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1997-04-16
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111 lines
Elektor Electronics RTC
-----------------------
Reviewed by Dave Price
----------------------
As a keen electronics DIYer I was extremely pleased to discover that
the monthly electronics magazine, Elektor Electronics, has published a few
useful projects for the Atari ST one of which is a Real Time Clock (RTC)
that connects to the ST printer port but still allows the printer to
work.
I should point out that this project could end up costing more than a
ready made unit such as the Frontier cartridge particularly if you don't
already have some of the components in your goody box. It would also be a
lot cheaper if you are able to make your own PCB and wrote your own
software.
One other thing to consider is that a small modification needs to be
made inside the ST which involves soldering two wires between the sound
chip (YM 2149) and the centronics printer port.
One wire is connected to pin 40 of the sound chip to provide a +5V
supply to the RTC via pin 16 of the printer port. This is perfectly safe
as pin 16 is the centronics RESET(low) line which is not used by the ST
and since the RESET signal is active low placing +5V on the line has no
effect on the printer(The ST is not capable of issuing a reset via this
line anyway).
The other wire is connected to an unused IN/OUT line found at pin 14
on the sound chip and fed to pin 17 of the printer port. This line is used
to direct data to the RTC or printer and is not used by either the ST or
printer but is supported by XBIOS thus enabling full control of data flow
by software.
The article, consisting of three A4 pages, was published in the June
1991 issue of Elektor Electronics and included a components list, circuit
diagrams, PCB track layout and other diagrams. The text explains how the
circuit works, construction details and a couple of short paragraphs on
the control software, I would like to have seen a bit more on the
software. The article was fairly easy to follow although there was a small
error in the construction details concerning the modification to the ST.
Pin 14 of the sound chip should be connected to pin 17 of the printer port
and NOT to pin 16 as stated in the text. Also pin 40 of the sound chip
should be connected to pin 16 of the printer port and NOT to pin 17 as
stated in the text. This is shown correctly in the circuit diagram but it
had me fooled at first.
The RTC is based around the Motorola MC146818 Real Time Clock chip and
has a PCB mounted rechargable battery to keep the clock going when the ST
is turned off. The battery is automatically charged when ever the ST is
powered up.
All the ICs used in the project are sensitive to static electricity
therefore normal E.S.D. precautions as described in previous articles
should be observed.
Most of the components are readily available from suppliers such as
Maplin Electronics etc. Maplin did not have the RTC chip but is available
from Viewcom who are Digital Integrated Circuit specialists.
The PCB supplied by Elektor is screen printed to allow easy component
location and at a cost of £6.17 is reasonable when one considers the high
quality of the board.
The software supplied on disk by Elektor is, I feel, a bit expensive
at £7.64 but does include the full C source code which enables you to
customise the program. I reckon it should have been printed as a listing
within the main article but maybe it would have taken up too many pages in
the magazine. The supplied program is a desk accessory of about 7K in
length. It automatically reads in the time/date data from the RTC and sets
the system clocks. When opening the accessory from the desk top you are
presented with a window containing the date and time which you are able to
adjust if necessary. On pressing return or clicking the SET button both
the RTC and system clocks are corrected accordingly.
I plan to write my own Auto run program that will read the RTC and
adjust the system clocks then erase it self from memory thus not using up
memory or taking up an accessory slot.
To summarize, I found great pleasure in making something useful for the
ST and have gained a little more knowledge about the inner workings of my
computer into the bargain. Anyone with a little practical experience of
soldering components to a PCB should be able to tackle this project with
out any real difficulties.
I have had the RTC up and running for about two months or so and have
not had any problems with my system at all. There may be problems with the
supplied desk accessory not getting on with other accessories but I
haven't come across any yet.
If you decide to build the RTC and come across any problems please get
in touch and I will try my best to help out.
*******************************************************
* Real Time Clock By F.Dossche *
* Published in June 1991 Issue of Elektor Electronics *
* *
* The following is available from Elektor *
* Article reprints @ £1.50 inc p&p per article *
* RTC Printed Circuit Board No. 910006 @ £6.17 *
* RTC Software on 3.5 Disk No. 1621 @ £7.74 *
* *
* Elektor Electronics (Publishing) *
* Down House, Broomhill Road, London SW18 4JQ *
* *
* VIEWCOM ELECTRONICS *
* 77 Upperton Road, West Plaistow, London E13 9LT. *
*******************************************************