ezsbc / samd21 Goto Github PK

Just a friendly FYI for any future PCB revisions:

At the moment, the EzSBC board has a 100k pull-down "R2-2" that is in conflict with the internal pull-up on SWCLK. One can see that the signal is mid-scale by probing the signal. CMOS inputs generally don't like mid-scale signals, and this resultant resistor divider circuit adds a bit more current consumption too.

A pull-down is not desirable, as the SAMD21 has a feature called "CPU reset extension" (see Section 13.6.2) where SWCLK is sampled when the external reset is triggered. That is why section 39.1.1 of the SAMD21 datasheet states "a pull-up resistor on the SWCLK pin is critical for reliable operations".

So, perhaps it is worth considering replacing the pull-down with a pull-up?