11. Timers

There are two timers--Timer A and Timer B. Timer A is intended mainly for generating the clock for various peripherals, baud clock for the serial ports, a periodic clock for clocking Parallel Ports D and E, or for generating periodic interrupts. Timers A1-A7 are general-purpose timers, and Timers A8-A10 are dedicated to specific peripherals. Timer B can be used for the same functions, but it cannot generate the baud clock. Timer B is more flexible when it can be used because the program can read the time from a continuously running counter and events can be programmed to occur at a specified future time.

Figure 11-1. Block Diagram of Timers A and B

11.1 Timer A

Timer A consists of ten separate countdown timers A1-A10 as shown in Figure 11-1.

Timers A1 and A2-A10 are 8-bit countdown registers as shown in Figure 11-2. The reload register can contain any number in the range from 0 to 255. The counter divides by (n+1). For example, if the reload register contains 127, then 128 pulses enter on the left before a pulse exits on the right. If the reload register contains zero, then each pulse on the left results in a pulse on the right, that is, there is division by one.

Figure 11-2. Reload Register Operation

The timer systems can be driven by the peripheral clock, or peripheral clock divided by two. This clock is always the same as the processor clock, or it is faster than the processor clock by a factor of eight. The output pulses are always one clock long. Clocking of the counters takes place on the negative edge of this pulse. When the counter reaches zero, the reload register is loaded on the next input pulse instead of a count being performed. The reload registers may be reloaded at any time since the peripheral clock is synchronous with the processor clock.

Timers A2, A3, A4, A5, A6 and A7 always provide the baud clock for Serial Ports E, F, A, B, C, and D respectively. Except for very low baud rates, clock A1 does not need to be used to prescale the input clock for timers A2-A7. For example, if the system clock is 11.0592 MHz, and the timer A4 divides by 144, an asynchronous baud rate of 2400 bps can be achieved in one step (assuming that the timer is clocked by peripheral clock divided by two). The clock input to the serial port can be 8 or 16 times the baud rate for asynchronous mode and 8 times the baud rate for synchronous mode. The maximum asynchronous baud rate with a 11.0592 MHz clock would be (11,059,200/(1*8) = 1,382,400.

For seven of the counters (A1-A7), the terminal count condition is reported in a status register and can be programmed to generate an interrupt. There is one interrupt vector for Timer A and a common interrupt priority. A common status register (TACSR) has a bit for each timer that indicates if the output pulse for that timer has taken place since the last read of the status register. When the status register is read, these bits are cleared. No bit will be lost. Either it will be read by the status register read or it will be set after the status register read is complete. If a bit is on and the corresponding interrupt is enabled, an interrupt will occur when priorities allow. However, a separate interrupt is not guaranteed for each bit with an enabled interrupt. If the bit is read in the status register, it is cleared and no further interrupt corresponding to that bit will be requested. It is possible that one bit will cause an interrupt, and then one or more additional bits will be set before the status register is read. After these bits are cleared, they cannot cause an interrupt. If any bits are on, and the corresponding interrupt is enabled, then the interrupt will take place as soon as priorities allow. However, if the bit is cleared before the interrupt is latched, the bit will not cause an interrupt. The proper rule to follow is for the interrupt routine to handle all bits that it sees set.

Although timers A8-A10 are part of Timer A, they are dedicated to the input pulse capture, PWM, and quadrature decoder peripherals respectively. The peripherals clocked by these timers can generate interrupts but the timers themselves cannot. Furthermore, these timers cannot be cascaded with Timer A1.

11.1.1 Timer A I/O Registers

The I/O registers for Timer A are listed in Table 11-1.

Table 11-1. Timer A I/O Registers

Register Name	Mnemonic	I/O address	R/W	Reset
Timer A Control/Status Register	TACSR	0xA0	R/W	00000000
Timer A Prescale Register	TAPR	0xA1	W	xxxxxxx1
Timer A Time Constant 1 Register	TAT1R	0xA3	W	xxxxxxxx
Timer A Control Register	TACR	0xA4	W	00000000
Timer A Time Constant 2 Register	TAT2R	0xA5	W	xxxxxxxx
Timer A Time Constant 8 Register	TAT8R	0xA6	W	xxxxxxxx
Timer A Time Constant 3 Register	TAT3R	0xA7	W	xxxxxxxx
Timer A Time Constant 9 Register	TAT9R	0xA8	W	xxxxxxxx
Timer A Time Constant 4 Register	TAT4R	0xA9	W	xxxxxxxx
Timer A Time Constant 10 Register	TAT10R	0xAA	W	xxxxxxxx
Timer A Time Constant 5 Register	TAT5R	0xAB	W	xxxxxxxx
Timer A Time Constant 6 Register	TAT6R	0xAD	W	xxxxxxxx
Timer A Time Constant 7 Register	TAT7R	0xAF	W	xxxxxxxx

The following table summarizes Timer A's capabilities.

