SPI CS not really dropping the voltage

General
1

tl;dr: I’m missing a connection

Hi! Brand noob to Bus Pirate (v5), and I’m trying to test an SPI master-to-sub connection. The BP5 is the master. Vout isn’t connected.** I have MOSI(7), MISO(4), CS(5), SCK(6) all connected, and double checked, and without connecting the CS probe to the sub’s SPI CS pin, (NXP LPC832, if it matters), I see the voltage rides around 3.5V on the 'scope. If I connect the CS probe, the voltage drops down to 1.5v, and then if I send “[ 2 ]” in the SPI mode prompt, the CS voltage doesn’t drop enough to trigger the sub. Best I can tell, it drops about .1 of a volt. Why isn’t it working?

** In the “your topic is similar panel” I saw “SPI Setup - how do I connect the BP,” and the OP had Vout connected. I misunderstood what “Vout” actually means - I thought it suggested a voltage out - so I didn’t connect it. When I connected it to my 3.3v supply, or what would be “voltage in,” the testing works. So, user error on my part. But I’m leaving this here so you can all point and laugh at the noob, and hopefully some other traveler will learn from my mistake.

2

Are you trying SPI in-circuit or the chip is desoldered? If its thedoldered than sure you need to connect Vout if you power it up externally (by powering the board) or by PSU then connect Vout to that power source but dont enable “W” in the BPv5 console so it finds the reference voltage

Also CS (chip select) needs to be connected. Could you show a picture of your connection setuo?

3

Glad you got it.

VREF_VOUT serves two purposes:

  1. Enable the onboard power supply with the W command and it is a power out for the the IO buffers and optionally the device you’re using.
  2. As a reference voltage from an external device which will power the IO buffers.
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