Datasheet

7. Clock Generation Circuit
page 47
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7.3 On-chip Oscillator Clock
This clock is supplied by a on-chip oscillator. This clock is used as the clock source for the CPU and
peripheral function clocks. In addition, if the PM22 bit in the PM2 register is “1” (on-chip oscillator clock for
the watchdog timer count source), this clock is used as the count source for the watchdog timer (Refer to
10.1 Count source protective mode).
The on-chip oscillator clock after reset oscillates. The on-chip oscillator clock f2(ROC) divided by 16 is used
for the CPU clock. It can also be turned off by setting the CM21 bit in the CM2 register to “0” (main clock or
PLL clock). If the main clock stops oscillating when the CM20 bit in the CM2 register is “1” (oscillation stop,
re-oscillation detection function enabled) and the CM27 bit is “1” (oscillation stop, re-oscillation detection
interrupt), the on-chip oscillator automatically starts operating, supplying the necessary clock for the micro-
computer.
7.4 PLL Clock
The PLL clock is generated from the main clock by a PLL frequency synthesizer. This clock is used as the
clock source for the CPU and peripheral function clocks. After reset, the PLL clock is turned off. The PLL
frequency synthesizer is activated by setting the PLC07 bit to “1” (PLL operation). When the PLL clock is
used as the clock source for the CPU clock, wait tsu(PLL) for the PLL clock to be stable, and then set the
CM11 bit in the CM1 register to “1”.
Before entering wait mode or stop mode, be sure to set the CM11 bit to “0” (CPU clock source is the main
clock). Furthermore, before entering stop mode, be sure to set the PLC07 bit in the PLC0 register to “0”
(PLL stops). Figure 7.4.1 shows the procedure for using the PLL clock as the clock source for the CPU.
The PLL clock frequency is determined by the equation below.
PLL clock frequency=f(XIN) X (multiplying factor set by the PLC02 to PLC00 bits in the PLC0 register)
(However, 10 MHz ≤ PLL clock frequency ≤ 20 MHz in M16C/26A and M16C/26T, 10 MHz ≤ PLL clock
frequency ≤ 24 MHz in M16C/26B)
The PLC02 to PLC00 bits can be set only once after reset. Table 7.4.1 shows the example for setting PLL
clock frequencies.
XIN
(MHz)
PLC02 PLC01 PLC00 Multiplying factor PLL clock
(MHz)
(1)
10 0 0 1 2
20
50 1 0
4
Table 7.4.1. Example for Setting PLL Clock Frequencies
NOTE:
1. 10 MHz ≤ PLL clock frequency ≤ 20 MHz in M16C/26A and M16C/26T, 10 MHz ≤ PLL clock
frequency ≤ 24 MHz in M16C/26B)