MuratUrsavas
Posts: 32
Joined: Tue Nov 10, 2020 6:45 am

Reserved pin usage on CM4IO

Fri Nov 13, 2020 8:22 am

Hi,

As I'm progressing on upgrading to CM4, I've seen that one of the reserved pins (pin 76 particularly) is used for SD Card detection.

Do we have to pull pin 76 up to be able to use SD Card, or this is something that had been put for just in case?

Regards,

dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 671
Joined: Thu Dec 29, 2011 5:46 pm

Re: Reserved pin usage on CM4IO

Fri Nov 13, 2020 8:46 am

It is reserved and must not be used.

MuratUrsavas
Posts: 32
Joined: Tue Nov 10, 2020 6:45 am

Re: Reserved pin usage on CM4IO

Fri Nov 13, 2020 12:40 pm

To be on the safe side I'll add a pull-up resistor and mark it as DNF.

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davidcoton
Posts: 6582
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: Reserved pin usage on CM4IO

Fri Nov 13, 2020 10:35 pm

MuratUrsavas wrote:
Fri Nov 13, 2020 12:40 pm
To be on the safe side I'll add a pull-up resistor and mark it as DNF.
Why is that "safe"?
What does "DNF" mean?
Just leave it unconnected. Mark it "NC" for "No Connection".
Location: 345th cell on the right of the 210th row of L2 cache

trejan
Posts: 3850
Joined: Tue Jul 02, 2019 2:28 pm

Re: Reserved pin usage on CM4IO

Fri Nov 13, 2020 11:32 pm

davidcoton wrote:
Fri Nov 13, 2020 10:35 pm
MuratUrsavas wrote:
Fri Nov 13, 2020 12:40 pm
To be on the safe side I'll add a pull-up resistor and mark it as DNF.
Why is that "safe"?
Their reasoning is because CM4IO schematic has that reserved "Do not Connect anything to this pin." pin going through a not fitted resistor to the SD card presence switch then another not fitted resistor to the 3.3V rail. If that pin shouldn't be used then the CM4IO schematics should have it removed as well.
davidcoton wrote:
Fri Nov 13, 2020 10:35 pm
What does "DNF" mean?
Do Not Fit. They're trying to account for that pin being possibly being required in the future. I expect it'll stay unused though.

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davidcoton
Posts: 6582
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Location: Cambridge, UK

Re: Reserved pin usage on CM4IO

Fri Nov 13, 2020 11:57 pm

trejan wrote:
Fri Nov 13, 2020 11:32 pm
davidcoton wrote:
Fri Nov 13, 2020 10:35 pm
MuratUrsavas wrote:
Fri Nov 13, 2020 12:40 pm
To be on the safe side I'll add a pull-up resistor and mark it as DNF.
Why is that "safe"?
Their reasoning is because CM4IO schematic has that reserved "Do not Connect anything to this pin." pin going through a not fitted resistor to the SD card presence switch then another not fitted resistor to the 3.3V rail. If that pin shouldn't be used then the CM4IO schematics should have it removed as well.
davidcoton wrote:
Fri Nov 13, 2020 10:35 pm
What does "DNF" mean?
Do Not Fit. They're trying to account for that pin being possibly being required in the future. I expect it'll stay unused though.
OK, so the OP's pull-up resistor won't be fitted. That just about makes sense.
Location: 345th cell on the right of the 210th row of L2 cache

MuratUrsavas
Posts: 32
Joined: Tue Nov 10, 2020 6:45 am

Re: Reserved pin usage on CM4IO

Mon Nov 16, 2020 12:22 pm

Sorry for being late to the party :) @trejan's answer is correct.

Think this: What if some day Raspberry Pi OS engineers decide to make SD Card detection obligatory and I had manufactured PCB's already? Then the design would not work and I'll be WTF? Instead of that I choose to place a resistor as DNF :)

This will make me ready for both choices.

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