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STM32VL-Discovery: Different behavior sending over UART for same model

Question asked by on Dec 6, 2013
Latest reply on Jan 13, 2014 by sung.chen_chung
Hi together,

I have a couple of STM32VL-Discovery boards, with each holding the same firmware. The firmware essentially does nothing more then sending 8kb of data over UART.
Furthermore, I have a custom controller board, to which up to 16 STM32s can be connected. The controller board iterates over each STM, receiving its data and transferring it to the desktop. Thus, I can read out the boards in an automatic fashion.
In particular,  the board under test gets powered, the controller then first waits 100ms, then it starts monitoring the board's serial output. As long as datais being received the board remains powered. When no data has been seenfor 30 time periods (which should be roughly equal to 22 bytes over theserial port, the controller assumes that transmissions have finishedand powers up the next board.

However, the interesting observation is: that out of 10 boards, there are 3 boards for which the whole 8 KB are received. The others would only transmit a fraction (around 440 byte) before the controller switches to the next one. Thus, there must be some kind of delay for some boards sending the data even though all boards have the same firmware image and all are the same model. The code is ASM and thus should be quiet fast to be executed.

My question now is: how can it be that for some boards this works an for some it does not (always the same boards, not changing)? Is there some kind of additional delay / buffer/ caching mechanism which can be disabled?