High-Performance SAB 80C517A/83C517A-5 8-Bit CMOS Single ...

SAB 80C517A/83C517A-5Pin Definitions and Functions (cont’d)Symbol Pin Number I/O *) FunctionPSEN 49 22 O The Program Store Enableoutput is a control signal that enables theexternal program memory to the bus duringexternal fetch operations. It is activatedevery six oscillator periodes except duringexternal data memory accesses. Remainshigh during internal program execution.ALE 50 23 O The Address Latch Enableoutput is used for latching the address intoexternal memory during normal operation.It is activated every six oscillator periodesexcept during an external data memoryaccessEA 51 24 I External Access EnableWhen held at high level, instructions arefetched from the internal ROM (SAB83C517A-5 only) when the PC is less than8000H. When held at low level, the SAB80C517A fetches all instructions from externalprogram memory. For the SAB80C517A this pin must be tied lowP0.0 - P0.7 52 - 59 26 - 27,30 - 35* I = InputO = OutputP-LCC-84P-MQFP-100-2I/O Port 0is an 8-bit open-drain bidirectional I/O port.Port 0 pins that have 1 s written to themfloat, and in that state can be used as highimpe-danceinputs. Port 0 is also themultiplexed low-order address and databus during accesses to external program ordata memory. In this application it usesstrong internal pull-up resistors whenissuing 1 s. Port 0 also out-puts the codebytes during program verification in theSAB 83C517A if ROM-Protection was notenabled. External pull-up resistors arerequired during program verification.Semiconductor Group 12