National Instruments PC-DIO-96 Manual Do Utilizador
Register-Level Programming
Chapter 4
PC-DIO-96 User Manual
4-4
© National Instruments Corporation
Warning:
During programming, note that each time a port is configured, output ports A
and C are reset to 0, and output port B is undefined.
and C are reset to 0, and output port B is undefined.
Table 4-2 shows the control words for setting or resetting each bit in port C. Notice that bit 7 of
the control word is cleared when programming the set/reset option for the bits of port C.
the control word is cleared when programming the set/reset option for the bits of port C.
Table 4-2. Port C Set/Reset Control Words
Bit
Bit Set
Bit Reset
The Bit Set or
Number
Control Word
Control Word
Reset in Port C
0
0xxx0001
0xxx0000
xxxxxxxb
1
0xxx0011
0xxx0010
xxxxxxbx
2
0xxx0101
0xxx0100
xxxxxbxx
3
0xxx0111
0xxx0110
xxxxbxxx
4
0xxx1001
0xxx1000
xxxbxxxx
5
0xxx1011
0xxx1010
xxbxxxxx
6
0xxx1101
0xxx1100
xbxxxxxx
7
0xxx1111
0xxx1110
bxxxxxxx
Register Description for the 8253
Figure 4-2 shows the control word format used to completely program the 8253. Bits 7 and 6 of
the control word select the counter to be programmed. Bits 5 and 4 select the mode by which the
count data is written to and read from the selected counter. Bits 3, 2, and 1 select the mode for
the selected counter. Bit 0 selects whether the counter counts in binary or BCD format.
the control word select the counter to be programmed. Bits 5 and 4 select the mode by which the
count data is written to and read from the selected counter. Bits 3, 2, and 1 select the mode for
the selected counter. Bit 0 selects whether the counter counts in binary or BCD format.
BCD
D2
D1
D0
D3
D7
D6
D5
D4
1 = count in BCD
0 = count in binary
0 = count in binary
Mode Select
000 = mode 0
001 = mode 1
010 = mode 2
011 = mode 3
100 = mode 4
101 = mode 5
110 = mode 2
111 = mode 3
000 = mode 0
001 = mode 1
010 = mode 2
011 = mode 3
100 = mode 4
101 = mode 5
110 = mode 2
111 = mode 3
Access Mode
00 = latch counter value
01 = access LSB only
10 = access MSB only
11 = access LSB, then MSB
01 = access LSB only
10 = access MSB only
11 = access LSB, then MSB
Counter Select
00 = counter 0
01 = counter 1
10 = counter 2
11 = illegal
00 = counter 0
01 = counter 1
10 = counter 2
11 = illegal
Figure 4-2. Control-Word Format for the 8253