MC912DG128A Motorola, MC912DG128A Datasheet - Page 35

• Manufacturer Part Number: MC912DG128A
• Description: Microcontrollers
• Manufacturer: Motorola

9-pins
MOTOROLA
part to reach a proper reset state even if the clocks have failed, while
allowing synchronized operation when starting out of reset.
It is important to use an external low-voltage reset circuit (such as
MC34064 or MC34164) to prevent corruption of RAM or EEPROM due
to power transitions.
The reset sequence is initiated by any of the following events:
External circuitry connected to the reset pin should not include a large
capacitance that would interfere with the ability of this signal to rise to a
valid logic one within nine bus cycles after the low drive is released.
Upon detection of any reset, an internal circuit drives the reset pin low
and a clocked reset sequence controls when the MCU can begin normal
processing. In the case of POR or a clock monitor error, a 4096 cycle
oscillator startup delay is imposed before the reset recovery sequence
starts (reset is driven low throughout this 4096 cycle delay). The internal
reset recovery sequence then drives reset low for 16 to 17 cycles and
releases the drive to allow reset to rise. Nine cycles later this circuit
samples the reset pin to see if it has risen to a logic one level. If reset is
low at this point, the reset is assumed to be coming from an external
request and the internally latched states of the COP timeout and clock
monitor failure are cleared so the normal reset vector ($FFFE:FFFF) is
taken when reset is finally released. If reset is high after this nine cycle
delay, the reset source is tentatively assumed to be either a COP failure
or a clock monitor fail. If the internally latched state of the clock monitor
fail circuit is true, processing begins by fetching the clock monitor vector
($FFFC:FFFD). If no clock monitor failure is indicated, and the latched
state of the COP timeout is true, processing begins by fetching the COP
vector ($FFFA:FFFB). If neither clock monitor fail nor COP timeout are
pending, processing begins by fetching the normal reset vector
($FFFE:FFFF).
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Power-on-reset (POR)
COP watchdog enabled and watchdog timer times out
Clock monitor enabled and Clock monitor detects slow or stopped
clock
User applies a low level to the reset pin
Pinout and Signal Descriptions
Pinout and Signal Descriptions
MC68HC912DT128A Rev 2.0
Signal Descriptions
35