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iSBC 80/30
Programming Information
Table 3-12. PIT Rate Generator Frequencies and Timer Intervals
Single Timer
1
Single Timer
2
Dual Timer
3
(Counter 0)
(Counter 1)
(0 and 1 in Series)
Minimum
Maximum
Minimum
Maximum
Minimum
Maximum
Rate Generator
18.75 Hz
614.4 kHz
2.344 Hz
76.8 kHz
0.00029 Hz
307.2 kHz
(Frequency)
Real-Time
Interrupt
1.63 p,sec
53.3 msec
13 p,sec
(Interval)
NOTES:
1. Assuming a 1.23-MHz clock input
2. Assuming a 153.6-kHz clock input.
3. Assuming Counter 0 has 1.23-MHz clock input.
3-27. 8255A PPI PROGRAMMING
The three parallel I/O ports interfaced to connector J 1 are
~ontrolled
by an Intel S255A Programmable Peripheral
Interface. Port A includes bidirectional data buffers and
Ports Band e include Ie sockets for installation of either
input terminators or output drivers depending on the
user's application.
Default jumpers set the Port A bidirectional data buffers to
the input mode. Optional jumpers allow the bidirectional
data buffers to be set to the output mode or to allow any
one of the eight Port e bits to selectively set the Port A
bidirectional data. buffers to the input or output mode.
Table 2-11 lists the various operating modes for the three
PPI parallel I/O ports. Note that Port A (ES) can be
operated in Modes 0, 1, or 2; Port B (E9) and Port e (EA)
can be operated in Mode
°
or 1.
3-28. CONTROL WORD FORMAT
The control word format shown in figure 3 -11 is used to
initialize the PPI to define the operating mode of the three
ports. Note that the ports are separated into two groups.
Group A (control word bits 3 through 6) defines the
Table 3-13. PIT Timer Intervals Vs Timer Counts
T
N*
10 p,sec
12
100 p,sec
123
1 msec
1229
10 msec
12288
50 msec
61440
*Count Values
(N)
assume clock is 1.23 MHz.
Double Count Values
(N)
for 2.46 MHz
clock. Count Values
(N)
are in decimal.
426.67 msec
3.26 p,sec
58.25 minutes
operating mode for Port A (ES) and the upper four bits of
Port e (EA). Group B (control word bits
°
through 2)
defines the operating mode for Port B (E9) and the lower
four bits of Port e (EA). Bit 7 of the control word controls
the mode set flag.
CONTROL WORD
~
0 7 \ 0 6
Os \ 0
4
\
0 3 \ O 2 \ 0 1 \ Do
I
LJ
/
GROUP B
\
PORT C (LOWER)
'----
1 = INPUT
0= OUTPUT
PORT B
l ' INPUT
0= OUTPUT
MODE SELECTION
0= MODE 0
1 = MODE 1
/
GROUP A
\
PORT C (UPPER)
1 = INPUT
0= OUTPUT
PORTA
1 = INPUT
0= OUTPUT
MODE SELECTION
00 = MODE 0
01 = MODE 1
1X = MODE 2
MODE SET FLAG
1 = ACTIVE
Figure 3-11. PPI Control Word Format
3-13
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