ESP32 – Stock Firmware Disables JTAG

I’ve been working on a project recently with the ESP32.

I purchased 4 raw ESP32-WROOM-32 modules from Digi-key, soldered them to my hardware, and then attempt to flash them over the JTAG interface. Every attempt of programming the module left me with the dreaded all ones seen blah blah blah error.

Error: JTAG scan chain interrogation failed: all ones
Error: Check JTAG interface, timings, target power, etc.
Error: Trying to use configured scan chain anyway...
Error: esp32.cpu0: IR capture error; saw 0x1f not 0x01

All of the existing documentation online suggested a hardware fault so I went into hardware troubleshooting mode until I became fully confident the hardware was not an issue. After much thrashing around, I finally noticed the module were pre-programmed as it was dumping data over the UART port. I managed to re-flash the hardware using the UART interface instead and then the JTAG interface was accessible.

This left me with the following understanding:
1. It’s possible for the firmware to “brick” the JTAG interface
2. ESP32 modules from espressif can potentially come with firmware pre-installed.

I’ve posted to the Espressif Forum here: https://www.esp32.com/viewtopic.php?f=2&t=12600&p=50119#p50119

 

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