MC68HC908AS32A Datasheet, PDF (120/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Computer Operating Properly (COP)
8.2 Functional Description
The COP counter is a free-running 6-bit counter preceded by a 12-bit prescaler. If not cleared by software,
the COP counter overflows and generates an asynchronous reset after 8176 or 262,128 CGMXCLK
cycles, depending on the state of the COP long timeout bit, COPL, in the CONFIG1. When COPL = 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 through 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_TST. During the break
state, V_TST 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.
8.3 I/O Signals
The following paragraphs describe the signals shown in Figure 8-1.
8.3.1 CGMXCLK
CGMXCLK is the crystal oscillator output signal. CGMXCLK frequency is equal to the crystal frequency.
8.3.2 STOP Instruction
The STOP instruction clears the COP prescaler.
8.3.3 COPCTL Write
Writing any value to the COP control register (COPCTL) clears the COP counter and clears stages 12
through 4 of the COP prescaler (refer to 8.4 COP Control Register). Reading the COP control register
returns the reset vector.
8.3.4 Power-On Reset
The power-on reset (POR) circuit clears the COP prescaler 4096 CGMXCLK cycles after power-up.
8.3.5 Internal Reset
An internal reset clears the COP prescaler and the COP counter.
MC68HC908AS32A Data Sheet, Rev. 2.0
120
Freescale Semiconductor