
18
EPSON
S1C88409 TECHNICAL MANUAL
CHAPTER 3: CPU AND MEMORY
3.6.2 Address bus
The S1C88409 has a 22-bit external address bus
(A0–A21). The terminals and output circuits of the
address bus A0–A21 are shared with the output
ports R00–R07 (=A0–A7), R10–R17 (=A8–A15) and
R20–R25 (=A16–A21), and the function switches
according to the bus mode setting.
In the single chip mode, the 22-bit terminals are all
set as the output port terminals R00–R07, R10–R17
and R20–R25.
In the expanded 64K mode, 16-bit terminals within
the 22 bits are set as the address bus A0–A15,
while the remaining 6 bits, A16–A21, are set as
output port R20–R25.
In the expanded 4M minimum and maximum
modes, all of the 22-bit terminals are set as the
address bus (A0–A21).
When the address bus is set, the data register and
high impedance control register of each output
port are disconnected from the output circuit and
can be used as general-purpose data registers with
the ability to read/write.
Output
port
Address
bus
R00
R01
R02
R03
R04
R05
R06
R07
R10
R11
R12
R13
R14
R15
R16
R17
R20
R21
R22
R23
R24
R25
A0
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
A11
A12
A13
A14
A15
A16
A17
A18
A19
A20
A21
64K
4M
(max.)
4M
(min.)
64K
Single
chip
Bus mode
Fig. 3.6.2.1 Correspondence between address bus
and output ports
3.6.3 Read (RD)/write (WR) signals
The output terminals and output circuits for the
read (RD)/write (WR) signals are shared with the
output ports R26 and R27, and the function
switches according to the bus mode setting.
In the single chip mode, both the terminals are set
as output port terminals and in other expanded
modes, they are set as read (RD)/write (WR)
signal output terminals. When they are set as read
(RD)/write (WR) signal output terminals, the data
register and high impedance control register for
each output port (R26, R27) are disconnected from
the output circuit and can be used as a general-
purpose data register with the ability to read/
write.
These two signals are output only when the
memory area for the external device is being
accessed. They are not output when the internal
memory is accessed.
Refer to Section 3.6.5, "WAIT control", for the
signal output timing.
Output
port
RD/WR
signal
R26
R27
RD
WR
64K
4M
(max.)
4M
(min.)
Single
chip
Bus mode
Fig. 3.6.3.1 Correspondence between read (RD)/write
(WR) signal and output port
3.6.4 Chip enable (CE) signal
The S1C88409 has a built-in address decoder
which can output up to three chip enable (CE)
signals. Consequently, three devices equipped
with a chip enable (CE) or chip select (CS) terminal
can be directly connected without an external
address decoder.
The three chip enable (CE0–CE2) signal output
terminals and output circuits are shared with
output ports R30–R32. In the expanded modes, the
function, either CE or output port, can indepen-
dently be selected by software according to the
chips to be expanded.
When the chip enable (CE) output is set, the data
register and high impedance control register of the
output port are disconnected from the output
circuit and can be used as general-purpose data
registers with the ability to read/write.
In the single chip mode, they can be used as the
output ports R30–R32.