nRF52840 SPI MISO is actively driven

RE: nRF52840 SPI MISO is actively driven

Dave Littell — Fri, 21 Nov 2025 18:01:29 GMT

Thanks for your attention. It turns out the board had some misbehaving SPI Slaves that were driving MISO in spite of their CS signals being negated.

RE: nRF52840 SPI MISO is actively driven

Håkon Alseth — Fri, 21 Nov 2025 13:35:09 GMT

Q1: Have you tried using other GPIOs than P1.08 to P1.10 ?

Q2: Have you tried to switch the MOSI to MISO (in firmware), to see if there is any other pin conflict that could explain why it does not sample?

Ie. if MOSI outputs on P1.10, and MISO does not sample correctly on P1.09;

will then MOSI output correctly on P1.09, and MISO sample correctly on P1.10?

Q3: What is being read by the nRF when you loopback? Please provide any runtime logs.

Q4: how did you measure that MISO is driven by the nRF? please share any details on your test setup.

Kind regards,

Håkon

RE: nRF52840 SPI MISO is actively driven

Dave Littell — Fri, 21 Nov 2025 13:25:05 GMT

OP second sentence: "In both cases there are no active SPI Slaves connected - just a loopback connection from MOSI to MISO."

RE: nRF52840 SPI MISO is actively driven

Håkon Alseth — Fri, 21 Nov 2025 09:15:08 GMT

Hi,

nRF SPIM module does not drive the MISO, but you can enable a pull in the pad itself (pull-up/down of approx. 13 kOhm).

[quote user="Dave Littell"]Thanks for the reply but 15K shouldn't be enough to defeat a loopback from MOSI, right?[/quote]

The loop-back should be able to drive such a load.

You have two cs-gpios defined, and two sensors, are these connected now? If yes, can you double-check the orientation of their cs-gpios to ensure that they are indeed idle when testing this loopback scenario?

Kind regards,

Håkon

RE: nRF52840 SPI MISO is actively driven

Turbo J — Fri, 21 Nov 2025 08:23:50 GMT

Only when MOSI is actively driven in an SPI transaction.

I don't remember whether the MCU drives MOSI at all after SPI transactions.

RE: nRF52840 SPI MISO is actively driven

Dave Littell — Thu, 20 Nov 2025 22:11:13 GMT

Hello?

RE: nRF52840 SPI MISO is actively driven

Dave Littell — Tue, 18 Nov 2025 20:04:59 GMT

Thanks for the reply but 15K shouldn't be enough to defeat a loopback from MOSI, right?

RE: nRF52840 SPI MISO is actively driven

Turbo J — Tue, 18 Nov 2025 19:13:45 GMT

You have the "bias-pull-down" option enabled. The pullup and pulldown resistors in NRF chips are unusually strong (see PS), around 15K Ohms.

This won't look like an open line in the oscilloscope but more like an inverted I²C signal. And yes, those pulldowns are always active when you configure the bus pins this way.