Bootloader hardware requirements

[var2]

Posted on March 15, 2014 at 12:11

In the AN2606 11.2 Bootloader hardware requirements, one can read:

''The RX pins of the peripheral unused in this bootloader have to be kept at a known (low or high) level, and should not be left floating during the detection phase''. Usually we connect pins not directly to GND or VCC, but with resistors. The question: what is the maximal value or resistance, not to be recognized as ''floating''?  

[Tesla DeLorean]

Posted on March 15, 2014 at 14:17

I'm sure 100K would be fine.

If you have other interfaces connected to the USART's etc, that would be fine too. The key is not generate transitioning signals the system loader would think are attempts to send something. The 0x7F sent to the system loader is never actually received by the USART, a TIM is used to measure the pulse width, and compute a baud rate for subsequent communication.