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C161U
AC/DC Characteristics
Data Sheet
447
2001-04-19
Figure 126
Generation mechanisms for the CPU Clock
The CPU clock signal can be generated via different mechanisms. The mechanism used
to generate the CPU clock is selected during reset via the logic levels on pins P0.15-13
(P0H.7-5) and is described in detail in
Chapter 3.3
, page 35. The duration of TCLs and
their variation (and also the derived external timing) depends on the mechanism used to
generate f
CPU
. This influence must be regarded when calculating the timings for the
C161U.
Note:
The example for PLL operation shown in
Figure 126
refers to a PLL factor of 4.
The PLL multiplies the input frequency by the factor
F
which is selected via the
combination of pins P0.15-13 (ie. f
CPU
= f
XTAL
*
F
). With every
F
’th transition of f
XTAL
the
PLL circuit synchronizes the CPU clock to the input clock. This synchronization is done
smoothly, i.e. the CPU clock frequency does not change abruptly.
Due to this adaptation to the input clock, the frequency of f
CPU
is constantly adjusted so
it is locked to f
XTAL
. The slight variation causes a jitter of f
CPU
which also affects the
duration of individual TCLs.
The timings listed in the AC Characteristics that refer to TCLs therefore must be
calculated using the minimum TCL that is possible under the respective circumstances.
The actual minimum value for TCL depends on the jitter of the PLL. As the PLL is
constantly adjusting its output frequency so it corresponds to the applied input frequency
(crystal or oscillator), the relative deviation for periods of more than one TCL is lower
than for a single TCL. This is especially important for bus cycles using wait-states and
for the operation of timers, serial interfaces, etc. For all slower operations and longer
periods (eg. pulse train generation or measurement, lower baudrates, etc.), the deviation
caused by the PLL jitter is negligible.
TCLTCL
f
CPU
f
XTAL
Phase Locked Loop Operation
TCL
TCL
f
CPU
f
XTAL
Prescaler Operation