SPI at 1.8v, MISO line lowers the voltage

[kamal]

Device nrf52832 Custom board and nrf52DK with nrf52832

SDK 17.0.0

IDE Segger

when SPI interface is initialized by nrf_drv_spi_init, then MISO line behaves as follows, 

when operating at 3.3v MISO line reads 3.3 v, and after spi init voltage drops to 2.9v and spi interface works, but I need to run system at 1.8v, and then MISO line reads 1.8v , but after spi init MISO line voltage drops to 1.05v.

MISO line tests I done at 1.8v

- MISO has 0v when nothing connected to it.

- SPI interface reads 0xFF when I connect a pullup of 4k7 between 1.8v and MISO line. (this means that spi is reading data from MISO). Note:- pullup volatge on MISO line drops from 1.8v to 1.35v (looks like some load is connected to MISO internally after SPI init)

- If I connect slave to MISO with pullup then SPI receives 0v

also dis the same test on nrf52dk board, spi works, but voltage drop happens there too. 3.3v to 2.9v

this voltage drop happens when I initialize spi. I am using spi example from sdk

Note:- MISO line itself always shows 0v, this voltage 1.05 and 2.9 comes when connected to slave

slave device is MAX6675 board, which works fine when nrf52832 running at 3.3v , and I am using the MAX6675 through sparkfun levelshifter all the time.

please guide

thanks

kamal

[haakonsh]

I think you're overloading the SPI lines. I suggest you look for abnormal current consumptions on your board.

[kamal]

thanks haakonsh for your prompt reply,

sorry I didn't understand how I am loading SPI, when only MISO from slave is connected to this pin.

MISO line droping voltage after spi init, this behaviour I tested on P0.30 and also P0.11

when nothing connected to MOSI line voltage is 0

when MOSI line pulled up with 4k7 from 1.8v power, then before spi init it shows 1.8v (normal), but after spi init voltage drops to 1.35, I am using spi example code.

As per SPI interface's common definition, MOSI is input to master so on master side, it should be load free, but in my case its droping voltage.

2 different pins but same behaviour, by spi init

Abnormal Current consumption, i didn't saw change in current consumption

I read at devzone, some people are talking about wrong intialization of spi, but didn't find more info

please guide

thanks

[haakonsh]

Why are you using a pull-up on the MISO line? 

Does the issue exist without the pull-up?

[kamal]

hello haakonsh

pull up is just to prove that MISO line is loading internally after spi init,

without pull up, and slave connected

- before spi_init, MISO line shows 1.8v

- after spi init, MISO line drops to 1.8v (without any ongoing transfers)

without pull up, slave MISO line disconnected from cpu

- slave side voltage is 1.8v

- CPU (nrf52832) side is 0v

one more observation

pin starts behaving normal after spi uninit

thanks

[kamal]

mistake in line 5 in last post

- after spi init, MISO line drops to 1.05v (without any ongoing transfers)

thanks

[haakonsh]

Have you configured the MISO pin on the nRF as a GPIO that you use for other things when you're not using it for SPI?

Do you have a scope of the SPI lines?

[kamal]

its working now, on custom board because of long lines, only 125K speed worked for me, thanks for your help