UPD784224 Datasheet, PDF (51/92 Pages) NEC – 16/8-BIT SINGLE-CHIP MICROCONTROLLERS

µPD784224, 784225, 784224Y, 784225Y
9. LOCAL BUS INTERFACE
The local bus interface can connect an external memory or I/O (memory mapped I/O) and support a memory
space of 1 Mbyte (refer to Figure 9-1).
Figure 9-1. Example of Local Bus Interface (Multiplexed bus)
µPD784225
RD
WR
A8 to A19
ASTB
AD0 to AD7
VDD1
Address latch
LE
Q0 to Q7
D0 to D7
OE
Address bus
SRAM
CS
OE
WE
I/O1 to I/O8
A0 to A19
Data bus
9.1 Memory Expansion
External program and data memory can be connected in two stages: 256 Kbytes and 1 Mbytes.
To connect the external memory, ports 4 to 6 are used.
The external memory is connected by using a time-division address/data bus. The number of ports used when
the external memory is connected can be reduced in this mode.
9.2 Programmable Wait
Wait state(s) can be inserted to the memory space (00000H to FFFFFH) while the RD and WR signals are active.
In addition, there is an address wait function that extends the active period of the ASTB signal to gain the address
decode time.
9.3 External Access Status Function
An active low external access status signal is output from the P37/EXA pin. This signal notifies other devices
connected to the external bus of the external access status, to disable data output to the external bus from other
devices, or enables reception.
The external access status signal is output during external access.
Data Sheet U12376EJ1V0DS00
51