MC68HC05C9A Datasheet, PDF (33/124 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 5
Resets
5.1 Introduction
The MC68HC05C9A microcontroller unit (MCU) can be reset four ways:
• Initial power-on reset function
• Active low input to the RESET pin
• Computer operating properly (COP)
• Clock monitor
A reset immediately stops the operation of the instruction being executed, initializes some control bits,
and loads the program counter with a user-defined reset vector address. Figure 5-1 is a block diagram of
the reset sources.
CLOCK MONITOR
COP WATCHDOG
V_DD
POWER-ON RESET
STOP
RESET
INTERNAL CLOCK
D ^R Q ^RST
RESET
LATCH
Figure 5-1. Reset Sources
TO CPU AND
SUBSYSTEMS
5.2 Power-On Reset (POR)
A power-on reset (POR) occurs when a positive transition is detected on V_DD. 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. The RESET pin will output a logic 0 during the 4064-cycle delay. If the RESET pin is low
after the end of this 4064-cycle delay, the MCU will remain in the reset condition until RESET is driven
high externally.
MC68HC05C9A Advance Information Data Sheet, Rev. 5.1
Freescale Semiconductor
33