System Integration Module (SIM)
MC68HC908QL4 MC68HC908QL3 MC68HC908QL2 Data Sheet, Rev. 4
120
Freescale Semiconductor
13.4 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
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
13.5 SIM Counter
), but an external reset does not. Each of
the resets sets a corresponding bit in the SIM reset status register (SRSR). See
13.8 SIM Registers
.
13.4.1 External Pin Reset
The RST pin circuits include 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 at
least the minimum t
RL
time.
Figure 13-3
shows the relative timing. The RST pin function is only available
if the RSTEN bit is set in the CONFIG1 register.
Figure 13-3. External Reset Timing
13.4.2 Active Resets from Internal Sources
The RST pin is initially setup as a general-purpose input after a POR. Setting the RSTEN bit in the
CONFIG1 register enables the pin for the reset function. This section assumes the RSTEN bit is set when
describing activity on the RST pin.
NOTE
For POR and LVI resets, the SIM cycles through 4096 BUSCLKX4 cycles.
The internal reset signal then follows the sequence from the falling edge of
RST shown in
Figure 13-4
.
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.
All internal reset sources actively pull the RST pin low for 32 BUSCLKX4 cycles to allow resetting of
external peripherals. The internal reset signal IRST continues to be asserted for an additional 32 cycles
RST
ADDRESS BUS
PC
VECT H VECT L
BUSCLKX2