W78C438C
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FUNCTIONAL DESCRIPTION
The W78C438C is a functional extension of the W78C58 microcontroller. It contains a 256
×
8 RAM,
64 KB program/1 MB data memory address or memory-mapped chip select logic, two 8-bit address
ports, one 8-bit data port, five general I/O ports, four external interrupts, three timers/counters, and
one serial port.
Dedicated Data and Address Port
The W78C438C provides four general-purpose I/O ports for W78C32 applications; the address and
data bus are separated from Port 0 and Port 2 so that these ports can be used as general-purpose I/O
ports. In this product, DP4 is the data bus for external ROM and RAM, AP5<7:0> are the low byte
address, AP6<7:0> are the high byte address, PSEN enables the external ROM to DP4, and P3.6
(
WR
) and P3.7 (
RD
) are the write/read control signals for the external RAM. The external latch for
multiplexing the low byte address is no longer needed in this product. The W78C438C uses AP5 and
AP6 to support 64 KB external program memory and 64 KB external data memory, just as a standard
W78C32 does.
The W78C438C provides four pins, AP7.3
−
AP7.0 (CS3
−
CS0), to support either 64 KB program/1 MB
data memory space or memory-mapped chip select logic. Bit 7 of the EPMA (Extended Program
Memory Address) register, which is described in Table 1 below, determines the functions of these
pins.
When this bit is "0" (the default value), AP7<3:0> support external program/data memory addresses
up to 64 KB/1 MB for applications which need additional external memory to store large amounts of
data.
Although there is 1M bytes memory space, instructions stored here can not be run at full range of this
area except the first 64 Kbytes. It is owing to the fact that during the instruction fetch cycle, AP7<3:0>
always output 0s to address lines A19
−
A16. This limits the program code to store at address 0
−
0FFFFH (64K). The rest of the area (10000H
−
FFFFFH) can be treated as ROM data storage which
can be read by "MOVC A, @A+DPTR" instruction.
When "MOVC A, @A+DPTR" is executed to read the external ROM data or "MOVX dest, src" is
executed to access the external RAM data, AP7<3:0> output address <19:16> from bits <3:0> of the
EPMA (Extended Program Memory Address) register. At other times, AP7<3:0> always output 0H to
ensure the instruction fetch is within the 64K program memory address. Different banks can be
selected by modifying the content of the EPMA register before the execution of "MOVC A,
@A+DPTR" or "MOVX dest, src."
[Example]. Access the external ROM/RAM data from external memory space.
CLR
A
; Clear Accumulator.
MOV
DPTR, #0H
; Clear DPTR.
MOV
0A2H, #02
; Initialize EPMA(0A2H). EPMA.7 = 0: extended memory space
; EPMA.<3:0> = 0010B, the address range: 20000
−
2FFFFH.
MOVC
A, @A+DPTR ; Read the external ROM data from location 20000H.
MOVX
A, @DPTR
; Read the external RAM data from location 20000H.
CLR
A
MOV
0A2H, #03H
; EPMA.<3:0> = 0011B, the address range: 30000H
−
3FFFFH.
MOVC
A, @A+DPTR ; Read the external ROM data from location 30000H.
MOVX
@DPTR, A
; Write the contents of Accumulator to external RAM data.
; location 30000H.
