Rabbit 6000, User Manual

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Rabbit 6000 User’s Manual
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16. T
IMER
A
16.1 Overview
The Timer A peripheral consists of 12 separate eight-bit countdown timers, A1–A12. Each counter counts
down from a programmed time constant, which is automatically reloaded into the respective counter when
the count reaches zero. For example, if the reload register contains 127, then 128 pulses enter on the left
before a pulse exits on the right (see Figure 16.1). 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. The reload register can contain any
number in the range from 0 to 255. The counter divides by (
n + 1).
Figure 16.1 Reload Register Operation
The output pulses are always one clock wide. Clocking of the timers takes place on the negative edge of
these pulses. When the counter reaches zero, the reload register is loaded into the counter on the next input
pulse instead of a count being performed. The terminal count condition for Timers A1–A7 is reported in a
status register and can be programmed to generate an interrupt. Six of these seven timers (A2–A7) have
the option of being cascaded from Timer A1, but the primary clock for all of the timers is the peripheral
clock either directly or divided by TAPR (the default = 2).
Timers A2–A7 can be used to generate baud rates for Serial Ports A–F, or they can be used as general-pur-
pose timers if the dedicated baud rate timers on the Rabbit 6000 serial ports are used. Timers A8, A9, A10,
and A11 serve as prescalers which determine the resolution of the input capture, PWM, quadrature
decoder, and Timers B/C peripherals respectively. The peripherals clocked by these timers can generate