TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

RE: TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

Joao David Silva — Mon, 12 Aug 2024 09:36:29 GMT

Hi Simon,

Probing the lines with a logic analyser didn't quite clear thing up. Everything looked as expected but the client devices' response didn't make much sense. We only got the expected response around 20% of the time.

However, we did manage to solve the issue. Using an oscilloscope, we saw a lot of ringing/reflections in pretty much every edge of every signal on the high voltage side of the voltage translator, as well as some cross talk from CLK into MISO and CS. The logic analyser wasn't catching the cross-talk and the ringing (glitch filter was on).

The TXB0108 seems to be particularly sensitive to either reflections, capacitive loads, cross-talk or cables with higher impedance. What eventually solved our issue was to shorten the cable connecting the high voltage side of the TXB0108 to the sensor from 50cm to 40cm, twisting the signal wires with GND and VCC (CLK with GND, MISO with VCC, MOSI with VCC, etc), and shielding the cable with copper tape. With this new cable, everything started working as expected with no ringing (perhaps the edge-rate accelerator was being triggered by it).

Ideally we would have just bypassed the cable and shortened the connection as much as possible, but the sensor we are using doesn't allow for that. We are stuck with using a cable for SPI communication with this specific sensor.

During our testing, we also found out that the TXS0108E worked flawlessly while using the longer unshielded cable. It seems to us that this voltage translator is a lot more forgiving than the TXB0108.

Moving forward, we will swap to the TXS0108E and a properly twisted/shielded slightly shorter cable.

On a side note, we didn't do any follow up to the OE signal suggestion. Keeping it tied high doesn't seem to be affecting our sensor.

Thank you all for your feedback!

RE: TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

Simonr — Thu, 08 Aug 2024 12:51:49 GMT

Nice catch Hugh! That is a good point.

RE: TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

hmolesworth — Wed, 07 Aug 2024 17:37:00 GMT

The pull-up on OE looks dodgy; maybe test tying low:

"The TXB0108 is designed so that the OE input circuit is supplied by VCCA. To ensure the high-impedance state during power-up or power-down, OE should be tied to GND through a pulldown resistor; the minimum value of the resistor is determined by the current-sourcing capability of the driver"

RE: TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

Simonr — Wed, 07 Aug 2024 06:04:06 GMT

The schematics looks okay to me, and I can't say I see why this should cause trouble for the SPI communication. Are you able to probe the data lines to see what's happening on the lines themselves with a logic analyzer?

On the DK, what states are the switches set to here? As the power source for example should be set to Li-Po if you power it on P27 for example.

Best regards,

Simon

RE: TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

Machado94 — Mon, 05 Aug 2024 13:44:17 GMT

Hi Simon, thanks for your prompt reply.

Yes, we are using the P27 (Li-Po connector) with the nRF power source switch set to Li-po to supply at 3.3V.

Follows attached the pdf and the voltage translator data sheet you requested.

devzone.nordicsemi.com/.../shield_5F00_schematic.pdf

devzone.nordicsemi.com/.../txb0108.pdf

Best regards,
Pedro

RE: TXB0108PWR Voltage Level Translator + nrf52840dk + SPI issues

Simonr — Mon, 05 Aug 2024 12:22:16 GMT

Hi Pedro

Can you upload the schematics as a .pdf instead of SchDoc file. Do you also have a link to the datasheet of the voltage translator you're using so we can check its specs? To power the nRF52840 SoC on the nRF52840 DK with up to 5.5V, you should power the board through J6 or P27 (Li-Po connector).

Best regards,

Simon