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User
’
s Manual
6-6
1999-08
Direct Drive
When direct drive is configured (CLKCFG=
’
011
’
) the C161PI
’
s clock system is directly
fed from the external clock input, i.e.
CPU
=
OSC
. This allows operation of the C161PI with
a reasonably small fundamental mode crystal. The specified minimum values for the
CPU clock phases (TCLs) must be respected. Therefore the maximum input clock
frequency depends on the clock signal
’
s duty cycle.
Prescaler Operation
When prescaler operation is configured (CLKCFG=
’
001
’
) the C161PI
’
s input clock is
divided by 2 to generate then CPU clock signal, i.e.
CPU
=
OSC
/2. This requires the
oscillator (or input clock) to run on 2 times the intended operating frequency but
guarantees a 50% duty cycle for the internal clock system independent of the input clock
signal
’
s waveform.
PLL Operation
When PLL operation is configured (via CLKCFG) the C161PI
’
s input clock is fed to the
on-chip phase locked loop circuit which multiplies its frequency by a factor of
F
= 1.5...5
(selectable via CLKCFG, see table below) and generates a CPU clock signal with 50%
duty cycle, i.e.
CPU
=
OSC
*F.
The on-chip PLL circuit allows operation of the C161PI on a low frequency external clock
while still providing maximum performance. The PLL also provides fail safe mechanisms
which allow the detection of frequency deviations and the execution of emergency
actions in case of an external clock failure.
When the PLL detects a missing input clock signal it generates an interrupt request. This
warning interrupt indicates that the PLL frequency is no more locked, i.e. no more stable.
This occurs when the input clock is unstable and especially when the input clock fails
completely, e.g. due to a broken crystal. In this case the synchronization mechanism will
reduce the PLL output frequency down to the PLL
’
s base frequency (2...5 MHz). The
base frequency is still generated and allows the CPU to execute emergency actions in
case of a loss of the external clock.
On power-up the PLL provides a stable clock signal within ca. 1 ms after
reached the specified valid range, even if there is no external clock signal (in this case
the PLL will run on its base frequency of 2...5 MHz). The PLL starts synchronizing with
the external clock signal as soon as it is available. Within ca. 1 ms after stable
oscillations of the external clock within the specified frequency range the PLL will be
synchronous with this clock at a frequency of
F
*
clock.
When PLL operation is selected the CPU clock is a selectable multiple of the oscillator
frequency, i.e. the input frequency.
DD
has
OSC
, i.e. the PLL locks to the external