M16C26A Datasheet, PDF (74/352 Pages) Renesas Technology Corp – 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/Tiny SERIES

M16C/26A Group (M16C/26A, M16C/26B, M16C/26T)
7. Clock Generation Circuit
7.7 System Clock Protective Function
When the main clock is selected for the CPU clock source, this function protects the clock from modifica-
tions in order to prevent the CPU clock from becoming halted by run-away.
If the PM21 bit in the PM2 register is set to “1” (clock modification disabled), the following bits are protected
against writes:
• CM02, CM05, and CM07 bits in CM0 register
• CM10, CM11 bits in CM1 register
• CM20 bit in CM2 register
• All bits in PLC0 register
Before the system clock protective function can be used, the following register settings must be made while
the CM05 bit in the CM0 register is “0” (main clock oscillating) and CM07 bit is “0” (main clock selected for
the CPU clock source):
(1) Set the PRC1 bit in the PRCR register to “1” (enable writes to PM2 register).
(2) Set the PM21 bit in the PM2 register to “1” (disable clock modification).
(3) Set the PRC1 bit in the PRCR register to “0” (disable writes to PM2 register).
Do not execute the WAIT instruction when the PM21 bit is set to “1”.
7.8 Oscillation Stop and Re-oscillation Detect Function
The oscillation stop and re-oscillation detect function allows the detection of main clock oscillation stop and
reoscillation. At oscillation stop or re-oscillation detection, reset or oscillation stop, re-oscillation detection
interrupt are generated. Depending on the CM27 bit in the CM2 register. The oscillation stop detection
function can be enabled and disabled by the CM20 bit in the CM2 register. Table 7.8.1 lists a specification
overview of the oscillation stop and re-oscillation detect function.
Table 7.8.1. Specification Overview of Oscillation Stop and Re-oscillation Detect Function
Item
Specification
Oscillation stop detectable clock and f(XIN) ≥ 2 MHz
frequency bandwidth
Enabling condition for oscillation stop, Set the CM20 bit to “1”(enable)
re-oscillation detection function
Operation at oscillation stop,
•Reset occurs (when the CM27 bit is set to "0")
re-oscillation detection
•Oscillation stop, re-oscillation detection interrupt occurs(when the CM27 bit is
set to "1")
Rev. 2.00 Feb.15, 2007 ^{page 57} of 329
REJ09B0202-0200