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PiModules
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RPiCM HDMI ESD Protection

Tue Sep 16, 2014 12:16 pm

Hi,
I have a question.
Why on the development board there are only protection on the HDMI control lines and not on the others? Any special reason?

My colleague doing the design just copied it, but, on my opinion there should be also on all HDMI lines.

Can anybody comment this.

Thank you in advance
PiMaster
www.pimodules.com

drb_oliver
Posts: 5
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Re: RPiCM HDMI ESD Protection

Thu Nov 06, 2014 5:37 pm

Hi PiMaster,
i agree with you. On the Compute Module itself is a common mode choke and on the CM IOBoard no protection for the differential TMDS lines. After some looking around i found the CM2020 from On Semiconductor most suitable. If you are interested in my actual schematics please download the user manual of the base board DRB_IPC_884_CAN from http://drb-electronic.de/index.php/downloads. Of course any comments are welcome.
Best regards
drb_oliver

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PiModules
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Re: RPiCM HDMI ESD Protection

Thu Nov 06, 2014 6:41 pm

Hi,

Thank you for your advice, I already used the ESD8104 little bit difficult to be soldered by hands, but for production is OK.

Warmest Regards
Pi Master
www.pimodules.com

mikeyoung
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Location: Melbourne Australia

Re: RPiCM HDMI ESD Protection

Mon Nov 10, 2014 9:46 am

Hi drb_oliver

Please please please forgive me if I am wrong, but I took a look at the HDMI static protection circuit you have linked to and I think it is incorrect. I am using a similar device to the CM2020, in my case a TPD12S521. The devices uses mosfets as level shifters on the 4 control lines, CEC, SDA, SCL and HPD. On your circuit when the external Hotplug detect signal (HPD) goes high the internal level shifting mosfet will turn off. In your schematic there is no pull up resistor on the RPi side of the CM2020 to turn on the DMG1012T mosfet, Q5, so the gate will just float. I think you need a resistor to 3v3 here.

Also I think there is a problem with the CEC circuit. Again there is an internal level shifting mosfet between the CEC_IN and CEC_OUT pins. The gate of the internal mosfet is connected to the LV supply which is 3v3. You have then connected a second level shifting mosfet , Q4, in series and connected the gate up to 5v. I'm not clear what the effect of this will be but it does not look right to me. Seems to me like the Q4 is not required.

Once again please accept my apologies if i'm wrong, which I have been many times in the past :oops:

drb_oliver
Posts: 5
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Re: RPiCM HDMI ESD Protection

Mon Nov 10, 2014 8:04 pm

Hi mikeyoung,
thanks a lot you found these two mistakes. First of all the pullup at Q5 is simply missing no question to that.
Concerning Q4 i copied the Raspberry Pi Compute Module IO Board design without fully understanding it. To save my honor i really had the intention to post this as a question. Now i googled "mosfet level shifter wiki" and think i got it - i know that sounds a bit lazy. Good luck for me that experts like you reviewed my design (and i did not order the board right now).
Thanks a lot - best regards
drb_oliver

mikeyoung
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Location: Melbourne Australia

Re: RPiCM HDMI ESD Protection

Mon Nov 10, 2014 11:10 pm

Hi drb_oliver

I'm pleased I could help.

I actually think that the CM IO board design is incorrect. First of all, the HDMI_SDA and HDMI_SCL signals are pulled up to 5v via the ESD5384. That means that the Broardcom chip is exposed to 5v on these inputs. I do not have the spec to check if the inputs are 5v tolerant, so cannot be sure. Additionally the circuit around CEC signal makes no sense to me. Assuming that it is intended as a level shifter, the gate of Q3 should be connected to 3v3 and the pullup in the ESD5384 device should be connected to a CEC voltage reference, but pin B1 is left disconnected? From what I have read I believe that the CEC reference voltage should be 3.3v so pin B1 could just be tied to the 3v3 power rail, in which case you may ask yourself, why the level shifter?

Can anyone confirm if this circuit is correct or a mistake?

drb_oliver
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Re: RPiCM HDMI ESD Protection

Tue Nov 11, 2014 6:42 pm

Hi mikeyoung,
looking at the RPi 2.0 and RPI Compute Module IOBoard schematics HDMI_SDA and HDMI_SCL "must" be 5V tolerant even if VBAT is set to 3,3V. But furthermore i am now afraid they must be pulled up to 5V and not to 3,3V as in my design.

Concerning the CEC circuit i also would like to ask anybody (best one person of the RPI-Team) to help us concerning the functionality.

Best regards
drb_oliver

gsh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
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Re: RPiCM HDMI ESD Protection

Wed Nov 12, 2014 10:20 pm

The BCM2835 already has built in ESD protection on the data lines, that's why you don't need the protection there...

It's just the I2C and CEC lines where they're needed

The HDMI I2C is 5V tolerant (and should be pulled to 5V)

CEC circuit is correct and tested... The reason it was changed was to avoid dragging the pin low when you power off the Pi (it will break CEC for everthing if you power off the old Pi)
--
Gordon Hollingworth PhD
Raspberry Pi - Director of Software Engineering

mikeyoung
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Location: Melbourne Australia

Re: RPiCM HDMI ESD Protection

Thu Nov 13, 2014 5:22 am

Hi Gorden

Thanks for clearing up the issue with the CEC signal. Now I know its for signal isolation it makes sense.

Regards
Mike

PlanB
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Re: RPiCM HDMI ESD Protection

Sun Jul 12, 2015 2:30 am

I've managed to blow up 3 or 4 RPis, models ranging from the B (just 2 USB ports) to the B+ to the quad core. It always seems to happen when I plug or unplug the HDMI without removing the Pi 5v power first. The symptoms are always the same, increased current consumption, hotter main processor, no more HMDI output. I'm curious why there is so little to be found when I google this topic? Wondering why I'm so cursed?
When I first saw the Pi I thought what a great idea the HDMI port would be for diagnostics in the field (leave verbose mode on in python, just plug a portable monitor in & see what the machine is up to) but it has proved to be a bit of millstone. Starting to think more & more about carrying a laptop & a cat5 cable around & SSHing into the PIs instead. Really interested to learn more about this problem. Have a growing pile of dead PIs I'm happy to post to anyone in the PI inner circle who might be interested in diagnosing what hardware is failing.

AlessandroP
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Joined: Thu Aug 20, 2015 10:31 am

Re: RPiCM HDMI ESD Protection

Thu Aug 20, 2015 10:56 am

Hi Gordon.

I'm developing a board wih the Compute Module. The HDMI I2C is 5V tolerant, but I would like to know if HDMI I2C works correctly with 3.3V. In particular, in the datasheet of BCM2835, what is the value of "VIHmin" for the I2C of HDMI?
Thank you for the support.

Regards,
Alessandro.

gsh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
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Re: RPiCM HDMI ESD Protection

Thu Aug 20, 2015 12:50 pm

It'll work fine for 3v3

This is how it was tested originally.

Gordon
--
Gordon Hollingworth PhD
Raspberry Pi - Director of Software Engineering

maltk
Posts: 2
Joined: Sat Dec 08, 2018 12:51 pm

Re: RPiCM HDMI ESD Protection

Fri Mar 08, 2019 3:17 pm

Hi,

4 years later, we have burnt out 4 RpiCM3's because of the same HDMI problem mentioned here.

Is the solution to fit the ESD protection circuit found on normal Raspberry Pis onto our CM3 host board ?

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