68HC705SP7 datasheet(62/230 Pages) FREESCALE

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Resets

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Resets

5.5 Internal Resets

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

function, the COP watchdog timer reset, the low-voltage reset, and the

illegal address detector. Only the COP watchdog timer reset, low-

voltage reset, and illegal address detector will also assert the pulldown

device on the RESET pin for the duration of the reset function or for three

to four internal bus cycles, whichever is longer.

5.5.1 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); that

function can be performed by the LVR. Depending on the DELAY bit in

the mask option register (MOR), there is an oscillator stabilization delay

of 16 or 4064 internal bus cycles after the LPO becomes active.

The POR will generate the RST signal which will reset the CPU. If any

other reset function is active at the end of the 16 or 4064 cycle delay, the

RST signal will remain in the reset condition until the other reset

condition(s) end.

POR will not activate the pulldown device on the RESET pin. VDD must

drop below VPOR for the internal POR circuit to detect the next rise of

VDD.

5.5.2 Computer Operating Properly (COP) Reset

A timeout of the COP watchdog generates a COP reset. The COP

watchdog is part of a software error detection system and must be

cleared periodically to start a new timeout period. To clear the COP

watchdog and prevent a COP reset, write a logic zero to the COPC bit

of the COP register at location $1FF0. The COP register, shown in

Figure 5-2, is a write-only register that returns the contents of EPROM

location $1FF0 when read.

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