Help in clock design - Page 1
Forum section: electronics / beginners

author: kaizhen
author_rank: Newbie
timestamp: April 08, 2017, 02:59:00 pm
content: I designed a D.clock, sec and min all no problem. My hrs segment also no problem in reseting on 125959.My problem now is my clock when reach 10 will direct jump to 11, im so lost now on the hrs segment load part. I try to load to 01 when hrs segment reaches 00 but it makes all the 0 now become 1 any suggestion to this newbie will be very needed. Thank you

author: Audioguru
author_rank: Super Contributor
timestamp: April 08, 2017, 03:17:58 pm
content: I used old fashioned 74xx TTL logic ICs about 42 years ago then used Cmos since then. Why are you using TTL logic today?

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2017, 03:38:33 pm
content: Hi, (and welcome)Why have you put in all of the 100k resistors in series with gate inputs? They aren't needed. Are you really using TTL 7400 series or a more modern CMOS 74HC00 series? I can't see it working at all with resistors that big and TTL logic.

author: rstofer
author_rank: Super Contributor
timestamp: April 08, 2017, 03:54:47 pm
content: You must have had a reason for adding all those 100k resistors but I can't quite figure out why.In any event, TTL requires a significant amount of sink current (2 mA) on the gate input while pulling low to 0.5V. Current you can't get with series resistors.See Section 6.7 here for switching voltages and currents:http://www.ti.com/lit/ds/symlink/sn7400.pdf

author: Nusa
author_rank: Super Contributor
timestamp: April 08, 2017, 03:55:42 pm
content: Quote from: kaizhen on April 08, 2017, 02:59:00 pmAlmost. Check the logic of the 7425. The strobe input to U31 is ignored when all the inputs are 0. That's the cause of the problem you asked about. The fix is an exercise for you.

author: kaizhen
author_rank: Newbie
timestamp: April 08, 2017, 04:19:18 pm
content: Quote from: Audioguru on April 08, 2017, 03:17:58 pmIm just more familiar with TTL..because always use TTL so naturally used TTL...is there any differences between TTL and CMOS?

author: kaizhen
author_rank: Newbie
timestamp: April 08, 2017, 04:22:55 pm
content: Quote from: Gyro on April 08, 2017, 03:38:33 pmYa i know is not working at all that was my before circuit..i try to use cmos more often next time i been using TTL a lot so automatically using it now...still thanks for reply...

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2017, 04:25:33 pm
content: Quote from: kaizhen on April 08, 2017, 04:19:18 pmOnly for the better. For 74HC compared the TTL, power consumption is a small fraction of TTL (almost zero when static). Input impedances are very high. Output drive is symetrical. Look up a few 74HC datasheets on the web.

author: kaizhen
author_rank: Newbie
timestamp: April 08, 2017, 04:41:52 pm
content: Quote from: Gyro on April 08, 2017, 04:25:33 pmOK thanks i will look it up next time try to use cmos now thanks for the info

author: kaizhen
author_rank: Newbie
timestamp: April 09, 2017, 01:25:45 am
content: Quote from: Nusa on April 08, 2017, 03:55:42 pmThanks for the reply, i looked into the function table of 7425...My u39 suppose to have H output as ABCDG=LLLLX then go into the strobe input of u31 why does my u31 strobe input still got ignored? Hmm......i still dont get it why it will be ignored sorry.....and very sorry if im asking too much

author: Audioguru
author_rank: Super Contributor
timestamp: April 09, 2017, 01:45:23 am
content: I agree that the 100k input resistors are way too high for high current TTL ICs. The datasheet for the 7425 and most other TTL inputs shows a maximum input low current of 1.6mA. Then the input voltage to the 100k resistor must be -160V for the input to be a logic low. Resistors are not needed anyway.

author: kaizhen
author_rank: Newbie
timestamp: April 09, 2017, 03:50:52 am
content: Quote from: Audioguru on April 09, 2017, 01:45:23 amYes, i already took it out thank you for reply

author: Nusa
author_rank: Super Contributor
timestamp: April 09, 2017, 05:51:31 am
content: Quote from: kaizhen on April 09, 2017, 01:25:45 amCorrect, ABCDG=LLLLX = H, where X means irrelvant, except examine that behavior on U31, not U39. As for U39, note that XXXXL = H, so it's never going to change with G tied low.Start with U6,U8=0,9. Increment U8 to 10. U25 detects 10 and causes U6 to increment and U8 to clear, so U6,U8 = 1,0. So far we're good.However, U31 now sees the 0 from U8 and causes U8 to load 1. So now U6,U8=1,1.Does that make sense now? The increment worked fine, then was overwritten by an unwanted load.

author: kaizhen
author_rank: Newbie
timestamp: April 09, 2017, 09:43:24 am
content: Quote from: Nusa on April 09, 2017, 05:51:31 amFirst paragraph i understand, second paragraph is already not a problem. As for the last paragraph, i know that when u31 sees the 0 from u8 will load 1. What do u mean by u6,u8=1,1? Did u mean that when it reach 10 the 0 will automatically change to 1? And what does you mean by overwritten by unwanted load....please forgive me this beginner for asking stupid question. I really dont understand im sorry

author: Nusa
author_rank: Super Contributor
timestamp: April 09, 2017, 12:24:50 pm
content: Quote from: kaizhen on April 09, 2017, 09:43:24 amThat's what I understood your original statement of the problem to be: "My problem now is my clock when reach 10 will direct jump to 11", where u6 is 1 and u8 is 1, when you wanted u6 to be 1 and u8 to be 0.I'm saying that u31 will not allow u8 to maintain a stable output of 0 even when you want to display 0! As soon as the CLR line is no longer asserted, it will change to 1.

author: kaizhen
author_rank: Newbie
timestamp: April 09, 2017, 02:35:23 pm
content: Quote from: Nusa on April 09, 2017, 12:24:50 pmOhh, i understand your meaning now ya tht what i wanted to do but till now i still cant figure out anyway to solve this problem....i tried other combination of logic gate result only getting 01 and jam there wont change to 02 at all...7425 is the only logic gate which i can gt it running but the problem now is u8 all 0 become 1...any suggestions on how can i solve this problem? Been stuck in this problem for 2 days...help me thank you

author: kulky64
author_rank: Regular Contributor
timestamp: April 09, 2017, 03:05:11 pm
content: I suggest the following:1. Delete U39A and U31A completely. You don't need them.2. Delete connection between output of U27A and input of U28.3. Delete U28. Output of U25A connect directly to CLR of U8.4. Feed output from U27A through invertor (you can use U26 for that which is now free after perferming step 1.) to LOAD pin of U8.

author: kaizhen
author_rank: Newbie
timestamp: April 10, 2017, 03:56:31 am
content: Quote from: kulky64 on April 09, 2017, 03:05:11 pmVery thanks for your suggestions, if i put an inverter on the output of u27a it will just stuck on 01 if I remove it then if will move when 125959 restarts to 01 and 10 won't become 11 but bad news is on the first run start at 00 not 01 need ran for one round then become 01