Beyond Brown

When brown just isn't enough

MK68901 MFP (Multi-Function Peripheral)

MK68901 MFP (Multi-Function Peripheral)

Introduction

The MK68901 MFP (Multi-Function Peripheral) is a combination of many of the necessary peripheral functions in a microprocessor system. Included are:

  • Eight parallel I/O lines
  • Interrupt controller for 16 sources
  • Four timers
  • Single channel full duplex USART

The use of the MFP in a system can significantly reduce chip count, thereby reducing system cost. The MFP is completely 68000 bus compatible, and 24 directly addressable internal registers provide the necessary control and status interface to the programmer.

The MFP is a derivative of the MK3801, a Z80 family peripheral.

Register MAP

Address   Abbreviation   Register Name
Port #

0         GPIP           General purpose I/O
1         AER            Active edge register
2         DDR            Data direction register

3         IERA           Interrupt enable register A
4         IERB           Interrupt enable register B
5         IPRA           Interrupt pending register A
6         IPRB           Interrupt pending register B
7         ISRA           Interrupt in-service register A
8         ISRB           Interrupt in-service register B
9         IMRA           Interrupt mask register A
A         IMRB           Interrupt mask register B
B         VR             Vector register

C         TACR           Timer A control register
D         TBCR           Timer B control register
E         TCDCR          Timers C and D control registers
F         TADR           Timer A data register
10        TBDR           Timer B data register
11        TCDR           Timer C data register
12        TDDR           Timer D data register

13        SCR            Sync character register
14        UCR            USART control register
15        RSR            Receiver status register
16        TSR            Transmitter status register
17        UDR            USART data register

Interrupts

The General Purpose I/O-Interrupt Port (GPIP) provides eight I/O lines that may be operated either as inputs or outputs under software control. In addition, each line may generate an interrupt on either a positive going edge or a negative going edge of the input signal.

The GPIP has three associated registers. One allows the programmer to specify the Active Edge for each bit that will trigger an interrupt. Another register specifies the Data Direction (input or output) associated with each bit. The third register is the actual data I/O register used to input or output data to the port. These three registers are illustrated below.

General Purpose I/O Registers

                    Active Edge Register
Port 1 (AER)  GPIP GPIP GPIP GPIP GPIP GPIP GPIP GPIP   1=Rising
               7    6    5    4    3    2    1    0     0=Falling

                    Data Direction Register
Port 2 (DDR)  GPIP GPIP GPIP GPIP GPIP GPIP GPIP GPIP   1=Output
               7    6    5    4    3    2    1    0     0=Input

                    General Purpose I/O Register
Port 3 (GPIP) GPIP GPIP GPIP GPIP GPIP GPIP GPIP GPIP
               7    6    5    4    3    2    1    0

The Active Edge Register (AER) allows each of the General Purpose Interrupts to produce an interrupt on either a 1-0 transition or a 0-1 transition. Writing a zero to the appropriate bit of the AER causes the associated input to produce an interrupt on the 1-0 transition, while a 1 causes the interrupt on the 0-1 transition. The edge bit is simply one input to an exclusive-or gate, with the other input coming from the input buffer and the output going to a 1-0 transition detector. Thus, depending upon the state of the input, writing the AER can cause an interrupt-producing transition, which will cause an interrupt on the associated channel, if that channnel is enabled. One would than normally configure the AER before enabling interrupts via IERA an IERB. Note: changing the edge bit, with the interrupt enabled, may cause an interrupt on that channel.

The Data Direction Register (DDR) is used to define I0-I7 as input or as outputs on a bit by bit basis. Writing a zero into a bit of the DDR causes the corresponding Interrupt I/O pin to be a Hi-Z Input. Writing a one into a bit of the DDR causes the corresponding pin to be configured as a push-pull output. When data is written into the GPIP, those pins defined as inputs will remain in the Hi-Z state while those pins defines as outputs will assume the state (high or low) of their corresponding bit in the PIP. When the GPIP is read, the data read will come directly from the corresponding bit of the GPIP register for all pins defines as output, while the data read on all pins defined as inputs will come from the input buffers.

