MC68HC908AT32 Datasheet, PDF (126/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

Computer Operating Properly Module (COP)
If not cleared by software, the COP counter overflows and generates an asynchronous reset after 8,176
or 262,128 CGMXCLK cycles divided by the crystal frequency, depending upon COPRS bit in the
configuration register ($001F). See Chapter 9 Configuration Register (CONFIG-1).
COP timeout period = 8,176 or 262,128 / f_osc
When COPRS = 0, a 4.9152-MHz crystal gives a COP timeout period of 53.3 ms. Writing any value to
location $FFFF before an overflow occurs prevents a COP reset by clearing the COP counter and stages
4–12 of the SIM counter.
NOTE
Service the COP immediately after reset and before entering or after exiting
stop mode to guarantee the maximum time before the first COP counter
overflow.
A COP reset pulls the RST pin low for 32 CGMXCLK cycles and sets the COP bit in the reset status
register (RSR).
In monitor mode, the COP is disabled if the RST pin or the IRQ pin is held at V_DD + V_Hi. During the break
state, V_DD + V_Hi on the RST pin disables the COP.
NOTE
Place COP clearing instructions in the main program and not in an interrupt
subroutine. Such an interrupt subroutine could keep the COP from
generating a reset even while the main program is not working properly.
13.3 I/O Signals
The following paragraphs describe the signals shown in Figure 13-1.
13.3.1 CGMXCLK
CGMXCLK is the crystal oscillator output signal. CGMXCLK frequency is equal to the crystal frequency.
13.3.2 STOP Instruction
The STOP instruction clears the COP prescaler.
13.3.3 COPCTL Write
Writing any value to the COP control register (COPCTL) (see 13.4 COP Control Register) clears the COP
counter and clears stages 12 through 4 of the COP prescaler. Reading the COP control register returns
the reset vector.
13.3.4 Power-On Reset
The power-on reset (POR) circuit clears the COP prescaler 4096 CGMXCLK cycles after power-up.
13.3.5 Internal Reset
An internal reset clears the COP prescaler and the COP counter.
MC68HC908AT32 Data Sheet, Rev. 3.1
126
Freescale Semiconductor