Freescale Semiconductor, Inc.
Pinout and Signal Descriptions
Signal Descriptions
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 time-out 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 time-out is true, processing begins by fetching the COP
vector ($FFFA:FFFB). If neither clock monitor fail nor COP time-out are
pending, processing begins by fetching the normal reset vector
($FFFE:FFFF).
3.4.4 Maskable Interrupt Request (IRQ)
The IRQ input provides a means of applying asynchronous interrupt
requests to the MCU. Either falling edge-sensitive triggering or level-
sensitive triggering is program selectable (INTCR register). IRQ is
always enabled and configured to level-sensitive triggering at reset. It
can be disabled by clearing the IRQEN bit (INTCR register). When the
MCU is reset, the IRQ function is masked in the condition code register.
This pin is always an input and can always be read. There is an active
pull-up on this pin while in reset and immediately out of reset. The pull-
up can be turned off by clearing PUPE in the PUCR register.
MC68HC912DG128 — Rev 3.0
Technical Data
Pinout and Signal Descriptions
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