MC68HC05P4A Datasheet, PDF (27/72 Pages) Advanced Analogic Technologies – Microcontrollers

Chapter 5
Resets
5.1 Introduction
The MCU can be reset three ways:
1. Initial power-on reset function
2. Active low input to the RESET pin
3. Computer operating properly (COP) watchdog timer timeout
5.2 Power-On Reset (POR)
An internal reset is generated on power-up to allow the internal clock generator to stabilize. The power-on
reset is strictly for power turn-on conditions and should not be used to detect a drop in the power supply
voltage.
There is a 4064 internal processor clock cycle (t_cyc) oscillator stabilization delay after the oscillator
becomes active. If the RESET pin is low at the end of this 4064-cycle delay, the MCU will remain in the
reset condition until RESET goes high.
5.3 RESET Pin
The MCU is reset when a logic 0 is applied to the RESET input for a period of one and one-half machine
cycles (t_cyc).
5.4 Computer Operating Properly (COP) Reset
The MCU contains a watchdog timer that automatically times out if not reset (cleared) within a specific
time by a program reset sequence. If the COP watchdog timer is allowed to time out, an internal reset is
generated to reset the MCU. Because the internal RESET signal is used, the MCU comes out of a COP
reset in the same operating mode it was in when the COP timeout was generated.
The COP reset function is enabled or disabled by a mask option.
Refer to Chapter 9 Computer Operating Properly (COP) for more information on the COP.
MC68HC05P4A Data Sheet, Rev. 7.1
Freescale Semiconductor
27