Rabbit Rabbit 4000, Руководство пользователя

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Chapter 13 Timer A 107
13. T
IMER
A
13.1 Overview
The Timer A peripheral consists of ten separate eight-bit countdown timers, A1–A10. 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 13-1). 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 13-1. Reload Register Operation
For Timers A1–A7 the terminal count condition 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 periph-
eral clock either directly or divided by 2 (the default). The output pulses are always one
clock wide. Clocking of the timers takes place on the negative edge of this pulse. When
the counter reaches zero, the reload register is loaded into the counter on the next input
pulse instead of a count being performed.
Timers A2–A7 can be used to generate baud rates for Serial Ports A–F, or they can be used
as general-purpose timers if the dedicated timers on the Rabbit 4000 serial ports are used.
The three remaining timers (A8–A10) serve as prescalers for the input capture, PWM, and
quadrature decoder peripherals respectively. The peripherals clocked by these timers can
8-bit Reload Register
8-bit Down Counter
Clock IN
Pulse on Zero Count
OUT
LOAD
Input Clock
Count Value
N - 1
N 0 1 2