10

Bits 6-7 (TM2 and TM1) of command register contents are

used to start and stop the counter. there are four commands

to choose from:

Note that while the counter is counting, you may load a new

count and mode into the count length registers. Before the

new count and mode will be used by the counter, you must

issue a START command to the counter. This applies even

thought you may only want to change the count and use the

previous mode.

In case of an odd-numbered count, the first half-cycle of the

squarewave output, which is high, is one count longer than

the second (low) half-cycle, as shown in Figure 10.

The counter in the HS-81C55/56RH is not initialized to any

particular mode or count when hardware RESET occurs, but

RESET does stop the counting. Therefore, counting cannot

begin following RESET until a START command is issued

via the C/S register.

Please note that the timer circuit on the HS-81C55/56RH

chip is designed to be a square-wave timer, not an event

counter. To achieve this, it counts down by twos twice in

completing one cycle. Thus, its registers do not contain

values directly representing the number of TIMER IN pulses

received. You cannot load an initial value of 1 into the count

register and cause the timer to operate, as its terminal count

value is 10 (binary) or 2 (decimal). (For the detection of

single pulses, it is suggested that one of the hardware

interrupt pins on the HS-80C85RH be used.) After the timer

has started counting down, the values residing in the count

registers can be used to calculate the actual number of

TIMER IN pulses required to complete the timer cycle if

desired. To obtain the remaining count, perform the following

operations in order:

1. Stop the count

2. Read in the 16-bit value from the count length registers

3. Reset the upper two mode bits

4. Reset the carry and rotate right one position all 16 bits

through carry

5. If carry is set, add 1/2 of the full original count (1/2 full

count - 1 if full count is odd).

NOTE: If you started with an odd count and you read the

count length register before the third count pulse occurs, you

will not be able to discern whether one or two counts has

occurred. Regardless of this, the HS-81C55/56RH always

counts out the right number of pulses in generating the

TIMER OUT waveforms.

TM2

TM1

0

0

NOP - Do not affect counter operation

0

1

STOP-NOP - If timer has not started; stop

counting if the timer is running

1

0

STOP AFTER TC - Stop immediately after

present TC is reached (NOP if timer has not

started)

1

1

START - Load mode and CNT length and start

immediately 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.

TIMER OUT WAVEFORMS:

START

COUNT

TERMINAL

COUNT

(TERMINAL

COUNT)

0

0

1

1

0

1

0

1

1. SINGLE SQ.

WAVE

2. CONTINUOUS

SQ. WAVE

4. CONTINUOUS

PULSES

MODE BITS

M2 M1

3. SINGLE PULSE

ON TERM. COUNT

FIGURE 9. TIMER MODES

5

4

FIGURE 10. ASYMMETRICAL SQUARE-WAVE OUTPUT

RESULTING FROM COUNT OF 9

HS-81C55RH, HS-81C56RH