Each individual functions in the MK68901 is provided with a unique interrupt vector that is presented to the system during the interrupt acknowledge cycle. The interrupt vector returned during the interrrupt acknowledge cycle is shown below.

Interrupt Vector
         V7   V6   V5   V4   V3   V2   V1   V0
         \-----------------/\----------------/
                  |                  |
                  |                  ------------ Vector bits 3-0 supplied
                  |                               by the MFP based upon the interrupting
                  |                               channel.
                  |
                  ------------------------------- 4 most significant bits. Copied
                                                  from the vector register.
Vector Register
         V7   V6   V5   V4    S    *    *    *
         \-----------------/  |
                  |           |
                  |           ------------------- S In-Service Register Enable
                  |
                  ------------------------------- Upper 4 bits of the Vector Register
                                                  Written into by the user.

There are 16 vector addresses generated internally by the MK68901, one for each of the 16 interrupt channels.

The Interrupt Control Registers provide control of interrupt processing for all I/O facilities of the MK68901. These registers allow the programmer to enable or disable any or all of the 16 interrupts, providing masking for any interrupts, and provide access to the pending and in-service status of the interrupts. Optional end-of-interrupt modes are availble under software control.

Interrupt Control Registers

                    Interrupt Enable Registers
Port 3 (IERA) GPIP  GPIP TIMER  RCV   RCV  XMIT   XMIT TIMER
               7     6     A    Full  Err  Empty  Err    B

Port 4 (IERB) GPIP  GPIP TIMER TIMER  GPIP  GPIP  GPIP  GPIP
               5     4     C     D     3     2     1     0

                    Interrupt Pending Registers
Port 5 (IPRA) GPIP  GPIP TIMER  RCV   RCV  XMIT   XMIT TIMER
               7     6     A    Full  Err  Empty  Err    B
Port 6 (IPRB) GPIP  GPIP TIMER TIMER  GPIP  GPIP  GPIP  GPIP
               5     4     C     D     3     2     1     0
                         Writing 0: Clear
                         Writing 1: Unchanged

                    Interrupt In-Service Registers
Port 7 (ISRA) GPIP  GPIP TIMER  RCV   RCV  XMIT   XMIT TIMER
               7     6     A    Full  Err  Empty  Err    B
Port 8 (ISRB) GPIP  GPIP TIMER TIMER  GPIP  GPIP  GPIP  GPIP
               5     4     C     D     3     2     1     0

                    Interrupt Mask Registers
Port 9 (IMRA) GPIP  GPIP TIMER  RCV   RCV  XMIT   XMIT TIMER
               7     6     A    Full  Err  Empty  Err    B
Port A (IMRB) GPIP  GPIP TIMER TIMER  GPIP  GPIP  GPIP  GPIP
               5     4     C     D     3     2     1     0
                    1: UnMasked    0: Masked

Interrupt Control Register Definitions

Priority   Channel   Description
Highest     1111      General Purpose Interrupt 7(I7)
            1110      General Purpose Interrupt 6(I6)
            1101      Timer A
            1100      Receive Buffer Full
            1011      Receive Error
            1010      Transmit Buffer Empty
            1001      Transmit Error
            1000      Timer B
            0111      General Purpose Interrupt 5(I5)
            0110      General Purpose Interrupt 4(I4)
            0101      Timer C
            0100      Timer D
            0011      General Purpose Interrupt 3(I3)
            0010      General Purpose Interrupt 2(I2)
            0001      General Purpose Interrupt 1(I1)
Lowest      0000      General Purpose Interrupt 0(I0)

Interrupts may be either polled or vectored. Each channel may be individually enabled or disabled by writing a one or a zero in the appropriate bit of the Interrupt Enable Registers (IERA,IERB). When disabled, an interrupt channel is completely inactive. Any internal or external action which would normally produce an interrupt on that channel is ignored and any pending interrupt on that channel will be cleared by disabling that channel. Disabling and interrupt channel has no effect on the corresponding bit in Interrupt in-Service Registers (ISRA,ISRB); thus, if the In-Service Registers are used and an interrupt is in service on that channel when the channel is disabled, it will remain in service until cleared in the normal manner. IERA and IERB are also readable.

