Hello
The question is: why does the TRF7960A EVM turn on???
When the board is powered on, the EN pin on the TRF7970 is low, since the MCU has not yet started (not supplied with power) and EN2 pin is also pulled low. According to the datasheet all part should be off, including external powersupply to the MCU, when both EN and EN2 are low. Further EN cannot become high before the MCU has started. So what makes the TRF7960A EVM turn and provide the supply for the MCU, which in turn will wake and pull the EN pin high?
The background for my question is, that I have made a PCB with the TRF7970A chip supplied from a medical powersupply. The circuit will work when I bypass the powersupply and use an external powersupply, so both the TRF7970A and the MCU is working and alive. Now the suspicion would be on the medical power supply, but when I try to connect the TRF7960EVM board to same output of the as my own circuit is using, the TRF7960EVM is powering on as expected while my own circuit stays at sleep… First step to solve the problem, might be to understand, why the TRF7960EVM (and for that matter the my own board with external powersupply) is coming to life, when both EN and EN2 is low. What is it, that I have forgotten or not paid attention to?
Hope someone has an idea.
Regards Henrik