Tandy 1000 Technical Reference Manual

Tandy 1000 Technical Reference Manual
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825318253·5
8253 READIWRITE PROCEDURE
Figure 8. Programming Format
Count Register byte
Counter n
MODE Control Word
Counter n
~
-
Al AO
MODE Control WOrd
1
1
Counter 0
MODE Control Word
1
1
Counter 1
MODE Control Word
1 1
Counter 2
f-------.---------
t-~
._.-
LSB
Count Reglstel Byte
0
1
Counter 1
Count Register Byte
fT~
MSB
Counter 1
LSB
Count Register Byte
~;
Counter 2
MSB C"O"' R",,,,, ""'1 '
0
Counter 2
---
r-
LSB Count Register Byte 0
0
Counter 0
----------- ---
c--
MSB Count Register Byte 0
0
Counter 0
L_
. ---
LSB
MSB
Count Register byte
Counter n
'-------------
Note: Format shown is a SImple example of loading the 8253 and
does not imply that It is the only format that can be Used.
NO,7
No.9
No.5
No,
No.1
No,
No,6
No.4
No.2
The systems software must program each counter of the
8253 with the mode and quantity desired. The program-
mer must write out to the 8253 a MODE control word and
the programmed number of count register bytes (lor 2)
prior to actually using the selected counter
The actual order of the programming is quite flexible
Writing out of the MODE control word can be in any
sequence of counter selection, e.g., counter #0 does not
have to be first or counter #2 last Each counter's MODE
control word register has a separate address so that Its
loading IS completely sequence independent (SCO, SC 1)
The loading of the Count Register with the actual count
value, however, must be done In exactly the sequence
programmed in the MODE control word (RLO, RL 1) ThiS
loading of the counter's count register IS still sequence
independent like the MODE control word loading, but
when a selected count register is to be loaded It must be
loaded with the number of bytes programmed~the
MODE control word (RLO, RL 1) The one or two bytes to
be loaded in the count register do not have to follow the
associated MODE control word. They can be programmed
at any time following the MODE control word loading as
long as the correct number of bytes is loaded in order
All counters are down counters. Thus, the value loaded
into the count register will actually be decremented
Loading all zeroes into a count register will result in the
maximum count (2" for Binary or 10' for BCD) In MODE °
the new count will not restart until the load has been
completed. It will accept one of two bytes depending on
how the MODE control words (RLO, RL 1) are program-
med. Then proceed with the restart operation
Write Operatlona
Note: The exclusive addresses of each counter's count register make
the task of PJogramming the 8253 a very simple matler, dnd
maximum effective use of the device will result If this fe;Hure
is fully ulil,zen
Figure 9. Alternate Programming Formats
6-145 AFN-00745C

Tandy 1000 Specifications

General

Processor	Intel 8088
Processor Speed	4.77 MHz
RAM	128 KB (expandable to 640 KB)
Operating System	MS-DOS 2.11
Manufacturer	Tandy Corporation
Release Year	1984
Storage	Single or double 5.25" floppy disk drive (360 KB), optional hard drive
Sound	Tandy 3-voice sound
Ports	Parallel, Serial
Graphics	CGA (Color Graphics Adapter)
Graphics Modes	320x200, 640x200