When an interrupt is received on an enabled channel, its corresponding bit in the pending register will be set. When that channel is acknowledged it will pass its vector, and the corresponding bit in the Interrupt Pending Register (IPRA or IPRB) will be cleared. IPRA and IPRB are readable; thus by polling IPRA and IPRB, it can be determind whether a channel has a pending interrupt. IPRA and IPRB are also writeable and a pending interrupt can be cleared without going through the acknowledge sequence by writing a zero to the appropriate bit. This allows any one bit to be cleared, without altering any other bits, simply by writing all ones except for the bit position to be cleared on IPRA or IPRB. Thus a full polled interrupt scheme is possible. Note: writing a one to IPRA, IPRB has no effect on the interrupt pending register.

The interrupt mask registers (IMRA and IMRB) may be used to block a channel from making an interrupt request. Writing a zero into the corresponding bit of the mask register will still allow the channel to receive and interrupt and latch it into its pending bit (if that channel is enabled), but will prevent that channel from making an interrupt request. If that channel is causing an interrupt request at the time the corresponding bit in the mask register is cleared, the request will cease. If no other channel is making a request, INTR will go inactive. If the mask bit is re-enabled, any pending interrupt is now free to resume its request unless blocked by a higher priority request for service. IMRA and iMRB are also readable.

A conceptual circuit of an interrupt

       Edge       Enable                     Mask
    Register    Register                   Register              S-Bit
       |           |                          |                    |
       |           |                          ----|---\            --|---\ |------------|
       |           o-----|---\ |-----------|      |    |---o---------|    ||S Interrupt |
       ---\\--\    |     |    ||S Pending Q|------|---/    |       --|---/ |   Service  |
          ||   |---|-----|---/ |     R     |               |       |       |------------|
 I7-------//--/    |           |-----------|               |       |
                   |                 |                 Interrupt   |
                 -----             /---\                Request    |
                 \   /             |   |                           |
                  \ /              /---\                           |
                   o                | |                            |
                   |                | |                            |
                   ------------------ -----------------------------o------ Pass Vector

There are two end-of-interrupt modes: the automatic end-of-interrupt mode and the software end-of-interrupt mode. The mode is selected by writing a one or a zero to the S bit of the Vector Register (VR). If the S bit of the VR is a one, all channels operate in the software end-of-interrupt mode. If the S bit is a zero, all channels operate in the automatic end-of-interrupt mode, and a reset is held on all in-service bits. In the automatic end-of-interrupt mode, the pending bit is cleared when that channel passes its vector. At that point, no further history of that interrupt remains in the MK68901 MFP. In the software end-of-interrupt mode, the in-service bit is set and the pending bit is cleared when the channel passes its vector. With the in-service bit set, no lower priority channel is allowed to request an interrupt or to pass its vector during an acknowledge sequence, however, a lower priority channel may still receive and interrupt and latch it into the pending bit. A higher priority channel may stillrequest an interrupt and be acknowledged, The in-service bit of a particular channel may be cleared by writing a zero to the corresponding bit in ISRA or ISRB. Typically, this will be done at the conclusion of the interrupt routine just before the return. Thus no lower priority channel will be allowed to request service until the higer priority channel is complete, while channels of still higher priority will be allowed to request service. While the in-service bit is set, a second interrupt on that channel may be received and latched into the pending bit, though no service request will be made in response to the second interrupt until the in-service bit is cleared. ISRA and ISRB may be read at any time. Only a zero may be written into any bit of ISRA and ISRB; thus the in-service may be cleared in software but cannot be set in software. This allows any one bit to be cleared, without altering any other bits, simply by writing all ones except for the bit position to be cleared to ISRA or ISRB, as with IPRA and IPRB.

