glue logic psen, read signal

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A standard 22v10 GAL provides general purpose “glue logic” on the board.

In electronics, glue logic is the custom logic circuitry used to interface a number of off-the-shelf integrated circuits.

This is often achieved using ordinary (cheap) 7400- or 4000-series components. In more complex cases, programmable logic devices like a CPLD or FPGA might be used. The falling price of programmable logic devices, combined with their reduced size and power consumption (compared to discrete components), is making them common even for simple systems.

출처 : http://en.wikipedia.org/wiki/Glue_logic

The 8051 has two separate read
signals, RD# (P3.7) and PSEN#. The first is activated when a byte is to be read from
external data memory, the other, from external program memory. Both of these
signals are so-called active low signals. That is, they are cleared to logic level 0 when activated.

출처 : http://gandharmujumdar.blogspot.kr/2010/05/about-8051-architecture-pinout.html

2012.9.28추가

The standard 8051 keeps the two memory

spaces separate by using different bus signals to access them; the read strobe for program memory is PSEN# (Program Store Enable), and the read and write strobes for data memory are RD# and WR#. The 8051 generates PSEN# strobes for instruction fetches and for the MOVC (move  code memory into the accumulator) instruction; it generates RD# and WR# strobes for all datamemory accesses.

출처 : EZ-USB_TRM