M32C8A Datasheet, PDF (110/372 Pages) Renesas Technology Corp – RENESAS MCU M16C FAMILY / M32C/80 Series

M32C/8A Group
9. Clock Generation Circuits
9.6 System Clock Protect Function
The system clock protect function prohibits changing the CPU clock source when the main clock is selected as the
CPU clock source. With this function, the CPU clock can continue running even if the program runs out of control.
When the PM21 bit in the PM2 register is set to 1 (disables a clock change), the following bits cannot be written:
• Bits CM02, CM05, and CM07 in the CM0 register
• Bits CM10 and CM17 in the CM1 register
• The CM20 bit in the CM2 register
• All bits in registers PLC0 and PLC1
The CPU clock continues running when the WAIT instruction is executed.
Figure 9.16 shows a procedure to use the system clock protect function. Follow the procedure while the CM05 bit
in the CM0 register is set to 0 (main clock oscillates) and the CM07 bit to 0 (main clock as CPU clock source).
Start
PRCR register: PRC1 bit = 1
PM2 register: PM21 bit = 1⁽¹⁾
PRCR register: PRC1 bit = 0
Enable writing to registers associated with clocks
Disable a clock change
Disable writing to registers associated with clocks
End
NOTE:
1. Execute the WAIT instruction when the PM21 bit in the PM2 register is set to 0.
Figure 9.16 Procedure to Use System Clock Protect Function
Rev.1.00 Jul 15, 2007 Page 93 of 352
REJ09B0385-0100