Each interrupt channel responds with a discrete 8-bit vector when acknowledged. The upper four bits of the vector are set by writing the upper four bits of the VR. The four low order bits (bit 3-bit 0) are generated by the interrupt channel.

To acknowledge an interrupt, IACK goes low, the IEI input must go low (or be tied low) and the MK68901 MFP must have acknowledgeable interrupt pending. The daisy chaining capability requires that all parts in a chain have a command IACK. When the command IACK goes low all parts freeze and prioritize interrupts in parallel. Then priority is passed down the chain, via IEI and IEO, until a part which has a pending interrupt is reached. The part with the pending interrupt, passes a vector, does not propagate IEO, and generates DTACK.

Timers

There are four timers on the MK68901 MFP. Two of the timers (Timer A and Timer B) are full function timers which can perform the basic delay function and can also perform event counting, pulse width measurement and waveform generation. The other two timers (Timer C and Timer D) are delay timers only. One or both of these timers can be used to supply the baud rate clocks for the USART. All timers are prescaler/counter timers with a common independent clock input (XTAL1, XTAL2). In addition, all timers have a time-out output function that toggles each time the timer times out.

The four timers are programmed via three Timer Control Registers and four Timer Data Registers. Timers A and B are controlled bu the control registers TACR and TBCR, respectively, and by the data registers TADR and TBDR. Timers C and D are controlled by the control register TCDCR and two data registers TCDR and TDDR. Bits in the control registers allow the selection of operational mode, prescale, and control, while data registers are used to read the timer or write into the time constant register. Timer A and B input pins, TAI and TBI, are used for the event and pulse width modes for timers A and B.

Timer Data Registers (A,B,C and D)

     Port F (TADR)    D7   D6   D5   D4   D3   D2   D1   D0

     Port 10 (TBDR)   D7   D6   D5   D4   D3   D2   D1   D0

     Port 11 (TCDR)   D7   D6   D5   D4   D3   D2   D1   D0

     Port 12 (TDDR)   D7   D6   D5   D4   D3   D2   D1   D0

Timer A and B Control Register

     Port C (TACR)    *    *    *  Reset AC3  AC2  AC1  AC0

     Port D (TBCR)    *    *    *  Reset BC3  BC2  BC1  BC0

              C3  C2  C1  C0
               0   0   0   0   Timer Stopped
               0   0   0   1   Delay Mode, /4 Prescale
               0   0   1   0   Delay Mode, /10 Prescale
               0   0   1   1   Delay Mode, /16 Prescale
               0   1   0   0   Delay Mode, /50 Prescale
               0   1   0   1   Delay Mode, /64 Prescale
               0   1   1   0   Delay Mode, /100 Prescale
               0   1   1   1   Delay Mode, /200 Prescale
               1   0   0   0   Event Count Mode
               1   0   0   1   Pulse Width Mode, /4 Prescale
               1   0   1   0   Pulse Width Mode, /10 Prescale
               1   0   1   1   Pulse Width Mode, /16 Prescale
               1   1   0   0   Pulse Width Mode, /50 Prescale
               1   1   0   1   Pulse Width Mode, /64 Prescale
               1   1   1   0   Pulse Width Mode, /100 Prescale
               1   1   1   1   Pulse Width Mode, /200 Prescale

Timer C and D Control Register

     Port E (TCDCR)   *   CC2  CC1  CC0   *   DC2  DC1  DC0

                  C2  C1  C0
                   0   0   0   Timer Stopped
                   0   0   1   Delay Mode, /4 Prescale
                   0   1   0   Delay Mode, /10 Prescale
                   0   1   1   Delay Mode, /16 Prescale
                   1   0   0   Delay Mode, /50 Prescale
                   1   0   1   Delay Mode, /64 Prescale
                   1   1   0   Delay Mode, /100 Prescale
                   1   1   1   Delay Mode, /200 Prescale

With the timer stopped, no counting can occur. The timer contents will remain unaltered while the timer is stopped (unless reloaded by writing the Timer Data Register), but any residual count in the prescaler will be lost.

