MC68HC912D60 Datasheet, PDF (46/432 Pages) Freescale Semiconductor, Inc – M68HC12 Microcontrollers

Freescale Semiconductor, Inc.
Pinout and Signal Descriptions
3.5.2 E-Clock Output (ECLK)
ECLK is the output connection for the internal bus clock and is used to
demultiplex the address and data and is used as a timing reference.
ECLK frequency is equal to 1/2 the crystal frequency out of reset. The E-
clock output is turned off in single chip user mode to reduce the effects
of RFI. It can be turned on if necessary. In single-chip special mode, the
E-clock is turned ON at reset and can be turned OFF. In special
peripheral mode the E-clock is an input to the MCU. All clocks, including
the E clock, are halted when the MCU is in STOP mode. It is possible to
configure the MCU to interface to slow external memory. ECLK can be
stretched for such accesses.
3.5.3 Reset (RESET)
An active low bidirectional control signal, RESET, acts as an input to
initialize the MCU to a known start-up state. It also acts as an open-drain
output to indicate that an internal failure has been detected in either the
clock monitor or COP watchdog circuit. The MCU goes into reset
asynchronously and comes out of reset synchronously. This allows the
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:
• 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
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.
Advance Information
46
Pinout and Signal Descriptions
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68HC(9)12D60 — Rev 4.0
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