Freescale Semiconductor MC68HC908MR16 Power-On Reset POR, Computer Operating Properly COP Reset

Reset and System Initialization

14.3.2.1 Power-On Reset (POR)

When power is first applied to the MCU, the power-on reset (POR) module generates a pulse to indicate that power-on has occurred. The external reset pin (RST) is held low while the SIM counter counts out 4096 CGMXCLK cycles. Sixty-four CGMXCLK cycles later, the CPU and memories are released from reset to allow the reset vector sequence to occur.

OSC1

PORRST

4096

CYCLES

CGMXCLK

CGMOUT

RST

IAB

32

CYCLES

32

CYCLES

$FFFE$FFFF

Figure 14-6. POR Recovery

At power-on, these events occur:

•A POR pulse is generated.

•The internal reset signal is asserted.

•The SIM enables CGMOUT.

•Internal clocks to the CPU and modules are held inactive for 4096 CGMXCLK cycles to allow stabilization of the oscillator.

•The RST pin is driven low during the oscillator stabilization time.

•The POR bit of the SIM reset status register (SRSR) is set and all other bits in the register are cleared.

14.3.2.2 Computer Operating Properly (COP) Reset

An input to the SIM is reserved for the COP reset signal. The overflow of the COP counter causes an internal reset and sets the COP bit in the SIM reset status register (SRSR). The SIM actively pulls down the RST pin for all internal reset sources.

To prevent a COP module timeout, write any value to location $FFFF. Writing to location $FFFF clears the COP counter and bits 12–4 of the SIM counter. The SIM counter output, which occurs at least every 213–24CGMXCLK cycles, drives the COP counter. The COP should be serviced as soon as possible out of reset to guarantee the maximum amount of time before the first timeout.

The COP module is disabled if the RST pin or the IRQ pin is held at VHI while the MCU is in monitor mode. The COP module can be disabled only through combinational logic conditioned with the high voltage

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

Freescale Semiconductor 185