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Is it OK to have pull up or pull down resistors which aren't all the same value (if you're running low on components for example)? I can't think of a reason it should affect simple logic circuits.

I'm mostly using 74HC series ICs but plan to experiment with some Z80-based projects. Is there a possibility that things might start to act strangely at higher clock speeds of around 4MHz?

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  • \$\begingroup\$ What resistor values do you have at hand/do you intend to use? \$\endgroup\$
    – stevenvh
    Commented Jun 23, 2011 at 13:36
  • \$\begingroup\$ So far I've been using 22K for pull up and parallel pairs of 330 ohm for driving LEDs. The rest depends on whatever is left in the big urnorganised pile :p \$\endgroup\$
    – Wolfy
    Commented Jun 23, 2011 at 13:53
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    \$\begingroup\$ the 330 \$\Omega\$ will be much too low. Try to use at least a few k\$\Omega\$. \$\endgroup\$
    – stevenvh
    Commented Jun 23, 2011 at 15:44
  • \$\begingroup\$ @Stevenvh: 330 ohms is the Bog-standard value for driving a red LED from 5V. It gives a nice 10 mA current through the LED, which is perfectly visible in indoor lighting. \$\endgroup\$ Commented Jun 24, 2011 at 19:43

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By today's standards 4MHz is by no means a high clock speed. Most often pull up resistor values are not that critical, people often use 10k for address or data lines without even thinking about it (just like 100nF is used for decoupling). Using for instance 6k8 instead of 10k won't do any harm. Just make sure the pull up is on the trace itself, and not on a stub.

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    \$\begingroup\$ An often times when optimizing a design for production, you will modify pull up resistors on purpose. With SMD parts placement, you are limited to number of reels of parts. So if you can standardize the values, all the better. \$\endgroup\$
    – Joe
    Commented Jun 24, 2011 at 3:33

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