
SECTION 7: POWER SUPPLY AND REGULATION
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MOTOROLA
Page 49
7.4
REGULATOR CONTROL LOGIC
The ignition state monitor pin, V
IGN
, or the MDLC BUS pin (provided the MDLCPU mask
option is selected) allows the user to wake-up the primary power supply and subsequently
the MCU. A positive transition applied to either pin will enable the primary regulator. The
state of the V
IGN
pin can be determined by reading the IGNS bit of the MISCELLANEOUS
register. The primary regulator can be disabled by clearing the PDC bit in the
MISCELLANEOUS register. The BUS or V
IGN
pin need not be low to write the PDC bit.
Once the regulator has been disabled, a rising edge on V
IGN
or BUS will re-enable the
regulator.
To ensure the validity of the PDC bit during a V
DD
transition, the PDC bit is always copied
into a latch powered by the secondary regulator. The bit is copied during a write to the
MISCELLANEOUS register. When the regulator is powered up by a rising edge of BUS or
V
IGN
, the latch that contains the copy of the PDC bit will be initialized to a 1 and may have
a different value than the PDC bit itself. Software should always initialize the PDC bit to a
1. The latch will be set on the rising edge of V
IGN
or BUS. This will give the primary
regulator, CPU, and software ample time to start up and initialize the PDC bit to a 1. At this
point, the MCU will transition to the Power Up Mode. (See Figure 7-2.)
In order to ensure that the primary regulator starts up when V
BATT
voltage is applied to the
MCU, a low to high transition should be provided on the V
IGN
pin once V
BATT
reaches
minimum level, shown in Figure 7-3.
Figure 7-3: Regulator Startup
NOTE:
Once enabled, the regulator can only be disabled through software.
Voltage
time
Min
V
BATT
<= 0.4*V
BATT
V
BATT
V
IGN
V
IGN
>min V
BATT
F
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