MC68HC705C9ACFN datasheet(52/157 Pages) FREESCALE

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General Release Speciﬁcation

Power-On Reset (POR)

MC68HC705C9A — Rev. 2.0

Resets

Figure 5-1. Reset Sources

5.3 Power-On Reset (POR)

A power-on-reset occurs when a positive transition is detected on VDD.

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 (tcyc) oscillator stabilization delay after the

oscillator becomes active. (When configured as a C9A, the RESET pin

will output a logic zero 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.

5.4 RESET Pin

The function of the RESET pin is dependent on whether the device is

configured as an MC68HC05C9A or an MC68HC05C12A. When it is in

the MC68HC05C12A configuration, the pin is input only. When in

MC68HC05C9A configuration the pin is bidirectional. In both cases the

MCU is reset when a logic zero is applied to the RESET pin for a period

of one and one-half machine cycles (t

rl). For the MC68HC05C9A

configuration, the RESET pin will be driven low by a COP, clock monitor,

or power-on reset.

VDD

RESET

LATCH

POWER-ON RESET

INTERNAL CLOCK

RST

TO CPU AND

SUBSYSTEMS

STOP

CLOCK MONITOR

COP WATCHDOG

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MC68HC705C9ACFN Datasheet(PDF) 52 Page - Freescale Semiconductor, Inc