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N80C51GB

CHMOS SINGLE-CHIP 8-BIT MICROCONTROLLER

Intel 

8XC51GB

ONCE MODE

The ONCE (‘‘On-Circuit Emulation’’) Mode facilitates

testing and debugging of systems using the

8XC51GB without removing it from the circuit The

ONCE Mode is invoked by

1) Pulling ALE low while the device is in reset and

PSEN is high

2) Holding ALE low as RESET is deactivated

While the device is in ONCE Mode the Port 0 pins

float and the other port pins and ALE and PSEN are

weakly pulled high The oscillator circuit remains ac-

tive While the 8XC51GB is in this mode an emula-

tor or test CPU can be used to drive the circuit Nor-

mal operation is restored when a normal reset is ap-

plied

Watchdog Timer (WDT)

The 8XC51GB contains a dedicated Watchdog Tim-

er (WDT) to allow recovery from a software or hard-

ware upset The WDT consists of a 14-bit counter

which is cleared on Reset and subsequently incre-

mented every machine cycle While the oscillator is

running the WDT will be incrementing and cannot

be disabled The counter may be reset by writing

1EH and E1H in sequence to the WDTRST Special

Function Register If the counter is not reset before

it reaches 3FFFH (16383D) the chip will be forced

into a reset sequence by the WDT This works out to

12 28 ms 16 MHz WDTRST is a write only regis-

ter The WDT does not force the external reset pin

low

While in Idle mode the WDT continues to count If

the user does not wish to exit Idle with a reset then

the processor must be periodically ‘‘woken up’’ to

service the WDT In Power Down mode the WDT

stops counting and holds its current value

Serial Expansion Port (SEP)

The Serial Expansion Port is a half-duplex synchro-

nous serial interface with the following features

Four Clock Frequencies

Four Interface Modes

Edges

Interrupt Driven

XTAL 12 24 48 96

High Low Falling Rising

Oscillator Fail Detect (OFD)

The Oscillator Fail Detect circuitry triggers a reset if

the oscillator frequency is lower than the OFD trig-

ger frequency It can be disabled by software by writ-

ing E1H followed by 1EH to the OFDCON register

Before going into Power Down Mode the OFD must

be disabled or it will force the GB out of Power

Down The OFD has the following features

OFD Trigger Frequency Below 20 KHz the

8XC51GB will be held in reset Above 400 KHz

the 8XC51GB will not be held is reset

Functions in Normal and Idle Modes

Reactivated by Reset (or External Interrupt Ze-

ro One Pins) after Software Disable

8XC51GB EXPRESS

The Intel EXPRESS products are designed to meet

the needs of those applications whose operating re-

quirements exceed commercial standards

With the commercial standard temperature range

operational characteristics are guaranteed over the

temperature range of 0 C to a70 C With the ex-

tended temperature range option operational char-

acteristics are guaranteed over the range of b40 C

to a85 C The 87C51GB EXPRESS is packaged in

the 68-lead PLCC package In order to designate a

part as an EXPRESS part a ‘‘T’’ is added as a prefix

to the part number TN87C51GB denotes an EX-

PRESS part in a PLCC package

All AC and DC parameters in this data sheet apply to

the EXPRESS devices

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