For CEC peripheral what kind of trigger can cause TXERR set to one

[NRAML.1]

I am developing product using STM32H7A3 series MCU.

In my product we configure CEC peripheral on the MCU in order to use it.

I follow the steps mention in RM.

I conduct a CEC compliance test, and some error detected on CEC_ISR register.

My I know, what can cause the bit TXERR to become 1(meaning transmission error)?

----explanation in RM------

Bit 11 TXERR: Tx-error

In transmission mode, TXERR is set by hardware if the CEC initiator detects low impedance on the CEC line while it is released. TXERR aborts message transmission and clears TXSOM and TXEOM controls.

TXERR is cleared by software write at 1

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It mentions "low impedance detected" but what can cause the low impedance detected?

[SofLit · Accepted solution]

Hello,

It could be something related to your Hardware. Did you connect a 27k pull-up resistor on the CEC line?

You can refer to the AN4066 "Developing an HDMI-CEC network using an STM32F0xx microcontroller " / Section 2 Hardware environment.

And maybe also to inspire from STM320518-EVAL HDMI-CEC schematics from here:

[Andrew Neil]

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@NRAML.1 wrote:

what can cause the low impedance detected?

---

A short circuit.

Something actively driving the line.

[NRAML.1]

I had confirmed the circuit for CEC line. Yes, I connected the line with 27k pull up. 

[NRAML.1]

The test is to check CEC line can response correctly withing the limits of the specification.

Test condition:

Connect DUT(Our Product) HDMI out to TE(test equipment) HDMI in. Result are shown as below.

Do you have any idea what is going on. Thank you for the advice in advance.

[SofLit]

Hello @NRAML.1 ,

Your request has been escalated to the ST Online Support Team for additional assistance.

[NRAML.1]

Thank you for this advice.

After discussing with the hardware designer, confirm that the PULL UP method need to be revise.

Problem solved after the rework done to CEC line.