In the delay mode, the prescaler is always active. A count pulse will be applied to the main timer unit each time the prescribed number of timer clock cycles has elapsed. Thus, if the prescaler is programmed to divide by ten, a count pulse will be applied to the main counter every ten cycles of the timer clock.

Each time a count pulse is applied to the main counter, it will decrement its contents. The main counter is initially loaded by writing to the Timer Data Register. Each count pulse will cause the current count to decrement. When the timer has decremented down to ‘01’, the next count pulse will not cause it to decrement to ‘00’. Instead the next count pulse will cause the timer to be reloaded from the Timer Data Register. Additionally, a ‘Time Out’ pulse will be produced. This Time Out pulse is coupled to the timer interrupt channel, and, if that channel is enabled an interrupt will be produced. The Time Out pulse is also coupled to the timer output pin and will cause the pin to change states. The output will remain in this new state until the next Time Out pulse occurs. Thus the output will complete one full cycle each two Time Out pulses.

If, for example, the prescaler were programmed to divide by ten, and the Timer Data Register were loaded will 100(decimal), the main counter would decrement once for every ten cycles of the timer clock. A Time Out pulse will occur(hence an interrupt if that channel is enabled) every 1000 cycles of the timer clock, and the timer output will complete one full cycle every 2000 cycles of the timer clock.

The main counter is an 8-bit binary down counter. If may be read at any time by reading the Timer Data Register. The information read is the information last clocked into the timer read register when the DS pin had last gone high prior to the current read cycle. When written, data is loaded into the Timer Data Register, and the main counter, if the timer is stopped. If the Timer Data Register is written while the timer is running, the new word is not loaded into the timer until it counts through H01. However, if the timer is written while it is counting through H01, an indeterminate value will be written into the time constant register. This may be circumvented by ensuring that the daata register is not written when the count is H01.

If the main counter is loaded with 01, a Time Out Pulse will occur every time the prescaler presents a count pulse to the main counter. If loaded with 00, a Time Out pulse will occur after every 256 count pulses.

Changing the prescale value with the timer running can cause the first Time Out pulse to occur at an indeterminate time, (no less than one nor more than 200 timer clock cycles times the number in the time constant register), but subsequent Time Out pulses will then occur at the correct interval.

In addition to the delay mode described above, Timers A and B can also function in Pulse Width Measurement mode or in the Event Count mode. In either of these two modes, an auxilary count signal is required. The auxilary control input for Timer A is TAI, and for Timer B, TBI is used. The interrupt channels associated with I4 and I3 are used for TAI and TBI, respectively, in Pulse Width mode.

The pulse width measurement mode functions much like the delay mode. However, in this mode, the auxiliary control signal on TAI or TBI acts as an enable to the timer. When the control signal on TAI or TBI is inactive, the timer will be stopped. When it is active, the prescaler and main counter are allowed to run. Thus the width of the active pulse on TAI or TBI is determined by the number of timer counts which occur while the pulse allows the timer to run. The active state of the signal on TAI or TBI is dependent upon the associated Interrupt Channels edge bit (GPIP4 for TAI and GPIP3 for TBI, see Active Edge Register). If the edge bit associated with the TAI or TBI input is a one, it will be active high, thus the timer will be allowed to run when the input is at a high level. If the edge bit is a zero, the TAI or TBI input will be active low. As previously stated, the interrupt channel (I3 or I4) associated with the input still functions when the timer is used in the pulse width measurement mode. However, if the timer is programmed for the pulse width measurement mode, the interrupt caused by transitions on the associated TAI or TBI input will occur on the opposite transition.

