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I'm using the 74HC595. The datasheet can be found here.

Everything works, just the only problem is the Master Reset button is not working. Pressing it to LOW doesn't do anything. I measured the voltage while HIGH and LOW. Which is MRh = 4.69V, MRl = 0.04V.

Looking at the datasheet. With Vcc at 4.5V (I'm using 4.98V). The HIGH level input voltage is 3.15vMIN. The LOW level input voltage using 4.5Vcc is 1.35vMAX. Here's the schematic I drew. And the breadboard, although not really helpful. Is it the pull up/down resistor values?

What could be the problem?

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    \$\begingroup\$ Could you draw the schematic using the sites own schematic editor tool? Makes it much easier to read. Also, write down what it is you are expecting it to do vs what it is actually doing \$\endgroup\$
    – MCG
    Commented Jul 25, 2018 at 7:37
  • \$\begingroup\$ from, your desctiption of the measured voltages you are using suitable pull-up/down resistors. \$\endgroup\$
    – Jasen Слава Україні
    Commented Jul 25, 2018 at 8:06

1 Answer 1

Reset to default
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The MR only clears the shift register. To affect the parallel outputs, this cleared state needs to be clocked into the latch by a positive-going transition on ST_CP. All this is shown in the function table.

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  • \$\begingroup\$ unfortunately, I have do downvote this. OP may have been doing as you suggest, but after clocking it in, it isn't clearing. This is why I asked for more information in the comments. As OP stated everything works, except the reset, thus this answer would be more of a guess as it stands. Hopefully we will get some more information soon! \$\endgroup\$
    – MCG
    Commented Jul 25, 2018 at 9:31
  • \$\begingroup\$ I have to upvote this as it seems to be the most reasonable guess. \$\endgroup\$
    – Richard the Spacecat
    Commented Jul 25, 2018 at 9:55
  • \$\begingroup\$ @RichardtheSpacecat yeah, I agree it is a very reasonable guess, and likely correct, but it is still a guess. It is the question that needs clarifying first. As there is a lack of information in the question, a guess is better put in the comments than as an answer, hence my downvote. If the required information is put in and this is indeed correct, I will happily change my vote. \$\endgroup\$
    – MCG
    Commented Jul 25, 2018 at 10:53
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    \$\begingroup\$ Although I will say these situations are frustrating when people ask a question then don't bother to return to look at answers or comments as this could well be the answer. \$\endgroup\$
    – MCG
    Commented Jul 25, 2018 at 10:55
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    \$\begingroup\$ I think this answer is correct - however your low input levels are a bit high (because of the LEDs/resistors) and worse likely to be noisy - the diodes (LEDs) will turn off leaving the upper side (your inputs) essentially with an iffy sort of state - essentially high impedance \$\endgroup\$
    – Taniwha
    Commented Jul 25, 2018 at 11:46

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