HCS08 datasheet(52/198 Pages) NXP

52 / 198 page

Chapter 5 Resets, Interrupts, and General System Control

MC9S08QD4 Series MCU Data Sheet, Rev. 6

Freescale Semiconductor

5.4

Computer Operating Properly (COP) Watchdog

The COP watchdog is intended to force a system reset when the application software fails to execute as

expected. To prevent a system reset from the COP timer (when it is enabled), application software must

reset the COP counter periodically. If the application program gets lost and fails to reset the COP counter

before it times out, a system reset is generated to force the system back to a known starting point.

After any reset, the COPE becomes set in SOPT1 enabling the COP watchdog (see Section 5.8.5, “System

Options Register 2 (SOPT2),” for additional information). If the COP watchdog is not used in an

application, it can be disabled by clearing COPE. The COP counter is reset by writing any value to the

address of SRS. This write does not affect the data in the read-only SRS. Instead, the act of writing to this

address is decoded and sends a reset signal to the COP counter.

The COPCLKS bit in SOPT2 (see Section 5.8.5, “System Options Register 2 (SOPT2),” for additional

information) selects the clock source used for the COP timer. The clock source options are either the bus

clock or an internal 32 kHz clock source. With each clock source, there is an associated short and long

time-out controlled by COPT in SOPT1. Table 5-1 summaries the control functions of the COPCLKS and

COPT bits. The COP watchdog defaults to operation from the 32 kHz clock source and the associated long

time-out (28 cycles).

Even if the application will use the reset default settings of COPE, COPCLKS and COPT, the user must

write to the write-once SOPT1 and SOPT2 registers during reset initialization to lock in the settings. That

way, they cannot be changed accidentally if the application program gets lost. The initial writes to SOPT1

and SOPT2 will reset the COP counter.

The write to SRS that services (clears) the COP counter must not be placed in an interrupt service routine

(ISR) because the ISR could continue to be executed periodically even if the main application program

fails.

In Background debug mode, the COP counter will not increment.

When the bus clock source is selected, the COP counter does not increment while the system is in stop

mode. The COP counter resumes once the MCU exits stop mode.

When the 32 kHz clock source is selected, the COP counter is re-initialized to zero upon entry to stop

mode. The COP counter begins from zero once the MCU exits stop mode.

Table 5-1. COP Configuration Options

Control Bits

Clock Source

COP Overflow Count

COPCLKS

COPT

~32 kHz

210 cycles (32 ms)1

1 Values are shown in this column based on t

RTI =1 ms. See tRTI in the Section A.8.1,

“Control Timing,” for the tolerance of this value.

~32 kHz

213 cycles (256 ms)1

Bus

213 cycles

Bus

218 cycles

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