7

Programming of the Command Register

The command register consists of eight latches. Four bits (0-

3) define the mode of the ports, two bit (4-5) enable or disable

the interrupt from port C when it acts as control port, and the

last two bits (6-7) are for the timer.

The command register contents can be altered at anytime by

using the I/O address XXXXX000 during a WRITE operation

with the Chip Enable active and IO/M = 1. The meaning of

each bit of the command byte is defined in Figure 3. The

contents of the command register may never be read.

Reading the Status Register

The status register consists of seven latches, one for each

bit six (0-5) for the status of the ports and one (6) for the

status of the timer.

The status of the timer and the I/O section can be polled by

reading the Status Register (Address XXXXX000). Status

word format is shown in Figure 4. Note that you may never

write to the status register since the command register

shares the same I/O address and the command register is

selected when a write to that address is issued.

Input/Output Section

The I/O section of the HS-81C55RH and HS-81C56RH

consists of five registers: (See Figure 5)

• Command/Status Register (C/S) - Both register are

assigned the address XXXXX000. The C/S address

serves the dual purpose.

When the C/S registers are selected during WRITE

operation, a command is written into the command

register. The contents of this register are not accessible

through the pins.

When the C/S (XXXXX000) is selected during a READ

operation, the status information of the I/O ports and

the timer becomes available on the AD0 - AD7 lines.

• PA Register - This register can be programmed to be

either input or output ports depending on the status of

the contents of the C/S Register. also depending on the

command, this port can operate in either the basic

mode or the strobed mode (See timing diagram). the

I/O pins assigned in relation to this register are PA0 -

PA7. The address of this register is XXXXX001.

• PB Register - This register functions the same as PA

Register. the I/O pins assigned are PB0 - PB7. The

address of this register is XXXXX010

• PC Register - This register has the address

XXXXX011 and contains only 6 bits. The 6 bits can be

programmed to be either input ports, output ports or as

CE (81C55RH)

OR

CE (81C56RH)

IO/M

AD0 - AD7

ALE

RD OR WR

ADDRESS

DATA

VALID

FIGURE 2. ON-BOARD MEMORY READ/WRITE CYCLE

TM2 TM1 IEB

IEA PC2 PC1

PB

PA

76543210

DEFINES

DEFINES

DEFINES

ENABLE PORT

A INTERRUPT

ENABLE PORT

B INTERRUPT

00 = NOP - DO NOT AFFECT COUNTER

OPERATION

01 = STOP - NOP IF TIMER HAS NOT

STARTED; STOP COUNTING IF

THE TIMER IS RUNNING

10 = STOP AFTER TC - STOP IMME-

DIATELY AFTER PRESENT TC

IS REACHED (NOP IF TIMER

HAS NOT STARTED)

11 = START - LOAD MODE AND CNT

LENGTH AND START IMMEDIATE-

LY AFTER LOADING (IF TIMER IS

NOT PRESENTLY RUNNING). IF

TIMER IS RUNNING, START THE

NEW MODE AND CNT LENGTH

IMMEDIATELY AFTER PRESENT

TC IS REACHED.

0 = INPUT

1 = OUTPUT

00 = ALT1

11 = ALT2

01 = ALT3

10 = ALT4

0 = INPUT

1 = OUTPUT

PA0 - PA7

PB0 - PB7

PC0 - PC5

TIMER

COMMAND

FIGURE 3. COMMAND REGISTER BIT ASSIGNMENT

TIMER

INTE

B

B

BF

INTR

B

INTE

A

A

BF

INTR

A

AD7

AD6

AD5

AD4

AD3

AD2

AD1

AD0

PORT A

INTERRUPT

REQUEST

PORT A BUFFER

FULL/EMPTY

(INPUT/OUTPUT)

PORT A INTERRUPT ENABLE

PORT B INTERRUPT REQUEST

PORT B BUFFER FULL/EMPTY

(INPUT/OUTPUT)

PORT B INTERRUPT ENABLE

TIMER INTERRUPT (THIS BIT IS LATCHED HIGH WHEN

TERMINAL COUNT IS REACHED, AND IS RESET TO LOW

READING OF THE C/S REGISTER & BY HARDWARE RESET).

FIGURE 4. STATUS REGISTER BIT ASSIGNMENT

HS-81C55RH, HS-81C56RH