Radio Shack 26-1145 - Page 19

Summary
of BRG Programming
Radio Shack
software
supports the baud
rates which have
a yes
in
the Switch
Selectable
column
of Table 5. You
can program
non-standard
baud
rates
by loading the BRG
with
the appropriate
constants.
An input
operation from
port
E9H
loads a register in the
CPU
with the bit
pattern
programmed
by the
Configuration Sense
Switches. As we
noted before, the
Sense Switch Block
in Figure 4
consists of eight
single-pole
single-throw
switches. When
a switch
is open, it pulls
up (+5V)
the
input of an octal
tri-state buffer;
when
a
switch
is closed,
it
pulls
an input low.
NIBBLE TRANSMIT OR
SWITCH
LOADED RECEIVE BAUD RATE lOx CLOCK FREQ.
%
ERROR
SELECTABLE
?
OH
50
0.8
kHz No
1H 75 1.2 kHz
No
2H
110 1.76
kHz
Yes
3H
134.5 2.1523 kHz 0.016
No
4H
150 2.4
kHz Yes
5H
130 4.8 kHz
Yes
6H 600
9.6
kHz
Yes
7H
1200 19.2
kHz Yes
8H
1800 28.8
kHz
No
9H
2000 32.081 kHz 0.253
No
AH
2400 38.4
kHz Yes
BH
3600
57.6
kHz
No.
CH
4800 76.8
kHz
Yes
DH 7200 115.2 kHz No
EH 9600
113.6
kHz Yes
FH
19,200
316.8
kHz
3.125
No
TABLE
5. SUMMARY OF BRG PROGRAMMING
The
decode
logic, in
response to
an input
operation
from port E9H,
provides a
low-going
strobe which
is connected
to the enable
input
of the
tri-state
buffer. This strobe, when
low,
allows the buffer
to
present
the
logic
state of its
inputs to the
data
bus for loading
into the CPU. This
operation
is
performed
by executing the
following
instructions:
IN A, (0E9H)
;
LOAD SWITCH
SELECTIONS
After execution
of this
instruction,
the A register
on the CPU will
contain the bit
pattern which
corresponds
to the switch
settings.
This information
must
now be interpreted
by the software and the
proper
sequence of commands
issued
by
the
CPU
to configure
the
Interface.
16