Table 11-2. Timer A Capabilities

Timer	Cascade	Interrupt	Dedicated connection
A1	none	yes	Parallel Ports D-G, Timer B
A2	from A1	yes	Serial Port E
A3	from A1	yes	Serial Port F
A4	from A1	yes	Serial Port A
A5	from A1	yes	Serial Port B
A6	from A1	yes	Serial Port C
A7	from A1	yes	Serial Port D
A8	none	no	Input Capture
A9	none	no	Pulse Width Modulator
A10	none	no	Quadrature Decoder

The control/status register for Timer A (TACSR) is laid out as shown in Table 11-3.

Table 11-3. Timer A Control and Status Register (adr = 0A0h)

	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
Read	A7 count done	A6 count done	A5 count done	A4 count done	A3 count done	A2 count done	A1 count done	This bit is write only.
Write	A7 interrupt enable	A6 interrupt enable	A5 interrupt enable	A4 interrupt enable	A3 interrupt enable	A2 interrupt enable	A1 interrupt enable	1--enable Timer A main clock (pclk/2)

Bits 1-7--Read/write, terminal count reached on timers A1-A7. Reading this status register clears any bits (bits 1-7) that are on. Writing to these bits enables the interrupts for the corresponding timer.

Bit 0--Write, set to a "1" to enable the clock (perclk/2) for Timer A, set to "zero" to disable the clock (perclk/2 in Figure 11-1). Bits 1-7 are written (write only) to enable the interrupt for the corresponding timer.

The control register (TACR) is laid out as shown in Table 11-4.

Table 11-4. Timer A Control Register (adr = 0A4h)

Bit 7 A7	Bit 6 A6	Bit 5 A5	Bit 4 A4	Bit 3 A3	Bit 2 A2	Bits 1, 0
Source A7 0-pclk/2 1-A1	Source A6 0-pclk/2 1-A1	Source A5 0-pclk/2 1-A1	Source A4 0-pclk/2 1-A1	Source A3 0-pclk/2 1-A1	Source A2 0-pclk/2 1-A1	00--Interrupt disabled 01--priority 1 interrupt 10--priority 2 interrupt 11--priority 3 interrupt

The Timer A Prescale Register (TAPR) specifies the main clock for Timer A. By default Timer A is clocked by peripheral clock divided by two.

The prescale register (TAPR) is laid out as shown in Table 11-4.

Table 11-5. Timer A Prescale Register (adr = 0A1h)

Bits 7:1	Bit 0
These bits are ignored.	0--The main clock for Timer A is the peripheral clock. 1--The main clock for Timer A is the peripheral clock divided by two.

The time constant register for each timer (TATxR) is simply an 8-bit data register holding a number between 0 and 255. This time constant will take effect the next time that the Timer A counter counts down to zero. The timer counts modulo (divide-by) n+1, where n is the programmed time constant. The time constant registers are write only. The time constant registers are listed in Table 11-1.

11.1.2 Practical Use of Timer A

Timer A is disabled (bit 0 in control and status register) on power-up. Timer A is normally set up while the clock is disabled, but the timer setup can be changed while the timer is running when there is a need to do so. Timers that are not used should be driven from the output of A1 and the reload register should be set to 255. This will cause counting to be as slow as possible and consume minimum power.

As for general-purpose timers, Timer A has seven separate subtimer units, A1 and A2-A7, that are also referred to as timers.

Most likely, if a serial port is going to be used and a timer is needed to provide the baud clock, that timer will be set up to be driven directly from the clock, and the interrupt associated with that timer will be disabled. (Serial port interrupts are generated by the serial port logic.)

The value in the reload register can be changed while the timer is running to change the period of the next timer cycle. When the reload register is initialized, the contents of the countdown counter may be unknown, for example, during power-up initialization. If interrupts are enabled, then the first interrupt may take place at an unknown time. Similarly, if the timer output is being used to drive the clock for a parallel port or serial port, the first clock may come at a random time. If a periodic clock is desired, it is probably not important when the first clock takes place unless a phase relationship is desired relative to a different timers.

A phase relationship between two timers can be obtained in several ways. One way is to set both reload registers to zero and to wait long enough for both timers to reload (maximum 256 clocks). Then both timers' reload registers can be set to new values before or after both are clocked.

11.2 Timer B

Figure 11-1 shows a block diagram of Timer B. The Timer B counter can be driven directly by perclk/2, by that clock divided by 8, or by the output of Timer A1. Timer B has a continuously running 10-bit counter. The counter is compared against two match registers, the B1 match register and the B2 match register. When the counter transitions to a value equal to a match register, an internal pulse with a length of 1 peripheral clock is generated. The match pulse can be used to cause interrupts and/or clock the output registers of Parallel Ports D and E.

The match registers are loaded from the match preload registers that are written to by an I/O instruction. The data byte in the match preload register is advanced to the next match register when the match pulse is generated.

Every time a match condition occurs, the processor sets an internal bit that marks the match value in TBLxR as invalid. Reading TBCSR clears the interrupt condition. TBLxR must be reloaded to re-enable the interrupt. TBMxR does not need to be reloaded every time.

If both match registers need to be changed, the most significant byte needs to be changed first.

The I/O registers for Timer B are listed in Table 11-6.

