MC68HC908GR8A_07 Datasheet, PDF (160/260 Pages) Freescale Semiconductor, Inc – Microcontrollers

System Integration Module (SIM)
14.3 Reset and System Initialization
The MCU has these reset sources:
• Power-on reset module (POR)
• External reset pin (RST)
• Computer operating properly module (COP)
• Low-voltage inhibit module (LVI)
• Illegal opcode
• Illegal address
• Forced monitor mode entry reset (MODRST)
All of these resets produce the vector $FFFE:$FFFF ($FEFE:$FEFF in monitor mode) and assert the
internal reset signal (IRST). IRST causes all registers to be returned to their default values and all
modules to be returned to their reset states.
An internal reset clears the SIM counter (see 14.4 SIM Counter), but an external reset does not. Each of
the resets sets a corresponding bit in the SIM reset status register (SRSR). See 14.7 SIM Registers.
14.3.1 External Pin Reset
The RST pin circuit includes an internal pullup device. Pulling the asynchronous RST pin low halts all
processing. The PIN bit of the SIM reset status register (SRSR) is set as long as RST is held low for a
minimum of 67 CGMXCLK cycles, assuming that neither the POR nor the LVI was the source of the reset.
See Table 14-2 for details. Figure 14-4 shows the relative timing.
CGMOUT
RST
IAB
PC
VECT H VECT L
Figure 14-4. External Reset Timing
14.3.2 Active Resets from Internal Sources
All internal reset sources actively pull the RST pin low for 32 CGMXCLK cycles to allow resetting of
external peripherals. The internal reset continues to be asserted for an additional 32 cycles at which point
the reset vector will be fetched. See Figure 14-5. An internal reset can be caused by an illegal address,
illegal opcode, COP timeout, LVI, or POR. See Figure 14-6.
NOTE
For LVI or POR resets, the SIM cycles through 4096 + 32 CGMXCLK
cycles during which the SIM forces the RST pin low. The internal reset
signal then follows the sequence from the falling edge of RST shown in
Figure 14-5.
The COP reset is asynchronous to the bus clock.
The active reset feature allows the part to issue a reset to peripherals and other chips within a system
built around the MCU.
MC68HC908GR8A • MC68HC908GR4A Data Sheet, Rev. 5
160
Freescale Semiconductor