HC705CT4GRSSLASHD datasheet(46/152 Pages) NXP

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Resets

General Release Specification

MC68HC705CT4 — Rev. 2.0

Resets

MOTOROLA

5.4 Internal Resets

The three internally generated resets are the initial power-on reset

function, the COP watchdog timer reset, and the illegal address detector.

Termination of the external RESET input or the internal COP watchdog

timer are the only reset sources that can alter the operating mode of the

MCU. The other internal resets do not have any effect on the mode of

operation when their reset state ends.

5.4.1 Power-On Reset (POR)

The internal POR is generated on power-up to allow the clock oscillator

to stabilize. The POR is strictly for power turn-on conditions and is not

able to detect a drop in the power supply voltage (brown-out). There is

an oscillator stabilization delay of 4064 internal processor bus clock

cycles (PH2) after the oscillator becomes active.

The POR generates the RST signal that resets the CPU. If any other

reset function is active at the end of this 4064-cycle delay, the RST

signal remains in the reset condition until the other reset condition(s)

end. During the POR, the RESET pin is forced low.

5.4.2 Computer Operating Properly Reset (COPR)

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. Regardless of an internal or external

RESET, the MCU comes out of a COP reset according to the standard

rules of mode selection.

The COP reset function is enabled or disabled by a mask option and is

verified during production testing.

5.4.2.1 Resetting the COP

Writing a zero to the COPF bit prevents a COP reset. This action resets

the counter and begins the timeout period again. The COPF bit is bit 0

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

HC705CT4GRS/D 데이터시트(PDF) 46 Page - NXP Semiconductors