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I am trying to expand the number of UART ports I have available on a nRF52832 microcontroller using an SC16IS752 attached by SPI. However I don't manage to get responses back from the SC16IS752.

When I set CS to a low level SC16IS752 raises the MISO line to a high level. It stays at that level until after CS went high again. So I'm always continuously reading 0xFF on the SPI MISO line. It kind of looks like SC16IS752 is not seeing any clock signal on CLK. However I can measure the CLK signal of 4 MHz is present..

Any idea what else could result in this behaviour of the SC16IS752?

Schematic

Example Communication

enter image description here

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  • \$\begingroup\$ MISO is the output .... you have to clock commands into the MOSI pin after CS goes low .... the chip does not "know" what to do until you tell it what to do \$\endgroup\$ – jsotola Aug 23 '18 at 16:29
  • \$\begingroup\$ It's a little difficult to tell form your screen cap, but it looks a lot to me as though you've configured your micro for the wrong SPI mode. The SC16IS752 is expecting SPI mode 0 where the MOSI data line is stable on rising edges of SCK, but it looks as though that's the edge the data is changing on. \$\endgroup\$ – brhans Aug 23 '18 at 16:40
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The problem was that I had low level at the RST line, so SC16IS752 was in an endless reset.

Actually I had already measured the level at pin 2 and thought it was high. But it seems, that I measured an incorrect pin. After redoing all the checks and measurements, I noticed that actually the RST line is low. So I added a 1K pull up and SC16IS752 is now responding to what I'm sending on SPI.

Now that I know that, it seems to be plausible, that there is no internal pull-up on this line. You may want to control several chips with the reset signal, and when all of these would have internal pull-ups it would be a problem.

Thanks for your help.

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