MC68HC908GR4MP datasheet(94/286 Pages) FREESCALE

94 / 286 page

Computer Operating Properly (COP)

MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

The COP counter is a free-running 6-bit counter preceded by a 12-bit prescaler counter. If not cleared by

software, the COP counter overflows and generates an asynchronous reset after 262,128 or 8176

CGMXCLK cycles, depending on the state of the COP rate select bit, COPRS, in the configuration

of 250 ms. Writing any value to location $FFFF before an overflow occurs prevents a COP reset by

clearing the COP counter and stages 12 through 5 of the prescaler.

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

In monitor mode, the COP is disabled if the RST pin or the IRQ is held at VTST. During the break state,

VTST 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.

9.3 I/O Signals

The following paragraphs describe the signals shown in Figure 9-1.

9.3.1 CGMXCLK

CGMXCLK is the crystal oscillator output signal. CGMXCLK frequency is equal to the crystal frequency.

9.3.2 STOP Instruction

The STOP instruction clears the COP prescaler.

9.3.3 COPCTL Write

Writing any value to the COP control register (COPCTL) (see COP Control Register) clears the COP

counter and clears bits 12 through 5 of the prescaler. Reading the COP control register returns the low

byte of the reset vector.

9.3.4 Power-On Reset

The power-on reset (POR) circuit clears the COP prescaler 4096 CGMXCLK cycles after power-up.

9.3.5 Internal Reset

An internal reset clears the COP prescaler and the COP counter.

9.3.6 Reset Vector Fetch

A reset vector fetch occurs when the vector address appears on the data bus. A reset vector fetch clears

the COP prescaler.

MC68HC908GR4MP Datasheet(PDF) 94 Page - Freescale Semiconductor, Inc