Yesterday I wanted to see, how much power my EFM8LB1 uses, when it has just this firmware with bootloader disabled:

.org 0
ljmp 1$
orl (REG01CN), #8
orl (PCON0), #2

Today I found, that I cannot access it via C2 anymore. I tried keeping it in reset when powering on and then sending C2 commands... without success...

How can I fix it without dumping the MCU and soldering a new one?

How does that EFM8LB1 decide if the reset means "now we do C2" or "now you run the reset ljmp"?

  • 1
    \$\begingroup\$ check the datasheet \$\endgroup\$
    – jsotola
    Commented Apr 23, 2021 at 21:35
  • \$\begingroup\$ @jsotola i did. but: the datasheet is not very clear about my questions. after the reset it seems like the MCU starts to turn everything off, before i can send the "halt core" byte. but i dont understand why, because i only wait less than 5usec after reset and the code execution starts allegedly after appr. 50usec. \$\endgroup\$
    Commented Apr 24, 2021 at 5:52

1 Answer 1


seems like it is possible to "brick" a EFM8LB1 and EFM8UB2... https://www.silabs.com/community/mcu/8-bit/forum.topic.html/efm8ub2_efm8lb1_noc2afterstop-PSub

not even a power-on-reset delay of >3msec helped in my case: https://skybluetrades.net/unbricking-efm8ub3-thunderboard/


  • \$\begingroup\$ Have you tried the suggestions in the last messages of: silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2015/… ? \$\endgroup\$
    – devnull
    Commented Apr 25, 2021 at 12:06
  • \$\begingroup\$ @vangelo sure... i even used a pnp BJT and observed the voltage rise (it took appr. 800usec) and then i wrote the magic bytes and then i tried to erase the chip, but even that failed, although it needed less than 1500usec. It seems like it does not initialize the SFRs immediately after RST/C2CK goes high. \$\endgroup\$
    Commented Apr 25, 2021 at 12:54

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