74LS92 - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: April 15, 2012, 01:44:23 pm
content: Howdy All,I am trying to reset the count on a 74LS92. I have read and reread the datasheet, but I am still not sure how to reset the count. The data sheet says pins 2 and 3 are the MR1 and MR2 respectively.Things I have tried:1. I have tried making MR1 go high using the output from a 74HC08 AND output. But the count does not reset.2. Then I thought, maybe the MR1 gets trigger by low voltage so I used an 74HC04 to invert the output from the 74HC08 AND output. But the count does not reset.3. Then I thought maybe I had to make both MR1 and MR2 low and or high, so I hooked up both the MR1 and MR2 to the output of the AND gate (and the inverted output on a second attempt). But the count does not reset.4. Then I thought maybe it was a TTL/CMOS issue, as in the voltage from the 74HC08 AND gate wasn't high or low enough to trigger the MR1 on the 74LS92, so I tried hooking up a 555 timer to MR1, just to check. But the count does not resent. I also tried inverting the output of the 555 timer just in case the MR1 needs a negative voltage as trigger - still no luck.I am not all that good at reading data sheets yet, but after reading functional description section it appears that MR1 and MR2 make and AND gate.So what I did is hook up Q0 and Q1 to MR1 and MR2 respective. But when I get to count 3, when Q0 and Q1 are both HIGH, the counter still does not reset.How do I reset the count on this 74LS92 chip? My ideas of reset is that the counter will go back to the count at zero when all the pins are LOW.Thanks for any help.Data Sheethttps://docs.google.com/viewer?url=http%3A%2F%2Fwww.datasheetcatalog.org%2Fdatasheets%2F90%2F335223_DS.pdf

author: Unknown
author_rank: Unknown
timestamp: April 15, 2012, 02:15:15 pm
content: Note that according to the datasheet, MR1 and MR2 are pins 6 and 7 for the LS92.The counter should reset when both MR1 and MR2 are high. The output from a TTL chip will probably be too low to trigger a CMOS AND gate, unless you use the 74HCT series. Try connecting both MR1 and MR2 to Q3, then Q1 and Q2 should count up when you feed a clock signal to CP1.Do you have access to a scope or logic analyzer to verify logic levels? Otherwise you might be able to feed it a really slow clock and measure the outputs with a DMM. Watch out for switch bounce if you use mechanical toggle switches, this may trigger the counter multiple times.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 15, 2012, 02:30:25 pm
content: According to the datasheet the reset function is asynchronous and occurs when both MR1 and MR2 go high. (See attached.)Being asynchronous then just setting these two pins high should do the trick. Like Alms says, be sure the voltage levels are appropriate.Interestingly, the Digikey parameters say the part's reset function issynchronous! At least for the TI equivalent. I don't know if I trust that or not, however.

author: Unknown
author_rank: Unknown
timestamp: April 15, 2012, 02:50:05 pm
content: Well, I am such a noob. When I searched for the 74LS92 datasheet, the first one I must have found that I could read well was the motorola version SN54/74LS92. The MR1 and MR2 on the motorola datasheet are Pins 2 and 3. Of course the part I have is a TI chip "Doh!".Looking at the TI datasheet, I can see that it is PINs 6 and 7 I should be trying to reset with! Thanks. I will try messing around with these for a little while now...

author: Bored@Work
author_rank: Super Contributor
timestamp: April 15, 2012, 03:23:41 pm
content: Quote from: OSHJunkies on April 15, 2012, 02:50:05 pmNo, you looked at the wrong figure. There is a reason the figures have labels like LS90, LS92 and LS93. And really, stop this "I am a noob" stuff.

author: Unknown
author_rank: Unknown
timestamp: April 16, 2012, 11:16:41 am
content: Thanks BoredAtWork,You are right, I did looked at the wrong figure. Which leads me to this question - when different manufactures make the same chip, can I expect the pins to be in the same location?

author: FreeThinker
author_rank: Frequent Contributor
timestamp: April 16, 2012, 12:13:00 pm
content: Quote from: OSHJunkies on April 16, 2012, 11:16:41 amWell the answer to that one is ALWAYS read the data sheet to be sure, but for most jellybean parts you can assume that they will be, alot will say 'pin compatable with xxx' in their description but really you need to check the data sheet or at least the product discription.

author: SeanB
author_rank: Super Contributor
timestamp: April 16, 2012, 07:49:18 pm
content: You can generally find that the same part number from different manufacturers will have the same pinout. however, this is often different in different cases, even if they have the same number of pins. You also find that different suffixes will sometimes be different in pin out. Best is to refer to the data sheet for the particular part and the replacement, and also remember that sometimes the same part from a different manufacturer will be slightly different characteristic wise, and this can sometimes bite you.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 16, 2012, 08:33:54 pm
content: Quote from: OSHJunkies on April 15, 2012, 01:44:23 pmThis clearly shows that you need a better understanding of basic logic level I/O. The 555 output is a square wave. It has both positive and negativegoing edges. If you invert the output, you haven't changed it at all. You've only added a phase shift of 180 degrees to the original clock.The 555 output, after the inverter, looks basically the same. It's been phase shifted and normalized to match the OH/OL of the inverter's logic leveloutputs.