Rabbit Rabbit 4000, User Manual

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12 Rabbit 4000 Microprocessor User’s Manual
2.3 Operation
2.3.1 Main Clock
The main clock is input on the CLKI pin, and is optionally sent through the spectrum
spreader and then the clock doubler. Both of these are described in greater detail below.
Different main clock modes may be selected via the GCSR, as shown in Table 2-1. Note
that one GCSR setting slows the processor clock while the peripheral clock operates at full
speed; this allows some power reduction while keeping settings like serial baud rates and
the PWM at their desired values.
When the 32 kHz clock is enabled in GCSR, it can be further divided by 2, 4, 6, or 8 to
generate even lower frequencies by enabling those modes in bits 0–2 of GPSCR. See
Table 2-4 for more details.
Table 2-1. Clock Modes
GCSR Setting Processor Clock Peripheral Clock
xxx010xx Main clock Main clock
xxx011xx Main clock / 2 Main clock / 2
xxx110xx Main clock / 4 Main clock / 4
xxx111xx Main clock / 6 Main clock / 6
xxx000xx Main clock / 8 Main clock / 8 (default on startup)
xxx001xx Main clock / 8 Main clock
xxx100xx 32 kHz clock (possibly divided)
32 kHz clock (possibly divided
via GPSCR)
xxx101xx
32 kHz clock (possibly divided);
main clock disabled via CLKIEN
output signal
32 kHz clock (possibly divided
via GPSCR)