When the DA14580 is connected to the SPI bus, it seems to be adding a weak pull-down on the MISO line. The MISO is shared with two other devices, has a 100k pullup resistor to 3.3V, and is also pulled high by the microcontroller's weak internal pull-up. The end result is that the line idles at 2.8V instead of 3.3. This is true whether or not the DA14580 is held in reset via the RST pin. This symptom disappears if I cut the DA14580's MISO trace, and comes back when I repair the trace.
This is happening in Dev Mode, without any external program source connected. I am intending to use Dev Mode to download code from an external microcontroller. I am using AN-B-001 for a reference. I have the SPI Master lines set up as shown in "Step 2" of this document, Table 1.
Is this behavior expected in Dev Mode? In normal operation, of course, the MISO line must be held at high-impedance by all (inactive) SPI slave devices. I haven't developed any program code for the DA14580 yet, so I am unable to test this myself.