OSCILLATOR CIRCUITS KS57C2308/P2308/C2316/P2316
6-10
SWITCHING THE CPU CLOCK
Together, bit settings in the power control register, PCON, and the system clock mode register, SCMOD,
determine whether a main system or a subsystem clock is selected as the CPU clock, and also how this
frequency is to be divided. This makes it possible to switch dynamically between main and subsystem clocks and
to modify operating frequencies.
SCMOD.3, SCMOD.2, and SCMOD.0 select the main system clock (fx) or a subsystem clock (fxt) and start or
stop main or sub system clock oscillation. PCON.1 and PCON.0 control the frequency divider circuit, and divide
the selected fx clock by 4, 8, 64, or fxt clock by 4.
NOTE
A clock switch operation does not go into effect immediately when you make the SCMOD and PCON
register modifications — the previously selected clock continues to run for a certain number of machine
cycles.
For example, you are using the default CPU clock (normal operating mode and a main system clock of fx/64)
and you want to switch from the fx clock to a subsystem clock and to stop the main system clock. To do this, you
first need to set SCMOD.0 to "1". This switches the clock from fx to fxt but allows main system clock oscillation
to continue. Before the switch actually goes into effect, a certain number of machine cycles must elapse. After
this time interval, you can then disable main system clock oscillation by setting SCMOD.3 to "1".
This same “stepped” approach must be taken to switch from a subsystem clock to the main system clock: first,
clear SCMOD.3 to "0" to enable main system clock oscillation. Until main osc is stabilized, system clock must not
be changed. Then, after a certain number of machine cycles has elapsed, select the main system clock by
clearing all SCMOD values to logic zero.
After RESET, CPU operation starts with the lowest main system clock frequency of 15.3 µs at 4.19 MHz after the
standard oscillation stabilization interval of 31.3 ms has elapsed. Table 6-6 details the number of machine cycles
that must elapse before a CPU clock switch modification goes into effect.