Jameco Electronics 2000, 3000 manual Timer B

11.2 Timer B

Figure 11-1shows 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 regis- ters, 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 gen- erated. 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	R/W	Reset
		address
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

156 Rabbit 3000 Microprocessor