I have multiple mcu's that are configured to communicate with bus topology. I have 10 pins (4 of them is occupied by now)

I need to debug (update firmware online, etc) all of these mcu's. I read this article but I didn't understand how to achieve the goal.

Question: How can I daisy chain these mcu's?


Debug protocol: SWD

  • \$\begingroup\$ Your development tools need to support it. \$\endgroup\$ Mar 8, 2014 at 12:24
  • 1
    \$\begingroup\$ I need the algorithm, I can build the development tool. \$\endgroup\$
    – ceremcem
    Mar 8, 2014 at 16:26
  • 1
    \$\begingroup\$ I think that you need to license that from ARM. \$\endgroup\$ Mar 8, 2014 at 17:06

2 Answers 2


How can I daisy chain these mcu's? MCU: STM32F407VG Debug protocol: SWD

These MCUs support JTAG and SWD. While JTAG supports a daisy chain configuration, SWD does not. There is a multi drop extension in SWD 2.0 protocol, but I am pretty sure your MCU does not implement this.

If you really want to debug multiple chips in a daisy chain, use JTAG. Beware: Tool configuration(s) may be non-trivial to do.

Note that you might want to separate a firmware update from your debugging scenario. Firmware updates can be done in multiple ways using a bootloader, so SWD or JTAG is not required in this case.

  • \$\begingroup\$ I need to be sure that I can not brick the device after a firmware update. I can fall back to the previous firmware if anything happens as far as I would use a debug port (JTAG or SWD). I'm not sure if bootloader option gives me that opportunity. My emergency plan is as follows: I can create point to point routes between mcu's and use every mcu as a proxy. \$\endgroup\$
    – ceremcem
    Mar 8, 2014 at 19:27
  • \$\begingroup\$ Here is the last solution we have: github.com/aktos-io/stm-flasher This is a command line utility that loads the firmware over uart. \$\endgroup\$
    – ceremcem
    Apr 15, 2016 at 0:21

I use mux chips, such as 74LVC1G18 or 74LVC1G19 for clock, and DG4157E for data. (Data requires an analog switch because it is bi-directional.)

There are of course higher-radix mux chips in case you want to program more devices from the same programmer.


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