Tandy 1000 Technical Reference Manual

Tandy 1000 Technical Reference Manual
inter
8253/8253·5
M - MODE:
OPERATIONAL DESCRIPTION
General
The complete functional definition of the 8253 IS
programmed by the systems software. A set of control
words must be sent out by the CPU to mltlalize each
counte~he 8253 with the desired MODE and quantity
information. Prior to initialization, the MODE, count, and
output of all counters is undefined. These control words
program the MODE, Loading sequence and selection of
binary or BCD counting.
Once programmed, the 8253 IS ready to perform whatever
tlmmg tasks it is assigned to accomplish
The actual counting operation of each counter IS
completely mdependent and additional logic IS provided
on-chip so that the usual problems associated with
efficient monitoring and management of external.
asynchronous events or rates to the microcomputer
system have been ellmmated
M2
M1
MO
0
D Mode D
Mode 1
X
Mode 2
X
Mode 3
Mode 4
Mode
5
BCD:
Binary Counter 16-blts
Binary Coded Decimal (BCD) Counter
(4 Decades)
Select Counter 1
Select Counter 2
Illegal
MODE Definition
MODE 0: Interrupt on Terminal Count. The output will
be initially low after the mode set operation. After the
count is loaded into the selected count register. the out-
put will remain low and the counter will count. When ter·
minal count is reached the output will go h:gh and re-
main high until the selected count register is reloaded
with the mode or a new count is loaded. ThEe counter
continues to decrement after terminal count has been
reached
Rewriting a counter register dUring counting results in
the following'
(1) Write 1st byte stops the current COunting
(2) Write 2nd byte starts the new count
Counter Loading
The count regis fer is not loaded until the count value is
written (one or two bytes. depending on the mode
selected by the RL bits), followed by a rising edge and a
falling edge of the clock. Any read of the counter prior to
that falling clock edge may yield invalid data
DO
BCD
0
1
MD
Definition of Control
SC - Select Counter.
Programming the 8253
All of the MODES for each counter are programmed by the
systems software by simple I/O operations
Each counter of the 8253 IS Individually programmed by
writing a control word Into the Control Word Register
(AD At 011)
Control Word Format
I SC1 I SCD I R 11 I R LD I M2 I M1
RL - Read/Load:
Rl1 RLO
Counter Latching
operation (see
R EADIWR ITE Procedure Section)
----+----+-- ---\
1 Read/Load most significant byte only
o 1 Read/Load least significant byte only.
---_.- -- I-~------~~-----
1 1 Read/Load least significant byte first,
then most significant byte.
MODE 1: Programmable One·Shot. The output will go
Iowan the count following the rising edge of t~,J gate In·
put
The output Will go high on the terminal count. If a new
count value is loaded while the output is low II will not
affect the duration of the one·shot pulse until the suc·
ceeding trigger. The current count can be read at any
time without affecling the one·shot pulse
The one·shot is retriggerable. hence :he output Will reo
main low for the full count after any rising edge of the
gate input.
6-142

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