A digital design question
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From: Gerhard on 14 Jan 2010 03:15 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:klsrk55bq84affa8n0frskn3hmucvkttnr(a)4ax.com... > On Wed, 13 Jan 2010 07:08:27 -0500, Bitrex > <bitrex(a)de.lete.earthlink.net> wrote: > >>I'm playing around with some 7400 series logic, trying to teach myself >>the basics of digital design. I've been able to come up with a working >>circuit for a 24 hour digital clock easily, but I'm struggling trying to >>think of a way to make a 12 hour clock. Specifically my problem is the >>reset of the hours counter - when the tens digit of the hours section is >> 0 I need the units counter to count from 1 to 9 and roll over to 0 >>(for example 09 pm to 10 pm) but when the tens digit of the hours >>counter is a 1 I need the units counter to start counting at 1 (for >>example 12 pm - 1 am). I'm using 74LS93 series plain binary counters >>and assorted logic, and after working on it for a long while I can't >>think of a way to get it to work without using something like a >>presetable counter. Anyone have any ideas? > > One caution: it's tempting, when using discrete logic, to build > klugey, asynchronous hairballs. That tends to be most efficient when > wiring up 7400-series logic, but it doesn't scale. Do that some, but > note that most "modern" logic is done in CPLDs and FPGAs and ASICs > with zillions of fast gates and flops available, and is usually fully > synchronous. I'd suggest you follow up this project with an identical > function, but implemented fully synchronously in a small FPGA. Dev > kits aren't expensive. > > There is a place for asynchronous hairball logic, so it's good to be > comfortable with both. > > John I would second the above ideas and suggest that you also try to do a clock that employ synchronous counters with a pre-load function such as 74LS160, 74LS161 ....(there are counters with pre-load). Please do not stop to live your dream of designing and building a TTL clock. I would suggest that you also try to design the clock using one of the inexpensive CPLD or FPGA kits. Having attained the skill to do that would serve you well in future. The Xilinx SK-CRIIL-G Coolrunnner Development Kit includes a 2X16 LCD display at a very reasonable cost. +-$50 See http://www.xilinx.com/products/boards/CR-II-SK/CR2-SK-SS.pdf http://dkc3.digikey.com/PDF/T091/p0587.pdf 3rd in list of develoment kits. Gerhard van den Berg Meraka CSIR
From: Sylvia Else on 14 Jan 2010 03:24 Bob Eld wrote: > "Sylvia Else" <sylvia(a)not.at.this.address> wrote in message > news:01970b1e$0$14953$c3e8da3(a)news.astraweb.com... >> Bob Eld wrote: >>> "Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message >>> news:Qq6dnXrFS-I2KtDWnZ2dnUVZ_s2dnZ2d(a)earthlink.com... >>>> I'm playing around with some 7400 series logic, trying to teach myself >>>> the basics of digital design. I've been able to come up with a working >>>> circuit for a 24 hour digital clock easily, but I'm struggling trying > to >>>> think of a way to make a 12 hour clock. Specifically my problem is the >>>> reset of the hours counter - when the tens digit of the hours section > is >>>> 0 I need the units counter to count from 1 to 9 and roll over to 0 >>>> (for example 09 pm to 10 pm) but when the tens digit of the hours >>>> counter is a 1 I need the units counter to start counting at 1 (for >>>> example 12 pm - 1 am). I'm using 74LS93 series plain binary counters >>>> and assorted logic, and after working on it for a long while I can't >>>> think of a way to get it to work without using something like a >>>> presetable counter. Anyone have any ideas? >>> I realize that you are trying to learn digital circuit design but this > is >>> really a job for a microprocessor. A PIC or some other processor can do > all >>> of the functions of xtal clock generation, timing, BCD to seven segment >>> conversion, latching as well as drive LEDs or LCD's for the display on a >>> single chip. It can all be done in a few lines of code and is trivial >>> compared to using 1970's TTL technology. You might as well try to do it > with >>> 6SN7 tubes if you want ancient methods. How about filing out your own > brass >>> gears to make a clock? >> And thus we end up with systems that use a programmable microcontroller >> to perform a three-way OR function. >> >> Sylvia. > > Somehow I suspect that a 12Hr clock requires a couple of more gates than a > three-way OR function. Do ya think? Actually I might use a less than a > dollar, 8 pin processor for a three input OR gate. Even TTL would be a 14 > pin part would it not? > > Yes. My point was that if people don't learn about basic logic gates, how are they going to judge whether a PIC is appropriate? If you all you have is a hammer... Sylvia.
