Tandy 1000 Technical Reference Manual

Tandy 1000
inter
R.ad Operations
Technical Reference Manual
825318253·5
R.ad Operation Chart
In most counter applications It becomes necessary to read
the value of the count In progress and make a
computational deCISion based on thiS quantity. Event
counters are probably the most common applicatIOn that
uses this function The 8253 contains logic that will allow
the programmer to easily read the contents of any of the
three counters Without disturbing the actual count In
progress
There are two methods that the programmer can use to
read the value of the counters The fllsl method Involves
the use of simple I/O read operatIOns 01 the selected
counter By controlling the AO. A1 Inpuls to the 8253 the
programmer can select the counter to be read (remember
that no read operation of the mode register IS allowed AO
Al-11) The only requllemenl wllh Ih,s method IS thai In
order to assure a Slable counl reading the actual operation
of the selected counter must ~ Inhlblled either by
coniroiling the Gate Input or by externallogle that Intllblts
the clock Input The contents of the counter selected will
be available as tallows
fllst 110 Read contains the least Significant byle (LSB)
second 110 Read contains the most Significant byte
(MSB)
Due to the Internal logiC of the 8253 II IS absolutely
necessary to complete the entlle reading procedure If two
bytes are programmed to be read then two bytes must be
read before any loading WR command can be se;U;-lhe
same counter
3MHz
elK t------l
8085
Al
AD
RD
0
0 0 Read Counter No. 0
a 1
a Read Counter No.1
1
a
a
Read Counter No. 2
1
1 0
Illegal
Reading While Counting
In order for the programmer to read the contents of any
counter Without etfectlng or disturbing the counting
operation the 8253 has special Internal logiC that can be
accessed uSing Simple WR commands to the MODE
register BaSically when the programmer Wishes to read
the contents of a selected counter "on the fly' he loads the
MODE register With a special code which latches the
present count value Into a storage register so that ItS
contents contain an accurate, stable quantlly. The
programmer Ihen Issues a normal read command to the
selected counter and t he contents of the latched register IS
available
MODE Register for Latching Count
AO,A1 = 11
SC 1 sca 0 a x x x x
SC1,SCO~ speCify counter to be latched
05,04 - 00 deSignates counter latching operation
don't care
The same limitation applies to this mode of reading the
counter as the previous method. That is, it is mandatory
to complete the entire read operation as programmed.
This command has no effect on the counter's mode,
, 1,5MHz
8253·5
-It an 8085 crOCk output is to drive an 8253·5 clock input, It must be reduced to 2 MHz or less
Figure 10. MCS-8S·· Clock Interface'
6-146
AFN..()C)14SC

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