Intel mcs-48 manual Timer Section

8155/8156/8155-2/8156-2

TIMER SECTION

The timer is a 14-bit down-counter that counts the TIMER IN pulses and provides either a square wave or pulse when terminal count (TC) is reached.

The timer has the I/O address XXXXX100forthe low order byte of the register and the I/O address XXXXX101 for the high order byte of the register. (See Figure 5).

To program the timer, the COUNT LENGTH REG is loaded first, one byte at a time, by selecting the timer addresses. Bits 0-13 of the high order count register will specify the length of the next count and bits 14-15 of the high order register will specify the timer output mode (see Figure 8). The value loaded into the count length register can have any value from 2H through 3FFH in Bits 0-13.

6	5	4	3	2	IT91	0
IM21 Ml	IT131	T121	Tll ITl0			Tal
I	II			I		I
I
TIMER MODE		MSB OF CNT LENGTH

65432o

I

LSB OF CNT LENGTH

Figure 8. Timer Format

There are four modes to choose from: M2 and M1 define the timer mode, as shown in Figure 9.

	TIMER OUT WAVEFORMS:
MOOE	START	TERMINAL	(TERMINAL)
BITS	COUNT	COUNT	COUNT
M2 M1	~ _____1____ _

o1. SINGLE

0

SQUARE WAVE

2.CONTINUOUS SQUARE WAVE

3.SINGLE---~----------

PULSE ON TERMINAL COUNT

4.CONTINUOUS -------ur------~

PULSES

Figure 9. Timer Modes

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:

TM2	TMl
o	0	NOP - Do not affect counter operation.
o		STOP - NOP if timer has not started;
		stop counting if the timer is running.
	o	STOP AFTER TC - Stop immediately
		after present TC is reached (NOP if timer
		has not started)
		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.

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 though you may only want to change the cOllnt 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.

NOTE; 5 AND 4 REFER TO THE NUMBER OF CLOCKS IN THAT TIME PERIOD

Figure 10; Asymmetrical Square-Wave Output Resulting from Count of 9

The counter in the 8155 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 CIS register.

Please note that tlie timer circuit on the 8155/8156 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 con- tain 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 8085A 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 com- plete 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 (112 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 8155/56 always counts out the right number of pulses in generating the TIMER OUT waveforms.

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