C8254 Datasheet, PDF (3/6 Pages) List of Unclassifed Manufacturers – PROGRAMMABLE TIMER /COUNTER MEGAFUNCTION

CAST C8254 Megafunction Datasheet
Functional Description
This section provides a short description of each element of the Block Diagram (Figure 1).
Data Bus Buffer
8-bit input and output data bus buffer is used to interface the C8254 to the system bus.
Read/Write Logic
The Read/Write Logic accepts inputs from the system bus and generates control signals for the other
functional blocks of the C8254. ADDR[1:0}select one of the three counters or the control word register to be
read from or written into. A “low” on RDN tells the C8254 that the CPU is reading one of the counters. A “low”
on the WRN input tells the C8254 that the CPU is writing either a Control Word or an initial count. Both RDN
and WRN are qualified by CSN. RDN and WRN are ignored unless the C8254 has been selected by holding CSN
“low”.
Control Word Register
The Control Word Register is selected by the Read/Write Logic when ADDR[1:0] = 11. During writing, the
data is stored in the Control Word Register and it is interpreted as a Control Word used to define the
operation of the Counters.
Control Word Format
D7 D6 D5 D4 D3 D2 D1 D0
SC1 SC0 RW1 RW0 M2 M1 M0 BCD
SC1 SC0
0 0 Select Counter 0
0 1 Select Counter 1
1 0 Select Counter 2
1 1 Read-Back Command
RW1
0
0
1
1
RW0
0
1
0
1
Counter Latch Command
Read/Write LSB only
Read/Write MSB only
Read/Write LSB first, then MSB
M2 M1 M0
0 0 0 Mode 0
0 0 1 Mode 1
X 1 0 Mode 2
X 1 1 Mode 3
1 0 0 Mode 4
1 0 1 Mode 5
BCD
0 Binary Counter 16-bits
1 BCD Counter (4 Decades)
CAST, Inc.
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