The 8253 Timer

1
The 8253 Timer
D7 - D0
CLK0
GATE0
OUT0
RD
WR
CLK1
GATE1
CS
OUT1
CLK2
A1
GATE2
A0
OUT2
2
The 8253 Timer
Data is communicated 8 bits at a time 2 address
lines 4 internal registers 00 Counter 0 01
Counter 1 10 Counter 2 11 Control Word
D7 - D0
CLK0
GATE0
OUT0
RD
WR
CLK1
GATE1
CS
OUT1
CLK2
A1
GATE2
A0
OUT2
3
8253 counters

• Each counter is a 16-bit counter
• There are 8 data lines
• There is only one physical address per counter
• Therefore to write/read 16 bits into a counter
  requires a low-byte/high-byte write or read to
  the same address

4
8253 Control Word
5
Homework for Tuesday, Nov. 10

• Write an 8088 procedure that will do the
  following
• Configure counter 0 in binary Mode 0 with an
  initial value of 0ABCDh
• Configure counter 1 in decimal Mode 1 with an
  initial value of 9801
• Assume that Counter 0 has been configured as
  noted above. Write an 8088 procedure that will
  read the current status of counter 0 and return
  with AX holding the number of negative edges that
  have occurred on clock 0.