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I while a ago I saw a circuit in a datasheet that automatically cleared a 74xx164 shift register on power-up. I don't exactly remember which logic series it was (might have been LS or HC), and the reset circuit was basically just a capacitor and a resistor which generated a reset pulse when power was applied to the circuit. I don't remember if it contained any further components, and I cannot find that datasheet anymore (because I didn't really pay attention to the circuit at that time).

Does anyone here remember having seen that circuit, which datasheet it is in, or possibly even used it? I have looked at RC reset pulse generators on the web, but I'm not sure if they are the same thing I have seen in the datasheet.

I'm basically asking for advice on how to build a reset circuit for a 74xx164.

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  • \$\begingroup\$ Are you asking for advice for a reset circuit for the 74XX164? RC plus a diode maybe the answer if you believe it was more complex than just RC. The diode is across the resistor and discharges C when power is removed. \$\endgroup\$ – Andy aka Mar 20 '13 at 10:57
  • \$\begingroup\$ Yes, I'm asking for advice. \$\endgroup\$ – Christoph Mar 20 '13 at 11:48
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10nF capacitor down to ground. Other end to the reset line. Resistor of 10k from reset line to positive supply (Vcc). 1N4148 (or BAS16) diode across the resistor, cathode on the Vcc line. The diode type is not particularly important other than it needs to be able to handle the discharge current from the cap as the power is removed. If power is removed by a switch, there should also be a 10k across the chips power rail to ensure the cap is fully discharged before the power restarts.

Let me know if you need a circuit diagram.

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  • \$\begingroup\$ Your description of the reset circuit is pretty clear. I'm not sure about the 10k resistor across the chip's power rail - its purpose is to draw current from the cap when the power supply is absent, correct? \$\endgroup\$ – Christoph Mar 20 '13 at 12:44
  • \$\begingroup\$ @Christoph - correct - it discharges the cap so that next time you apply power it is primed to do a proper reset \$\endgroup\$ – Andy aka Mar 20 '13 at 12:49
  • \$\begingroup\$ In that case I can also rely on a nearby voltage divider to discharge the cap. I'll just try it... \$\endgroup\$ – Christoph Mar 20 '13 at 13:01
  • \$\begingroup\$ @Christoph - yes you can \$\endgroup\$ – Andy aka Mar 20 '13 at 13:03
  • \$\begingroup\$ Another approach I've seen for devices with an on/off switch is to use a double-throw switch with one throw wired to short the board supply [maybe through a resistor if there's a big filter cap, but generally not] when the board is turned "off". I've observed that many such devices won't restart reliably if unplugged and replugged without the switch being turned off in the interim, but the switch improves start-up reliability to 100%. \$\endgroup\$ – supercat Mar 20 '13 at 16:47

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