User manual

21 PCI-1750 User Manual
Appendix A Function of 8254 Counter Chip
M2, M1 & M0 Select Operating Mode
BCD Select Binary or BCD Counting
If you set the module for binary counting, the count can be any number from 0 up to
65535. If you set it for BCD (Binary Coded Decimal) counting, the count can be any
number from 0 to 9999.
If you set both SC1 and SC0 bits to 1, the counter control register is in read-back
command mode. The control register data format then becomes:
CNT = 0 Latch count of selected counter(s).
STA = 0 Latch status of selected counter(s).
C2, C1 & C0 Select counter for a read-back operation.
C2 = 1 select Counter 2
C1 = 1 select Counter 1
C0 = 1 select Counter 0
If you set both SC1 and SC0 to 1 and STA to 0, the register selected by C2 to C0
contains a byte which shows the status of the counter. The data format of the counter
read/write register then becomes:
M2 M1 M0 Mode
0 0 0 0 Programmable one shot
0 0 1 1 Programmable one shot
X 1 0 2 Rate generator
X 1 1 3 Square wave generator
1 0 0 4 Software triggered strobe
1 0 1 5 Hardware triggered strobe
BCD Type
0 Binary counting 16-bits
1 Binary coded decimal (BCD) counting
BASE + 27(Dec) 8254 control, read-back mode
Bit D7D6D5D4D3D2D1D0
Value 1 1 CNT STA C2 C1 C0 X
BASE+24/25/26(Dec) Status read-back mode
BitD7D6D5D4D3D2D1D0
Value OUT NC RW1 RW0 M2 M1 M0 BCD
OUT Current state of counter output
NC Null count is 1 when the last count written to the counter register has been
loaded into the counting element