Internal Register Of 81 C55 - Tandy 102 Service Manual

Function
81 C55 has 3 functions:
• 2K bit, static RAM (256 words x 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
following.
8-bit Internal Data Bus
Com-
PC PB PA
Timer Timer
mand t MSB LSB
Status Timer Mode
6 bits 8 bits
8 bits
Figure C-7. Internal Register of 81 C55
I/O Address
Selecting Register
A7 A6 A5 A4 A3 A2
Al AO
X X X X X 0 0 0 Internal command/status register
X X X X X 0 0 1 Universal I/O port A (PA)
X
X
X X X 0 1 0 Universal I/O port B (PB)
X X X X X 0
1 1 1/O port C (PC)
X X X X X 1 0 0 Timer count lower position 8 bits (LSB)
X X X X X 1
0
1 Timer count upper position 6 bits and timer mode 2 bits (MSB)
X : Don't care.
Table C-4. I/O Address of 81 C55