During a low-power stop, unless the system clock signal is supplied by an external
source and that source is removed, the SIM clock control logic and the SIM clock sig-
nal (SIMCLK) continue to operate. The periodic interrupt timer and input logic for the
RESET and IRQ pins are clocked by SIMCLK. The SIM can also continue to generate
the CLKOUT signal while in low-power mode.
The stop mode system integration module clock (STSIM) and stop mode external
clock (STEXT) bits in SYNCR determine clock operation during low-power stop. Table
4-9 is a summary of the effects of STSIM and STEXT. MODCLK value is the logic level
on the MODCLK pin during the last reset before LPSTOP execution. Any clock in the
off state is held low. If the synthesizer VCO is turned off during LPSTOP, there is a
PLL relock delay after the VCO is turned back on.
Table 4-9 Clock Control
Mode
LPSTOP
No
Pins
SYNCR Bits
STSIM STEXT
Clock Status
CLKOUT
MODCLK
EXTAL
SIMCLK
ECLK
0
External
Clock
X
X
External
Clock
External
Clock
External
Clock
Yes
Yes
Yes
Yes
No
0
0
0
0
1
1
1
1
1
External
Clock
0
0
1
1
X
0
0
1
1
0
1
0
1
X
0
1
0
1
External
Clock
Off
Off
4
External
Clock
External
Clock
External
Clock
External
Clock
External
Clock
External
Clock
Off
Off
External
Clock
External
Clock
External
Clock
External
Clock
Crystal or
Reference
VCO
VCO
VCO
Yes
Yes
Yes
Yes
Crystal or
Reference
Crystal or
Reference
Off
Off
Crystal or
Reference
Crystal or
Reference
Crystal/
Reference
Off
Crystal or
Reference
VCO
Off
Off
Crystal or
Reference
VCO
VCO
VCO
4.3.5 Loss of Reference Signal
The state of the reset enable (RSTEN) bit in SYNCR determines what happens when
clock logic detects a reference failure.
When RSTEN is cleared (default state out of reset), the clock synthesizer is forced
into an operating condition referred to as limp mode. Limp mode frequency varies
from device to device, but maximum limp frequency does not exceed one half max-
imum system clock when X = 0, or maximum system clock frequency when X = 1.
When RSTEN is set, the SIM resets the MCU.
The limp status bit (SLIMP) in SYNCR indicates whether the synthesizer has a refer-
ence signal. It is set when a reference failure is detected.
MOTOROLA
4-16
SYSTEM INTEGRATION MODULE
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
