8XC51GB
ONCE MODE
Serial Expansion Port (SEP)
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:
The Serial Expansion Port is a half-duplex synchro-
nous serial interface with the following features:
Four Clock FrequenciesÐ XTAL/12, 24, 48, 96.
1) Pulling ALE low while the device is in reset and
PSEN is high;
Four Interface ModesÐ High/Low/Falling/Rising
Edges.
2) Holding ALE low as RESET is deactivated.
Interrupt Driven.
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.
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.
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
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.
@
12.28 ms 16 MHz. WDTRST is a write only regis-
ter. The WDT does not force the external reset pin
low.
With the commercial standard temperature range,
operational characteristics are guaranteed over the
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.
a
tended temperature range option, operational char-
temperature range of 0 C to
70 C. With the ex-
§
§
b
acteristics are guaranteed over the range of 40 C
to 85 C. The 87C51GB EXPRESS is packaged in
§
a
§
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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