From: Raveninghorde on 14 Jan 2010 05:40 On Wed, 13 Jan 2010 21:38:40 -0800, "Bob Eld" <nsmontassoc(a)yahoo.com> wrote: > >"Sylvia Else" <sylvia(a)not.at.this.address> wrote in message >news:01970b1e$0$14953$c3e8da3(a)news.astraweb.com... >> Bob Eld wrote: >> > "Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message >> > news:Qq6dnXrFS-I2KtDWnZ2dnUVZ_s2dnZ2d(a)earthlink.com... >> >> I'm playing around with some 7400 series logic, trying to teach myself >> >> the basics of digital design. I've been able to come up with a working >> >> circuit for a 24 hour digital clock easily, but I'm struggling trying >to >> >> think of a way to make a 12 hour clock. Specifically my problem is the >> >> reset of the hours counter - when the tens digit of the hours section >is >> >> 0 I need the units counter to count from 1 to 9 and roll over to 0 >> >> (for example 09 pm to 10 pm) but when the tens digit of the hours >> >> counter is a 1 I need the units counter to start counting at 1 (for >> >> example 12 pm - 1 am). I'm using 74LS93 series plain binary counters >> >> and assorted logic, and after working on it for a long while I can't >> >> think of a way to get it to work without using something like a >> >> presetable counter. Anyone have any ideas? >> > >> > I realize that you are trying to learn digital circuit design but this >is >> > really a job for a microprocessor. A PIC or some other processor can do >all >> > of the functions of xtal clock generation, timing, BCD to seven segment >> > conversion, latching as well as drive LEDs or LCD's for the display on a >> > single chip. It can all be done in a few lines of code and is trivial >> > compared to using 1970's TTL technology. You might as well try to do it >with >> > 6SN7 tubes if you want ancient methods. How about filing out your own >brass >> > gears to make a clock? >> >> And thus we end up with systems that use a programmable microcontroller >> to perform a three-way OR function. >> >> Sylvia. > >Somehow I suspect that a 12Hr clock requires a couple of more gates than a >three-way OR function. Do ya think? Actually I might use a less than a >dollar, 8 pin processor for a three input OR gate. Even TTL would be a 14 >pin part would it not? > The problem with using a PIC for everything is when you need something faster such as memory map decoding for a processor. In the end you need to understand hardware logic. But I agree if speed isn't an issue it is easier to throw in a small PIC. Then when the 3 input OR needs to have an input inverted it is just a code change.
From: Martin Brown on 14 Jan 2010 06:15 Raveninghorde wrote: > On Wed, 13 Jan 2010 21:38:40 -0800, "Bob Eld" <nsmontassoc(a)yahoo.com> > wrote: > >> "Sylvia Else" <sylvia(a)not.at.this.address> wrote in message >> news:01970b1e$0$14953$c3e8da3(a)news.astraweb.com... >>> Bob Eld wrote: >>>> "Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message >>>> news:Qq6dnXrFS-I2KtDWnZ2dnUVZ_s2dnZ2d(a)earthlink.com... >>>>> I'm playing around with some 7400 series logic, trying to teach myself >>>>> the basics of digital design. I've been able to come up with a working >>>>> circuit for a 24 hour digital clock easily, but I'm struggling trying >> to >>>>> think of a way to make a 12 hour clock. Specifically my problem is the >>>>> reset of the hours counter - when the tens digit of the hours section >> is >>>>> 0 I need the units counter to count from 1 to 9 and roll over to 0 >>>>> (for example 09 pm to 10 pm) but when the tens digit of the hours >>>>> counter is a 1 I need the units counter to start counting at 1 (for >>>>> example 12 pm - 1 am). I'm using 74LS93 series plain binary counters >>>>> and assorted logic, and after working on it for a long while I can't >>>>> think of a way to get it to work without using something like a >>>>> presetable counter. Anyone have any ideas? The traditional design task for students with 74 series logic is usually a less ambitious digital dice 1-6, 0-5, 0-F you choose 7 segment or LED. >>>> I realize that you are trying to learn digital circuit design but this >> is >>>> really a job for a microprocessor. A PIC or some other processor can do >> all >>>> of the functions of xtal clock generation, timing, BCD to seven segment >>>> conversion, latching as well as drive LEDs or LCD's for the display on a >>>> single chip. It can all be done in a few lines of code and is trivial >>>> compared to using 