Table 11-6. Timer B Registers

Register Name	Mnemonic	I/O address	R/W	Reset
Timer B Control/Status Register	TBCSR	0xB0	R/W	xxxxx000
Timer B Control Register	TBCR	0xB1	W	xxxx0000
Timer B MSB 1 Register	TBM1R	0xB2	W	xxxxxxxx
Timer B LSB 1 Register	TBL1R	0xB3	W	xxxxxxxx
Timer B MSB 2 Register	TBM2R	0xB4	W	xxxxxxxx
Timer B LSB 2 Register	TBL2R	0xB5	W	xxxxxxxx
Timer B Count MSB Register	TBCMR	0xBE	R	xxxxxxxx
Timer B Count LSB Register	TBCLR	0xBF	R	xxxxxxxx

The control/status register for Timer B (TBCSR) is laid out as shown in Table 11-7.

Table 11-7. Timer B Control and Status Register (TBCSR) (adr = 0B0h)

Bits 7:3	Bit 2	Bit 1	Bit 0
Not used	1--A match with match register 2 was detected. This bit is cleared when this register is read; setting this bit to 1 enables the interrupt.	1--A match with match register 1 was detected. This bit is cleared when this register is read; setting this bit to 1 enables the interrupt.	1--Enable the main clock for this timer.

The control register for Timer B (TBCR) is laid out as shown in Table 11-8.

Table 11-8. Timer B Control Register (TBCR)

Bits 7:4	Bits 3:2	Bits 1:0
Not used	00--Counter clocked by perclk/2 01--Counter clocked by output of timer A1 1x--Timer clocked by perclk/2 divided by 8	00--Interrupt disabled xx--Interrupt priority xx enabled.

The MSB x registers for Timer B (TBM1R/TBM2R) are laid out as shown in Table 11-9.

Table 11-9. Timer B MSB x Register (TBM1R/TBM2R = 0B2h/0B4h)

Bits 7:6	Bits 5:0
Two most significant bits of timer match preload register.	Not used.

11.2.1 Using Timer B

Normally the prescaler is set to divide perclk/2 by a number that provides a counting rate appropriate to the problem. For example, if the clock is 22.1184 MHz, then perclk/2 is 11.0592 MHz. A Timer B clock rate of 11.0592 MHz will cause a complete cycle of the 10-bit clock in 92.6 µs.

Normally an interrupt will occur when either of the comparators in Timer B generates a pulse. The interrupt routine must detect which comparator is responsible for the interrupt and dispatch the interrupt to a service routine. The service routine sets up the next match value, which will become the match value after the next interrupt. If the clocked parallel ports are being used, then a value will normally be loaded into some bits of the parallel port register. These bits will become the output bits on the next match pulse. (It is necessary to keep a shadow register for the parallel port unless the bit-addressable feature of ports D and E is used.)

If it is desired to read the time from the Timer B counter, either during an interrupt caused by the match pulse or in some other interrupt routine asynchronous to the match pulse, a special procedure needs to be used to read the counter because the upper 2 bits are in a different register than the lower 8 bits. The following method is suggested.

1. Read the lower 8 bits.

2. Read the upper 2 bits

3. Read the lower 8 bits again

4. If bit 7 changed from 1 to 0 between the first and second read of the lower 8 bits there has been a carry to the upper 2 bits. In this case read the upper 2 bits again and decrement those 2 bits to get the correct upper 2 bits. Use the first read of the lower 8 bits.

This procedure assumes that the time between reads can be guaranteed to be less than 256 counts. This can be guaranteed in most systems by disabling the priority 1 interrupts, which will normally be disabled in any case in an interrupt routine.

It is inadvisable to disable the high-priority interrupts (levels 2 and 3) as that defeats their purpose.

If speed is critical, the three reads of the registers can be performed without testing for the carry. The three register values can be saved and the carry test can be performed by a lower priority analysis routine. Since the upper 2 bits are in the register TBCMR at address 0BEh, and the lower 8 bits are in TBCLR at address 0BFh, both registers can be read with a single 16-bit I/O instruction. The following sequence illustrates how the registers could be captured.

; enter from external interrupt on pulse input transition
; 19 clocks latency plus 10 clocks interrupt execution
push af  ; 7
push hl
ioi ld a,(TBCLR)	   ; 11 get lower 8 bits of counter
ioi ld hl,(TBCMR)  ;13  get l=upper, h=lower

Timer B can be used for various purposes. The 10-bit counter can be read to record the time at which an event takes place. If the event creates an interrupt, the timer can be read in the interrupt routine. The known time of execution of the interrupt routine can be subtracted. The variable interrupt latency is then the uncertainty in the event time. This can be as little 19 clocks if the interrupt is the highest priority interrupt. If the system clock is 20 MHz, the counter can count as fast as 10 MHz. The uncertainty in a pulse width measurement can be nearly as low as 38 clocks (2 x 19), or about 2 µs for a 20 MHz system clock.

Timer B can be used to change a parallel port output register at a particular specified time in the future. A pulse train with edges at arbitrary times can be generated with the restriction that two adjacent edges cannot be too close to each other since an interrupt must be serviced after each edge to set up the time for the next edge. This restriction limits the minimum pulse width to about 5 µs, depending on the clock speed and interrupt priorities.