A conceptual circuit of the MFP timer in the pulse width measurement mode

                            |-----|
                            | TAI |
                            |-----|
                               |
                               -----------|---\
  Timer A                                 |    |----
  Pulse Width Mode ------------o----------|---/    |
                               |                   |---\---\      |-----------|
                               |                        |   |-----| Interrupt |
           |----|              |   |\              |---/---/      |  Channel  |
           | I4 |-------|      ----| o----|---\    |              |-----------|
           |----|       |          |/     |    |----
                        ------------------|---/


                        ------------------|---\
           |----|       |          |\     |    |----
           | I3 |-------|      ----| o----|---/    |
           |----|              |   |/              |---\---\      |-----------|
                               |                        |   |-----| Interrupt |
  Timer B                      |                   |---/---/      |  Channel  |
  Pulse Width Mode ------------o----------|---\    |              |-----------|
                                          |    |----
                               -----------|---/
                               |
                            |-----|
                            | TBI |
                            |-----|

For example, if the edge bit associated with the TAI input (AER-GPIP 4) is a one, an interrupt would normally be generated on the 0-1 transition of the I4 input signal. If the timer associated with this input (Timer A) is placed in the pulse width measurement mode, the interrupt will occur on the 1-0 transition of the TAI signal instead. Because the edge bit (AER-GPIP4) is a one, Timer A will be allowed to count while the input is high. When the TAI input makes the high to low transition, Timer A will stop, and it is at this point that the interrupt will occur (assuming that the channel is enabled). This allows the interrupt to signal the CPU that the pulse being measured has terminated this Timer A may now be read to determine the pulse width. (Again note that I3 and I4 may still be used for I/O when the timer is in the pulse width measurement mode). If Timer A is re-programmed for another mode, interrupts will again occur on the transition, as normally defined by the edge bit. Note that, like changing the edge bit, placing the timer into or taking it out of pulse width mode can produce a transition on the signal to the interrupt channel and may cause an interrupt. If measuring consecutive pulses, it is obvious that one must read the contents of the timer and reinitalize the main counter by writing to the timer data register. If the timer data register is written while the pulse is going to the active state, the write operation may result in an indeterminate value being written into the main counter. If the timer is written after the pulse goes active, the timer counts from the previous contents, and when it counts through H01, the correct value is written into the timer. The pulse width then includes counts from before the timer was reloaded.

In the event count mode, the prescaler is disabled. Each time the control input on TAI or TBI makes an active transition as defined by the associated Interrupt Channel’s edge bit, a count pulse will be generated, and the main counter will decrement. In all other respects, the timer functions as previously described. Altering the edge bit while the timer is in the event count mode can produce a count pulse. The interrupt channel associated with the input (I3 for TBI or I4 for TAI) is allowed to function normally. To count transitions reliably, the input must remain in each state (/1/0) for a length of time equal to four perioids of the timer clock; thus signals of a frequency up to one fourth of the timer clock can be counted.

The manner in which the timer output pins toggle states has previously been described. All timer outputs will be forced low by a device RESET. The output associated with Timers A and B will toggle on each Time Out pulse regardless of the mode the timers are programmed to. In addition, the outputs from Timers A and Timers B can be forced low at any time by writing a 1 to the reset location in TACR and TBCR, respectively. The output will be forced to the low state during the WRITE operation, and at the conclusion of the operation, the output will again be free to toggle each time a Time Out pulse occurs. This feature will allow waveform generation.

During reset, the Timer Data Registers and the main counters are not reset. Also, if using the reset option on Timers A or B, one must make sure to keep the other bits in the correct state so as not to affect the operation of Timers A and B.