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Reset_on_Connect.md

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Fixing Reset-on-Connect Issue / Broken Reset Circuits

Regarding to Issue #14

Build-In Reset Circuit

The original NodeMCU DEVKIT includes a build-in circuit to pull down the reset (RST) and/or flash (GPIO0) pins via the serial connection using the DTR/RTS lines. This allows the user to execute a hard-reset or flash command via software.

Reset Circuit (from NodeMCU DEVKIT Schematics)

Reset Circuit of NodeMCU DEVKIT

The Issue

It's a very useful feature, but on some modules these circuits cause a reset on connect which make the module inaccessible with NodeMCU-Tool: on opening the serial connection software side, the module will be reset immediately and it reboots. Directly after reboot, the auto-baudrate detection of NodeMCU is capturing the serial input to determinate the used baudrate. And this is the issue: NodeMCU-Tool sends a few hearbeat messages to the module to check its state, but these messages are lost because of the auto-baudrate detection. Therefore the module is threatened as unavailable and a error message is raised.

This behaviour can be caused by a hardware issue: wrong transistors, wrong series-resistors or a usb driver failure, which invokes the DTR/RTS lines asynchronous

Timeline

  1. NodeMCU-Tool is establishing the serial connection
  2. DTR/RTS lines are invoked
  3. NodeMCU will do a hard-reset or go into flash mode
  4. NodeMCU-Tool sends heartbeat sequence
  5. NodeMCU Auto-Baudrate Detection is capturing the input or the module is not ready yet the receive incoming data
  6. NodeMCU-Tool will not receive the hearbeat response - the module is threatened as unavailable

How to fix it

NodeMCU-Tool v2.0.0 includes a special delay flag --connection-delay N which waits N milliseconds after establishing the connection and sends additional dummy sequences to trigger the auto-baudrate detection.

Timeline

  1. NodeMCU-Tool establishes the serial connection
  2. NodeMCU-Tool is waiting N milliseconds until the hard-reset/reboot is done
  3. NodeMCU becomes ready and starts the auto-baudrate detection
  4. NodeMCU-Tool sends dummy sequence 1 to trigger the auto-baudrate detection
  5. NodeMCU-Tool is waiting N milliseconds for baudrate detection
  6. NodeMCU-Tool sends dummy sequence 2 to trigger the auto-baudrate detection
  7. NodeMCU-Tool is waiting N milliseconds for baudrate detection
  8. NodeMCU set its baudrate based on the previous dummy sequences
  9. NodeMCU-Tool starts the regular heartbeat detection

The connection delay is applied 3 times, between each step. A value between 300...500ms has been approved as best option.

Example 1

$ ./nodemcu-tool upload helloworld.lua --connection-delay 300 --baud 115200