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TerryC65
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Pi 3.3 V Specification

Thu May 09, 2019 11:35 am

Hi,

I'm writing a test spec for a bit of hardware that we will be producing in some quantities for a system that we are installing at a our local model town. In the spec, I am instructing the maker to check the voltage going to a particular IC and this voltage is the 3.3 V from the Pi. I'd like to put a realistic set of limits on this measurement, but I've been unable to find any official specification for this output in the Pi specs.

Can anyone point me at the right place?

hippy
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Re: Pi 3.3 V Specification

Thu May 09, 2019 11:56 am

Assuming a Pi 3B+; looking at the MxL7704-AQB/XQB datasheet the 3V3 voltage which goes to the GPIO connector pins can range from 3V0 to 3V6 in 20mV steps. The MxL7704-R3 has no publicly available datasheet but could be presumed to be similar, approximately 3V3+/-10%. Other Pi would probably be similar.

But it's not what the Pi puts out though which matters, it's what things using that 3V3 require.

You would need to know what all those things require before being able to say what range should be checked for to deem that 3V3 output acceptable.

If just looking at what the voltage could be to ensure safe operation of the monitoring circuit then 0V to 5V would seem a reasonable range to choose. No more than 5V should go into the Pi so no more should ever come out, even under a failure mode. Though all bets are off if more than 5V is allowed into the Pi.

aBUGSworstnightmare
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Joined: Tue Jun 30, 2015 1:35 pm

Re: Pi 3.3 V Specification

Thu May 09, 2019 12:02 pm

TerryC65 wrote:
Thu May 09, 2019 11:35 am
Hi,

I'm writing a test spec for a bit of hardware that we will be producing in some quantities for a system that we are installing at a our local model town. In the spec, I am instructing the maker to check the voltage going to a particular IC and this voltage is the 3.3 V from the Pi. I'd like to put a realistic set of limits on this measurement, but I've been unable to find any official specification for this output in the Pi specs.

Can anyone point me at the right place?
Why don't you use the DC characteristics and power supply operating range of the Compute Module?

In case your GPIO0-27_VDD is 3.3V +/-5% your GPIO output voltage will be the same.
https://www.raspberrypi.org/documentati ... us_1p0.pdf

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TerryC65
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Location: Wimborne, Dorset, UK

Re: Pi 3.3 V Specification

Thu May 09, 2019 2:04 pm

hippy wrote:
Thu May 09, 2019 11:56 am
Assuming a Pi 3B+; looking at the MxL7704-AQB/XQB datasheet the 3V3 voltage which goes to the GPIO connector pins can range from 3V0 to 3V6 in 20mV steps. The MxL7704-R3 has no publicly available datasheet but could be presumed to be similar, approximately 3V3+/-10%. Other Pi would probably be similar.
Thanks for that.
hippy wrote:
Thu May 09, 2019 11:56 am
But it's not what the Pi puts out though which matters, it's what things using that 3V3 require.

You would need to know what all those things require before being able to say what range should be checked for to deem that 3V3 output acceptable.
Actually the IC that is using this supply has a Vcc range of 2 V to 6 V and is being used as a level shifter between the external logic high (~12 V) and the GPIO input. This device works by shifting the output Hi to the value of the input Hi of the Pi.

By using the 3.3 V out of the Pi to power the chip, I'm pretty sure that the logic signals to the Pi are in spec and that's the voltage that I need the maker to test.
hippy wrote:
Thu May 09, 2019 11:56 am
If just looking at what the voltage could be to ensure safe operation of the monitoring circuit then 0V to 5V would seem a reasonable range to choose. No more than 5V should go into the Pi so no more should ever come out, even under a failure mode. Though all bets are off if more than 5V is allowed into the Pi.
Actually, I believe that the specified input voltage of the Pi is 5.1 V. ;) (I've seen 5.2 V from a standard Pi PSU.)

The reason that I want my test to specify the voltage range expected from the Pi is because anything else would indicate that something has gone wrong during construction.

I'm no longer trying to design this circuit; that was done many months ago and the first prototype has been deployed and working since around February. This is about production test - does the item meet it's specification?

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TerryC65
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Re: Pi 3.3 V Specification

Thu May 09, 2019 2:06 pm

aBUGSworstnightmare wrote:
Thu May 09, 2019 12:02 pm
Why don't you use the DC characteristics and power supply operating range of the Compute Module?

In case your GPIO0-27_VDD is 3.3V +/-5% your GPIO output voltage will be the same.
https://www.raspberrypi.org/documentati ... us_1p0.pdf
Thanks. That's exactly what I needed to know.

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