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I've seen datasheets for certain ICs recommend using a 100 kOhm (or so) pulldowns or pullups for setting configuration inputs. What could be a typical reason for this? Normally I would just tie the pin directly low or high.

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  • \$\begingroup\$ the reasons could be many and varied. I might be wise to provide an example so the possible reason could be addressed for that part. Generally, you'd use a resistor so you could easily change the state when debugging or testing. If the input is tied hard to a rail, then you'd have no chance without creating a short-circuit. \$\endgroup\$
    – Kartman
    Dec 23, 2021 at 0:23
  • \$\begingroup\$ 1) If the pin can be configured as an output in other modes, hardwiring to +V or GND can do some damage. 2) With a weak pulldown. your PCB test procedure can pull it high to test the alternate mode of operation. \$\endgroup\$
    – user16324
    Dec 23, 2021 at 15:57
  • \$\begingroup\$ Please, can you provide at least one specific example (IC part number)? As it stands it is extremely vague. The reasons could well depend on the specific IC. \$\endgroup\$ Dec 23, 2021 at 19:00

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The modern reason is a pull resistor lets you override it for debugging or change your mind and not populate it. A hard wired connection lets you do neither.

The historical reason was that old designs which run off BJTs instead of MOSFETs need pull resistors for current limiting since a direct connection to a power rail directly injects current into the diode-like base-emitter junction.

CORRECTION: Kevin White states:

" TTL ICs would recommend a pull-up resistor because the absolute maximum voltage for logic inputs was 5.5V whereas the maximum voltage for the power input pins was 7V. A surge on the supply rail to a voltage between 5.5v and 7v could damage the input transistor while the majority of the IC survived. It was considered good practice to provide current limiting to the inputs in the form of a resistor so that the device as a whole would survive a 7V surge."

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  • \$\begingroup\$ A bit misleading. TTL ICs would recommend a pull-up resistor because the absolute maximum voltage for logic inputs was 5.5V whereas the maximum voltage for the power input pins was 7V. A surge on the supply rail to a voltage between 5.5v and 7v could damage the input transistor while the majority of the IC survived. It was considered good practice to provide current limiting to the inputs in the form of a resistor so that the device as a whole would survive a 7V surge. \$\endgroup\$ Dec 23, 2021 at 1:38
  • \$\begingroup\$ @KevinWhite Is that the reason? I'll have to defer to you there. \$\endgroup\$
    – DKNguyen
    Dec 23, 2021 at 1:39
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This may not be what the OP is seeing but I’ve seen datasheets for some microprocessors that overlay configuration pins on pins for other input functions. The configuration is sampled only on reset, after which the pins assume their normal functions.

Using pull up/down resistors for configuration allows the configuration to be read but not interfere with the normal functioning once the chip is out of reset.

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  • \$\begingroup\$ I'm using one of those micros, at work. The complexity with this is making sure the circuitry on the other end isn't driving those lines whilst the micro is resetting, which can be "interesting". \$\endgroup\$
    – Graham
    Dec 23, 2021 at 9:56
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    \$\begingroup\$ I have encountered configuration pins read at reset that were outputs in normal operation. \$\endgroup\$ Dec 23, 2021 at 12:16
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Probably the main reason a pull up or pull down resistor is required is because there isn't on on the device and leaving the input floating (open) means it would have an indeterminate logical state.
Consequently it's effect on the operation of the device is undefined.
An open input could also pick up stray signals and noise and it would appear to be switching from 0 to 1 at random; again with undefined consequences.

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    \$\begingroup\$ I think you may have missed the point of the question. The OP isn't suggesting that the input should be floating, they are asking why we don't tie the input directly to ground or power, without the resistor. \$\endgroup\$ Dec 23, 2021 at 1:49
  • \$\begingroup\$ @ElliotAlderson Yes, you are correct, I misinterpreted the question. I think that Kevin White's correction to DKNguyen's answer (above) is probably nearest the truth. in most cases. \$\endgroup\$ Dec 23, 2021 at 10:41

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