NEC PD78078 用户手册
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CHAPTER 23 EXTERNAL DEVICE EXPANSION FUNCTION
23.1 External Device Expansion Functions
The external device expansion functions connect external devices to areas other than the internal ROM, RAM,
and SFR.
The external device expansion function can be used in the following two modes:
• Multiplexed bus mode
• Separate bus mode
(1) Multiplexed bus mode
External devices are connected via the multiplexed address/data bus. Connecting external devices using
this mode may reduce the number of ports to be used. In this mode, ports 4 through 6 are used for control
of address/data, reafd/write strobe, wait, address strobe as shown below.
Table 23-1. Pin Functions in External Memory Expansion Mode
Pin function at external device connection
Alternate function
Name
Function
AD0 to AD7
Multiplexed address/data bus
P40 to P47
A8 to A15
Upper Address bus
P50 to P57
RD
Read strobe signal
P64
WR
Write strobe signal
P65
WAIT
Wait signal
P66
ASTB
Address strobe signal
P67
Table 23-2. State of Port 4 to Port 6 Pins in External Memory Expansion Mode
Ports and bits
Port 4
Port 5
Port 6
Modes
0 to 7
0 1 2 3 4 5 6 7
0 to 3
4 to 7
Single-chip mode
Port
Port
Port
Port
256-byte expansion mode
Address/data
Port
Port
RD, WR, WAIT, ASTB
4-Kbyte expansion mode
Address/data
Address
Port
Port
RD, WR, WAIT, ASTB
16-Kbyte expansion mode
Address/data
Address
Port
Port
RD, WR, WAIT, ASTB
Full address mode
Address/data
Address
Port
RD, WR, WAIT, ASTB
Caution When the external wait function is not used, the WAIT pin can be used as a port in all modes.