NEC 22.3 External Device Expansion Function Timing

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CHAPTER 22 EXTERNAL DEVICE EXPANSION FUNCTION
22.3 External Device Expansion Function Timing
Timing control signal output pins in the external memory expansion mode are as follows.
(1) RD pin (Alternate function: P64)
Read strobe signal output pin. The read strobe signal is output in data accesses and instruction fetches from
external memory.
During internal memory access, the read strobe signal is not output (maintains high level).
(2) WR pin (Alternate function: P65)
Write strobe signal output pin. The write strobe signal is output in data access to external memory.
During internal memory access, the write strobe signal is not output (maintains high level).
(3) WAIT pin (Alternate function: P66)
External wait signal input pin.
When the external wait is not used, the WAIT pin can be used as an input/output port.
During internal memory access, the external wait signal is ignored.
(4) ASTB pin (Alternate function: P67)
Address strobe signal output pin. Timing signal is output without regard to the data accesses and instruction
fetches from external memory.
The ASTB signal is also output when the internal memory is accessed.
(5) AD0 to AD7, A8 to A15 pins (Alternate function : P40 to P47, P50 to P57)
Address/data signal output pin. Valid signal is output or input during data accesses and instruction fetches
from external memory.
These signals change when the internal memory is accessed (output values are undefined).
Timing charts are shown in Figure 22-4 to 22-7.