SIM Counter
If the stop enable bit, STOP, in the mask option register is logic 0, the SIM treats the STOP instruction as
an illegal opcode and causes an illegal opcode reset. The SIM actively pulls down the RST pin for all
internal reset sources.
7.3.2.4 Illegal Address Reset
An opcode fetch from an unmapped address generates an illegal address reset. The SIM verifies that the
CPU is fetching an opcode prior to asserting the ILAD bit in the SIM reset status register (SRSR) and
resetting the MCU. A data fetch from an unmapped address does not generate a reset. The SIM actively
pulls down the RST pin for all internal reset sources.
7.3.2.5 Low-Voltage Inhibit (LVI) Reset
The low-voltage inhibit module (LVI) asserts its output to the SIM when the V voltage falls to the
DD
LVI
voltage. The LVI bit in the SIM reset status register (SRSR) is set, and the external reset pin
TRIPF
(RST) is held low while the SIM counter counts out 4096 + 32 ICLK cycles. Thirty-two ICLK cycles later,
the CPU is released from reset to allow the reset vector sequence to occur. The SIM actively pulls down
the RST pin for all internal reset sources.
7.3.2.6 Monitor Mode Entry Module Reset
The monitor mode entry module reset asserts its output to the SIM when monitor mode is entered in the
condition where the reset vectors are blank ($FF). (See Chapter 8 Monitor ROM (MON).) When MODRST
gets asserted, an internal reset occurs. The SIM actively pulls down the RST pin for all internal reset
sources.
7.4 SIM Counter
The SIM counter is used by the power-on reset module (POR) and in stop mode recovery to allow the
oscillator time to stabilize before enabling the internal bus (IBUS) clocks. The SIM counter also serves as
a prescaler for the computer operating properly module (COP). The SIM counter overflow supplies the
clock for the COP module. The SIM counter is 13 bits long and is clocked by the falling edge of
CGMXCLK.
7.4.1 SIM Counter During Power-On Reset
The power-on reset module (POR) detects power applied to the MCU. At power-on, the POR circuit
asserts the signal PORRST. Once the SIM is initialized, it enables the clock generation module (CGM) to
drive the bus clock state machine.
7.4.2 SIM Counter During Stop Mode Recovery
The SIM counter also is used for stop mode recovery. The STOP instruction clears the SIM counter. After
an interrupt, break, or reset, the SIM senses the state of the short stop recovery bit, SSREC, in the mask
option register. If the SSREC bit is a logic 1, then the stop recovery is reduced from the normal delay of
4096 CGMXCLK cycles down to 32 CGMXCLK cycles. This is ideal for applications using canned
oscillators that do not require long start-up times from stop mode. External crystal applications should use
the full stop recovery time, that is, with SSREC cleared.
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
103
FREESCALE [ Freescale ]