1970's TTL technology. You might as well try to do it >> with >>>> 6SN7 tubes if you want ancient methods. How about filing out your own >> brass >>>> gears to make a clock? People do it - usually to replace worn out or damaged components on antique clocks. It is quite relaxing making your own gear wheels with single point tooling if a bit tedious towards the end. >>> And thus we end up with systems that use a programmable microcontroller >>> to perform a three-way OR function. >>> >>> Sylvia. >> Somehow I suspect that a 12Hr clock requires a couple of more gates than a >> three-way OR function. Do ya think? Actually I might use a less than a >> dollar, 8 pin processor for a three input OR gate. Even TTL would be a 14 >> pin part would it not? > > The problem with using a PIC for everything is when you need something > faster such as memory map decoding for a processor. In the end you > need to understand hardware logic. > > But I agree if speed isn't an issue it is easier to throw in a small > PIC. Then when the 3 input OR needs to have an input inverted it is > just a code change. Not only that but the 16877 has just enough output pins to direct drive a 4 segment LCD display and draws a grand total of 10uA at 3v on a 32kHz watch crystal. Digital trimming the crystal to get exact seconds is handy to avoid having any manual trimmer adjustments too. Regards, Martin Brown
From: John Larkin on 14 Jan 2010 07:17
On Wed, 13 Jan 2010 08:11:54 -0800, "Bob Eld" <nsmontassoc(a)yahoo.com> wrote: > >"Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message >news:Qq6dnXrFS-I2KtDWnZ2dnUVZ_s2dnZ2d(a)earthlink.com... >> I'm playing around with some 7400 series logic, trying to teach myself >> the basics of digital design. I've been able to come up with a working >> circuit for a 24 hour digital clock easily, but I'm struggling trying to >> think of a way to make a 12 hour clock. Specifically my problem is the >> reset of the hours counter - when the tens digit of the hours section is >> 0 I need the units counter to count from 1 to 9 and roll over to 0 >> (for example 09 pm to 10 pm) but when the tens digit of the hours >> counter is a 1 I need the units counter to start counting at 1 (for >> example 12 pm - 1 am). I'm using 74LS93 series plain binary counters >> and assorted logic, and after working on it for a long while I can't >> think of a way to get it to work without using something like a >> presetable counter. Anyone have any ideas? > >I realize that you are trying to learn digital circuit design but this is >really a job for a microprocessor. A PIC or some other processor can do all >of the functions of xtal clock generation, timing, BCD to seven segment >conversion, latching as well as drive LEDs or LCD's for the display on a >single chip. It can all be done in a few lines of code and is trivial >compared to using 1970's TTL technology. I know a lot of programmers who haven't the faintest idea of how computers work or how electricity behaves. Programming a PIC will teach you close to zero about digital logic. Wiring up a heap of 7400-series logic involves all the fundamentals of logic design: clocking, prop delays, races, noise, thermal issues, packaging, grounding, bypassing. You might as well try to do it with >6SN7 tubes if you want ancient methods. How about filing out your own brass >gears to make a clock? Digital logic design is not ancient quite yet. Gears aren't either, come to think of it. >I believe your time would be better spent learning a programming language >and how to accomplish various tasks in a processor than fooling around with >TTL. This would give you a foundation for a whole range of neat projects for >the 21st century. Plus the versatility of processors allows a virtual >infinity of designs in one type of device. Other than learning how to work >with a few NAND gates and D-Flops, there isn't any future in complicated TTL >methods for projects like yours. Umm, where will future computers and ASICs come from if nobody knows how to design logic? There's also the fundamental difference between computing (sequential execution) and logic (parallel, usually synchronous processing.) If a person starts out learing C, and progresses to C++ and Java and such, they may well be ruined for ever designing in VHDL. And a serious FPGA or ASIC designer will earn a lot more than a script kiddie, roughly 3x or so at current consulting rates. John |