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Programming Information
I
SC11
sca
I
a
I
a
I
x
I
x
I
x
450·19A
I '
,
L "
L
Don"t Care
Selects Counter Latching
Operation
Specifies Counter to be Latched
Figure 3-10. PIT Counter Register
Latch Control Word Format
where N is the count value,
B is the desired Baud rate, and
C is 1.23 MHz, the input clock frequency.
Thus, for a 4800 Baud rate, the required count value
(N)
is:
N
1.23
X
10
6
=
256
4800
----.:
If the binary equivalent of count value N
=
256 is loaded
into Counter 2, then the output frequency is 4800 Hz,
which is the desired clock rate for synchronous mode
operation.
3-24. ASYNCHRONOUS MODE. In the asynchronous
mode, the TXC and/or RXC rates equal the Baud rate
times one of the following multipliers: Xl, X16, orX64.
Therefore, the count value is determined by:
N
=
C/BM
where N is the count value,
B is the desired Baud rate,
M is the Baud rate multiplier (1, 16, or 64), and
C is 1.23 MHz, the input clock frequency.
Thus, for a 4800 Baud rate, the required count value (N) is
1.23
X
10
6
N
=
4800 x 16
=
16.
If the binary equivalent of count value N
=
16 is loaded
into Counter 2, then the output frequency is 4800 x 16
Hz, which is the desired clock rate for asynchronous mode
operation. Count values (N) versus rate multiplier (M) for
each Baud rate are listed in table 3 -11.
3-12
iSBC 80/30
NOTE
During initialization, be sure to load the count
value (N) into the appropriate counter and the
Baud rate multiplier (M) into the 8251 A
USART.
Table 3-11. PIT Count Value Vs Rate Multiplier for
Each Baud Rate
Baud Rate
*Count Value (N) For
(8)
M = 1
M=16
M=64
75
16384
1024
256
110
11171
698
175
150
8192
512
128
300
4096
256
64
600
2048
128
32
1200
1024
64
16
2400
512
32
8
4800
256
16
4
9600
128
8
2
19200
64
4
38400
32
2
76800
16
*Count Values (N) assume clock is 1.23 MHz.
Double Count Values (N) for 2.46 MHz clock.
Count Values (N) and Rate Multipliers (M) are in
decimal.
3-25. RATE GENERATOR/INTERVAL
tIMER.
Table 3-12 shows the maximum and minimum rate
generator frequencies. and timer intervals for Counters 0
and 1 when these counters, respectively, have 1.23-MHz
and 153.6-KHz clock inputs. The table also provides the
maximum and minimum generator frequencies and time
intervals and may be obtained by connecting Counters 0
and 1 in series.
3-26. INTERRUPT TIMER.
To program an interval
timer for an interruption terminal count, program the
appropriate timer for the correct operating mode (Mode 0)
in the control word. Then load the count value
(N),
which
is derived by
where
N
=
TC
N is the count value for Counter 0,
T is the desired interrupt time interval in seconds,
and
C is the internal clock frequency (Hz).
Table 3-13 shows the count value (N) required for several
time intervals (T) that can be generated for Counters 0
and 1.
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