olimex ESP32-EVB does not always boot correcty

[jorn]

sometimes after power on or after programming the EVB will not load the programmed code but hang on the UART boot.
Via the terminal I'll get: rst:0x1 (POWERON_RESET),boot:0xf (UART_BOOT(UART0))
read somewhere it could be because of pin 2 not being held low during boot.
I've got a REV.k here any ideas?

[LubOlimex]

What kind of code is loaded to the board? Can you try to insert some software delay around the start of your code?

[jorn]

In short the code reads in a sensor on gpio 16, when the sensor is high it will send and recieve data over serial to/from a barcode reader (RXD gpio 36 and TXD gpio 4, serial port 2). After that a UDP message will be send over ethernet and relay 1 will be triggered.
adding a delay to the start does not seem to work. I've tried 1s and 10s and in both cases I could still replicate the problem by pressing the reset button.
I'll try other code to see if I can replicate the problem.
-edit-
able to replicate problem with: https://github.com/espressif/arduino-esp32/blob/master/libraries/Ethernet/examples/ETH_LAN8720/ETH_LAN8720.ino
by pressing the reset button
-edit2-
only seems to happen when the barcode reader is (powered and) connected to the RX and TX pins

[jorn]

I've made a (temporary) fix/bodge in hardware to disconnect the RX and TX lines until the code is loaded.
This is done by putting a 2n2222 transistor in series with the TX and RX line. Then these transistors are connected to another 2n2222 acting as a not-gate.
The not-gate input is connected to gpio 14.
During boot the RX and TX will be 'disconnected' and when the code is ready to use the UART it wil pull GPIO 14 low.
I needed the not gate because all the pins I could use are pulled high on boot.
This seems to solve my problem for now.
p.s. I'm using these pins because I wanted to use the UEXT connector and not have to solder to the EVB

[LubOlimex]

Hmmmm maybe it is voltage difference between two grounds that affects the board?

Is the barcode reader powered form another source? Can you power it from the board (or power the board from the same source as the barcode reader)?

[LubOlimex]

Also you can try to use some of the other pins from the UART if you need just two out of 8 there. Just as test. UART can be re-defined to pins #5 and #6 for example (GPIO16 and GPIO17).

[JohnS]

only seems to happen when the barcode reader is (powered and) connected to the RX and TX pins

Sounds like it may be feeding power (aka backfeeding) the board.

Add some schottky diodes?

John

[jorn]

Thanks for all the sugestions. Changing the TX pin to GPIO15 seems to fix the problem as well.