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Subject: Re: In response to Brian Snell

Date: 12/30/02 at 10:50 PM
Posted by: Brian Snell
E-mail: bsnell@lara.on.ca
Message Posted:

In Reply to: In response to Brian Snell posted by Imdad Saiyed on 12/29/02 at 2:59 PM:

Greetings. When I replied to your query on your clock circuit I had not read down the listing of earlier queries-- one of which was an earlier appeal from you on the identical subject, and to which you had received two replies already. I can only assume that the suggestions contained in both therein did not meet your needs, or, somehow, you were unable to work out how they might best be implemented.

I understand now, from your circuit description, that you have managed to construct a working clock, but lack a simple, practical, method of arranging for a rapid pre-setting of the displayed digit displays in order to re-set the display to some desired reference set-point. You indicate that the "hours" display uses a 74192 counter chip, and I believe that you may not be aware how to work out how to use the "preset" option for pre-loading this portion of the display. You also refer to a 7493 counter chip for the "mins" and "secs" section of the clock -- I assume that you are using 4 such chips in order to employ a "divide by 10, divide by 6" configuration for both these display sections. I note that the chips you have used are traditional TTL 7493 types -- a simple counter, but lacking a lot in the form of useful options such as you now obviously require when re-setting. I am also going to assume that any additional mods which you may now wish to add will be involve further TTL chip types.

The 7493 counters lack any form of pre-setting -- hence any "pre-loading" can be done only by pulsing each counter with enough pulses for each counter to increment to the desired final count. The reply by Russ Kincaid is very practical and sound. Trying to increment a display with a maximum capacity of 12 hours, and at a rate of 1 pulse per second, is like watching a download from Microsoft, or viewing the progress of a "defrag", so the idea of speeding up the pulse-rate is usually employed. I would also recommend that you alter the circuit constants of your 555 clock-generator to 100 Hz. then use two additional 7490 "divide-by-10" (or use one 72390) chip to divide this frequency by 100 to end up with your basic 1 Hz timing interval. If this seems stupid, it also means that you have also divided any timing errors in your 555 circuit by a factor of 100 as well as now providing three separate frequencies (100Hz, 10 Hz, 1 Hz) which are available as a "Fast", "Slow" and "Freeze"options for controlling the rate of pre-loading the display. The switching can be done with 3 SPST push buttons arranged to steer each of these pulse rates via a 74153 Data Multiplexer. The "Freeze" button would freeze the count when depressed, and the count would proceed again at the 1 Hz rate when the button was released. There is some additional logic involved in providing the control for the data selection, but this is relatively simple. At least the maximum cycle-time from "00-00" to "59-59" would take only 36 seconds to go through, and if you go past, then just use the "Freeze" button to "hold" the count until the reference catches up.

The 74192 chip for "Hours" is different, as it requires a BCD pre-loading on the "A,B.C,and D" pre-load lines. The preferred method for this would be a small "BCD-Decimal" screwdriver-slot rotary switch which would provide the correct BCD coding for a "0-9" selection. I don't know whether you are using only one 74192 for the "Hours x 1" with some form of binary control for the "Hours x 10" digit, or whether you have full duplication of the "Hours x 10" with its own chip. I would need to know more of what you may have done in this section before advising further. If I were proposing a construction project of this nature I think I would have included a 0-9 "mini-keypad" by use of which the full "HH MM SS" would be keyed in, and the display would track it.

If I might make an observation -- the use of TTL chips for such an application has limited you to rather inelegant means of achieving easy interfacing. As an educational project, I can but commend you for managing to create, at least, a working system, and I am sure that the experience thus gained will prove a valuable asset in the future. That a complete clock can be purchased new for under $10.00 retail is not the way to defend work and effort which hopefully has given you great insight and satisfaction in its genesis.

Please advise if you require further details of circuitry and logic to accomplish the ideas set out above.


Brian Snell.

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