Rabbit 6000 User’s Manual
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176
18. T
IMER
C
18.1 Overview
The Timer C peripheral is a 16-bit up-counter clocked by the peripheral clock divided by 2, by the periph-
eral clock divided by 16, or by the output of countdown Timers A1 or A11. The counter counts from zero
to the limit programmed into the Timer C divider registers and then restarts at zero, so the overall cycle
count is the value in the divider registers plus one. There are four Timer C outputs that are called Timers
C0–C3. Each output is controlled by a 16-bit set value and a 16-bit reset value. Each output is set to one
when the count matches the value in the corresponding set register and is cleared when the count matches
the value programmed in the corresponding reset register. This allows the creation of quadrature signals or
three-phase signals with a variable frequency for motor-control applications. The values in all of the Timer
C registers are transferred to holding registers for use during the count cycle when the counter rolls over,
allowing the control registers to be reloaded at any time during the count cycle.
Timer C can generate an interrupt when the count limit value is reached.
A separate Timer C Block Access Register (TCBAR) and Timer C Block Pointer Register (TCBPR) are
available to allow DMA control of Timer C. The pointer register contains the address of the Timer C regis-
ter to be accessed via the access register. Each read or write of the access register automatically increments
the pointer register through the sequence shown below. Note that only the lower five bits of the pointer reg-
ister actually change. This allows the DMA to write to a fixed internal I/O location but still program all of
the relevant timer registers. The pointer register can be written and read if necessary. Normally the pointer
register TCBPR is initialized to 0x02 (the offset of the Timer C Divider Low Register), and the DMA then
transfers blocks of 18 bytes to completely reprogram Timer C.
0x502 -> 0x503 -> 0x508 -> 0x509 -> 0x50A -> 0x50B ->
0x50C -> 0x50D -> 0x50E -> 0x50F -> 0x518 -> 0x519 ->
0x51A -> 0x51B -> 0x51C -> 0x51D -> 0x51E -> 0x51F ->
When the DMA destination address is the TCBAR, the DMA request from Timer C is connected automati-
cally to the DMA.
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