BlastFX
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ESD protection diodes on the BCM2711

Wed Jul 10, 2019 6:54 pm

I can't seem to find the BCM2711 datasheet, so I'll ask here.
I'm assuming the BCM2711 has clamping diodes on all inputs like pretty much every digital IC manufactured in the last couple of decades. What is their current rating?
And while we're at it, what about previous generations of the SoC?

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Burngate
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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 9:03 am

Unless it's significantly different from previous SoCs, it doesn't have protection diodes as such - merely parasitic elements that sort of work as clamp diodes, and if significant current passes through one, it's likely to cause latchup and damage the silicon.
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BlastFX
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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 3:44 pm

Too bad, I was hoping to interface the GPIO to a TTL level device with only series resistors.
Although the page you linked says that sinking up to 500 μA into the “diodes” is fine (despite saying sinking any current into them is not acceptable just a paragraph earlier), I'm not risking my brand new Pi 4 to test that assertion.
Guess I'll squeeze a proper level shifter in somehow.
On a side note, isn't it a bit weird that the RPF supports, nay, encourages interfacing the Pi with other hardware, but refuses to provide proper electrical specification of the GPIO?

jamesh
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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 3:50 pm

BlastFX wrote:
Thu Jul 11, 2019 3:44 pm
Too bad, I was hoping to interface the GPIO to a TTL level device with only series resistors.
Although the page you linked says that sinking up to 500 μA into the “diodes” is fine (despite saying sinking any current into them is not acceptable just a paragraph earlier), I'm not risking my brand new Pi 4 to test that assertion.
Guess I'll squeeze a proper level shifter in somehow.
On a side note, isn't it a bit weird that the RPF supports, nay, encourages interfacing the Pi with other hardware, but refuses to provide proper electrical specification of the GPIO?
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BlastFX
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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 4:46 pm

I've read that page (as well as the registry maps “datasheets” for all 5 generations of the SoC and the specs of Compute Module 3).
First off, it's for the BCM2835. I can't know whether it still applies 4 generations later, especially after a major revision.
Second, it doesn't contain the information I was asking for (which has been present in pretty much every datasheet I've ever seen; most even include a diagram of the output driver).

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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 5:17 pm

BlastFX wrote:
Thu Jul 11, 2019 4:46 pm
I've read that page (as well as the registry maps “datasheets” for all 5 generations of the SoC and the specs of Compute Module 3).
First off, it's for the BCM2835. I can't know whether it still applies 4 generations later, especially after a major revision.
Second, it doesn't contain the information I was asking for (which has been present in pretty much every datasheet I've ever seen; most even include a diagram of the output driver).
It's still valid for Pi4, AFAIK, and all I have I am afraid. Seems to be enough for the majority of projects out there.
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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 5:47 pm

BlastFX wrote:
Thu Jul 11, 2019 3:44 pm
Guess I'll squeeze a proper level shifter in somehow.
A couple of resistors as a voltage divider should work on an input to the Pi, or a FET appropriately wired will work in both directions.

BlastFX
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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 7:15 pm

jamesh wrote:
Thu Jul 11, 2019 5:17 pm
It's still valid for Pi4, AFAIK, and all I have I am afraid. Seems to be enough for the majority of projects out there.
That's good to know.
As a Raspberry Pi Engineer you wouldn't happen to have access to the actual datasheet by any chance?
Burngate wrote:
Thu Jul 11, 2019 5:47 pm
A couple of resistors as a voltage divider should work on an input to the Pi, or a FET appropriately wired will work in both directions.
Yup, that's what I was thinking of, FETs with pull-up resistors. If I go for SOT23 FETs and 0603 resistors, with a bit of luck, I should be able to squeeze it in. If that doesn't work, I'll look for a dedicated chip in a small package (SSOP or QFN).

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Re: ESD protection diodes on the BCM2711

Thu Jul 11, 2019 8:22 pm

BlastFX wrote:
Thu Jul 11, 2019 7:15 pm
jamesh wrote:
Thu Jul 11, 2019 5:17 pm
It's still valid for Pi4, AFAIK, and all I have I am afraid. Seems to be enough for the majority of projects out there.
That's good to know.
As a Raspberry Pi Engineer you wouldn't happen to have access to the actual datasheet by any chance?
We don't have access to much more than we have actually published. Brcm datasheets are not great. What we do have is access to the engineers so we just ask them when we need to know stuff. Sometimes they even know the answers.
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