MC912DG128ACPVE Datasheet, PDF (144/478 Pages) Freescale Semiconductor, Inc – Upward compatible with M68HC11 instruction set

Freescale Semiconductor, Inc.
Resets and Interrupts
source of reset in a system. The POR circuit only initializes internal
circuitry during cold starts and cannot be used to force a reset as system
voltage drops.
It is important to use an external low voltage reset circuit (for example:
MC34064 or MC33464) to prevent power transitions or corruption of
RAM or EEPROM.
10.8.2 External Reset
The CPU distinguishes between internal and external reset conditions
by sensing whether the reset pin rises to a logic one in less than nine E-
clock cycles after an internal device releases reset. When a reset
condition is sensed, 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 a clock monitor error, a 4096 cycle oscillator
start-up 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 E-clock cycles later the reset pin is
sampled. If the pin is still held low, the CPU assumes that an external
reset has occurred. If the pin is high, it indicates that the reset was
initiated internally by either the COP system or the clock monitor.
To prevent a COP reset from being detected during an external reset,
hold the reset pin low for at least 32 cycles. To prevent a clock monitor
reset from being detected during an external reset, hold the reset pin low
for at least 4096 + 32 cycles. An external RC power-up delay circuit on
the reset pin is not recommended — circuit charge time can cause the
MCU to misinterpret the type of reset that has occurred.
10.8.3 COP Reset
The MCU includes a computer operating properly (COP) system to help
protect against software failures. When COP is enabled, software must
write $55 and $AA (in this order) to the COPRST register in order to keep
a watchdog timer from timing out. Other instructions may be executed
between these writes. A write of any value other than $55 or $AA or
Technical Data
144
MC68HC912DT128A — Rev 4.0
Resets and Interrupts
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA