Chapter 5 External Bus Interface; External Bus Interface - NEC 78K0 User Manual

5.1

External Bus Interface

The external bus interface connects external devices to areas other than the internal ROM, RAM, and SFR areas.
Connection of external devices uses ports 4 to 6. Ports 4 to 6 control address/data, read/write strobe, wait, address
strobe, etc.
The external bus interface is usable only when the X1 clock is selected as the CPU clock.
Caution The external bus interface function cannot be used in (A1) grade products and (A2) grade
products.
Table 5-1. Pin Functions in External Memory Expansion Mode
Name
AD0 to AD7
A8 to A15
RD
WR
WAIT
ASTB
Table 5-2. State of Ports 4 to 6 Pins in External Memory Expansion Mode
External Port
Expansion Mode
Single-chip mode
256-byte expansion mode
4 KB expansion mode
16 KB expansion mode
Full-address mode
Caution When the external wait function is not used, the WAIT pin can be used as a port in all modes.

CHAPTER 5 EXTERNAL BUS INTERFACE

Pin Function When External Device Is Connected
Multiplexed address/data bus
Address bus
Read strobe signal
Write strobe signal
Wait signal
Address strobe signal
Port 4
0 to 7 0 1
Port Port
Address/data Port
Address/data Address
Address/data Address
Address/data Address
User's Manual U15947EJ2V0UD
Function
P40 to P47
P50 to P57
P64
P65
P66
P67
*
Port 5
2 3 4 5 6 7
Port
Port
Alternate Function
Port 6
0 1 2 3 4 5
Port
Port RD, WR, WAIT, ASTB
Port RD, WR, WAIT, ASTB
Port RD, WR, WAIT, ASTB
Port RD, WR, WAIT, ASTB
6 7
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