u/Fancy-Delivery5081

Hi everyone, I'm stuck on a thermal monitoring project and have run out of ideas..
I'm building a thermal monitoring system for a BGA rework station. The goal is to monitor up to 4 Type-K thermocouples simultaneously during BGA reflow/rework operations and display live temperature curves on a connected monitor. The system runs on a Raspberry Pi Zero W v1.1 on a custom PCB, boots directly into a Python/matplotlib dashboard via systemd autostart, and is powered by a single 5V/5A supply.

Hardware:

• Raspberry Pi Zero W v1.1
• Custom PCB with 4x MAX31856MUD (TSSOP-14), assembled by JLCPCB (standard assembly service, no functional testing)
• AMS1117-3.3V LDO regulator
• Raspberry Pi OS Lite (Trixie, fully updated)

Symptoms:

• SPI is enabled (/dev/spidev0.0 and /dev/spidev0.1 present)
• All registers read 0x00 – including register 0x01 (CR1) which should default to 0x03 according to the datasheet
• Write operations have no effect (write 0x55, read back 0x00)
• adafruit_max31856 library throws no exceptions but always returns 0.0°C
• fault['open_tc'] shows False even when no thermocouple is connected (which is physically impossible)

What I have measured/verified:

• AVDD (Pin 5) → AGND (Pin 1): stable ~3.3V ✅
• DVDD (Pin 8) → DGND (Pin 14): stable ~3.3V ✅
• CS pin (GPIO8) at idle: ~3.3V (HIGH = correct) ✅
• Routing verified: Pi Pin 19 (MOSI/GPIO10) → MAX31856 Pin 11 (SDI) ✅
• Routing verified: Pi Pin 21 (MISO/GPIO9) → MAX31856 Pin 12 (SDO) ✅
• IC marking: MAX31856MUD ✅
• Pin 1 orientation (square pad): matches between PCB footprint and physical IC ✅

What I have tried:

1. Tested all 4 SPI modes (0-3) – all return 0x00
2. Hardware CS vs. software CS (manual GPIO control) – no difference
3. Bit-banging with RPi.GPIO – no difference
4. Swapped MOSI/MISO jumpers at the Pi header – briefly got realistic-looking values (once 16°C, otherwise 8-9°C) but write operations still had no effect and values never changed with temperature
5. Enabled SPI1 (dtoverlay=spi1-3cs) – OSError: Invalid argument
6. External 3.3V source (Arduino) directly on AVDD/DVDD – no difference
7. start_autoconverting() + unpack_temperature() – returns values but they never change regardless of temperature applied (heat gun, fingers, etc.)

Known design issue: Pin 1 of the Pi GPIO header was accidentally connected to AMS1117 VOUT, meaning the Pi's internal 3.3V regulator and the AMS1117 were fighting each other on the 3V3 rail. I have since cut this connection. Tested with external 3.3V supply as well – no change in behavior.

Test code:

python

import spidev
import time

for mode in range(4):
    spi = spidev.SpiDev()
    spi.open(0, 0)
    spi.max_speed_hz = 500000
    spi.mode = mode
    result = spi.xfer2([0x01, 0x00])
    print(f"Mode {mode}: Reg 0x01 = 0x{result[1]:02X}")
    spi.close()
    time.sleep(0.1)

Output: 0x00 on all modes.

My question: What could cause the IC to seemingly respond (no exceptions, open_tc False) while all registers read 0x00 and write operations have zero effect? Could this be a footprint issue even though Pin 1 orientation appears correct? Is it possible the IC is damaged from the 3V3 rail conflict? Any ideas appreciated – this has been a very long debug session! 😅

(Text translated with AI - not native english, excuse me)