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N87C51GB Datasheet(PDF) 7 Page - Intel Corporation |
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N87C51GB Datasheet(HTML) 7 Page - Intel Corporation |
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7 / 22 page  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 1228 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 XTAL12 24 48 96 Four Interface Modes HighLowFallingRising Edges Interrupt Driven 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- roOne 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 Cto 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 7 |
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