¡ Semiconductor
MSM81C55-5RS/GS/JS
OPERATION
Description
The MSM81C55-5 has three functions as described below.
• 2K-bit static RAM (256 words ¥ 8 bits)
• Two 8-bit I/O ports (PA and PB) and a 6-bit I/O port (PC)
• 14-bit timer counter
The internal register is shown in the figure below, and the I/O addresses are described in the
table below.
8 Bit Internal Data Bus
Command
Status
PC
PB
PA
Timer Timer
MSB LSB
Timer Mode
6 Bit
8 Bit
8 Bit
I/O Address
A7 A6 A5 A4 A3 A2 A1 A0
¥¥¥¥¥000
¥¥¥¥¥001
¥¥¥¥¥010
¥¥¥¥¥011
¥¥¥¥¥100
¥¥¥¥¥101
¥: Don't care.
Selecting Register
Internal command/status register
Universal I/O port A (PA)
Universal I/O port B (PB)
I/O port C (PC)
Timer count lower position 8 bits (LSB)
Timer count upper position 6 bits and timer mode
2 bits (MSB)
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