    Decode of the port address                    ANDed with the IORD signal.WR0074: Write Port 0074H.                    Decode of the port address                    ANDed with IOWR. Note that                    this signal is active high to                    be used with a latch rather                    than an edge triggered                    register.WR0075: Write Port 0075H.                    Decode of the port address                    ANDed with IOWR. Note that                    this signal is active high to                    be used with a latch rather                    than an edge triggered                    register.WR0076N: Write Port 0076H.                    Decode of the port address                    ANDed with IOWR.IRQSEL:                  Selects either latched or                    unlatched versions of some of                    the interrupt lines. Must be                    pulled high during normal                    system operation.MOUSEINT: Mouse Interrupt.                    Input from the system board                    supported pointing device.                    Normally would come from the                    8042.INCDEC: Incomplete Decode.                    When this input is high, it                    allows incomplete decodes of                    the selected IO ports in order                    to allow exact compatibility.                    If low, it forces exact                    decodes.DBENAN: Data Bus Enable. This active-low output provides                    the enable signal to the                    buffers for the data bus                    between the Peripheral                    Controller and the                    MicroChannel.DEC010X: Decode Addresses 010X.                    This output is asserted when an                    I/O address in the range 0100H-0107                    is selected. These                    addresses represent the                    registers used for the                    Programmable Option Select                    (POS)FASTREFN: Fast Refresh Select.                    Comes from the programmable                    port and goes to the DMA chip                    to indicate that fast refresh                    mode is active. Causes an 835                    ns refresh period rather than                    the normal 15 microsecond                    period.IOOUTN:                  Controls system buffer chips                    when an I/O write cycle is in                    progress.FDCPCSN: Floppy Disk Select.                    This output is asserted when an                    I/O address in the range 03F8H-93FFH                    is selected. This                    address range corresponds to                    the space for the floppy disk.FDACKN: Floppy Disk Acknoweldge.                    Input which controls the buffer                    when a DMA cycle for transfer                    of floppy disk data is in                    progress.KYBDCSN: Keyboard Select.                    This output is asserted when an                    I/O address 0060H or 0064H is                    selected. These addresses                    correspond to the space for the                    keyboard.CMDDLY:                  Output of the delay for the                    keyboard read. Provides the                    necessary recovery time for                    this slow peripheral device.                    Normally paired with STBDLY                    input.STBDLY:                  Input for the keyboard access                    recovery time. Normally                    connected to CMDDLY.LITEAN, LITEBN: Light Drivers.                    These active low outputs are                    used to control the fixed disk                    activity light drivers. These                    signals correspond to bits 7                    and 4 of the register at                    address 0092H.MPCENA: Memory Parity Check Enable.                    This output signal indicates                    that the memory parity check                    information is to be used.MPCKN: Memory Parity Check.                    This active low input indicates                    the results of the memory                    parity check. This signal is a                    0 when a memory parity error                    occurs.PASSA20: System Control Port A, bit 1.                    This bit can be used to control                    address bit A20.POSENA: POS Setup Enable Register Bit 7.                    This active high output signal                    is bit 7 of the System Board                    Setup Enable Register (I/O                    Address 0094).RTCAS:                   Real-Time CMOS RAM Address                    Strobe: This output strobes                    the address to the real-time                    CMOS RAM.RTCDS:                   Real-Time CMOS RAM Data Strobe:                    This output strobes data to the                    real-Time CMOS RAM.RTCRD:                   Real-Time CMOS RAM Read/Write.                    This output signal controls the                    real-time CMOS RAM data                    direction during data accesses.SERICSN: Serial Port 1 Decode.                    This output is asserted when                    I/O addresses 03F8-03FF are                    selected. These addresses                    correspond to the space for the                    serial port.TESTN: Test Input.       This input is used for                    functional testing of the                    circuit and is not used in the                    system operation.VGASETN: Set up VGA.     This active low output directs                    the VGA circuit into the setup                    mode. This signal reflects the                    status of bit 5 of the register                    at address 0094H.4.5 - MISCELLANEOUSVDD: Power.VSS: Ground.Reserved: Reserved for future use. Must not be used in system.NC: No connection.__________________________________________________________________________ 
    
     Parallel printer port 106 comprises control and data latches for interfacing to various printer interfaces, including the centronics-type interface, in response to read status information received from such interfaces. 
     With continuing reference to FIG. 1, bus interface 107 comprises gating and multiplexing necessary to control the data path within the controller of the present invention for receiving and processing data, address, and control signals from the MCA. Finally, register section 108 comprise 8 registers, R61, 70, 91, 92, 94, 96, 102 and 103. These registers comprise registers of varying data width, and are used for controlling and configuring various system parameters. 
     FIGS. 2A and 2B compare the control signals and their relationships produced by prior art peripheral device controllers with control signals and their relationships produced by the controller of the present invention. In FIG. 2A, MCA bus control signals ADDRESS, MEM, SxN and CMDN are processed into IORDN, IOWRN, and KYBDCSN for a keyboard access according to the prior art. Since this figure shows typical prior art operation of the bus and the signals derived from the bus, the function is shown as if there were no access recovery time compensation at all. Thus, none of the recovery times, designated &#34;t-r&#34;, are modified or extended. 
     FIG. 2B shows the operation of two &#34;back-to-back&#34; MCA accesses to the keyboard controller with the recovery time circuit of the present invention in operation. Typically, the minimum recovery time between any two consecutive MCA bus operations is 100 ns. The recovery time required by the keyboard controller is 1000 ns minimum. 
     Recovery times, also designated &#34;t-r&#34; in FIG. 2B, are modified or extended according to the present invention by activating the STBDLY signal. In operation, the recovery time compensation circuit senses when the MCA bus access to the keyboard controller has ended and uses this information to start a counter. The counter will count by a selectable amount which relates to the specific amount of recovery time required by the peripheral device of interest, in this case, 1000 ns. At the end of this time, the IORDN or IOWRN signal will be generated. The command signal CMDN, must remain active during this entire time. Thus, the PERRDY signal must be generated to assure that the command signal remains active, and the MCA bus cycle is not terminated too soon. 
     Referring now to FIG. 3A, the PERRDY signal is produced by circuit components U201-U212 as shown. PERRDY effectively extends the duration of the CMDN strobe on the MCA bus. The PERRDY signal is also generated by U208, 210 and 212 when the STBDLY signal is active. Thus, the CMDN signal will be stretched or extended for as long as STBDLY is active. 
     With reference to FIG. 3B, flip-flops U908, 918 and 917 sense the end of an access controller signal, IORDN or IOWRN to the keyboard controller and initiates counter U911. Counter U911 will count until it reaches a preselected terminal count value which then indicates to the system that the keyboard controller is ready to process another access operation. Since CMDDLY signal is tied directly to and follows the STBDLY signal, the IORDN or IOWRN access to the keyboard controller will be allowed. Thus, as long as STBDLY signal is active, premature IORDN or IOWRN access to the keyboard controller is prohibited. The STBDLY signal may be connected externally to the CMDDLY output, or driven by another signal relating to a different peripheral device. 
     A detailed functional specification of a peripheral device controller constructed according to the preferred embodiment of the present invention is given in Exhibit A hereto, which is incorporated by reference herein. While preferred forms and arrangements have been described with reference to the drawing for illustrating the invention, it should be understood that such illustrations should not be taken as limitations on the invention. Various changes in and modifications of detail and arrangement may be made by designers of ordinary skill in the art without departing from the spirit and scope of this disclosure or the claimed invention. The features and advantages of the present invention may be employed in any microcomputer system desired.