AnsweredAssumed Answered

STM32VL Discovery -> Strange Voltages

Question asked by ro.dru on Nov 16, 2013
Latest reply on Nov 17, 2013 by Proteus
So far from the help here (mainly clive1) I got my timers running, GPIO initialized and some demo code running.  The device I am trying to control is not recognizing the data I am trying to send to it so I dug a little further with the scope.  The timing is alright but the voltages are strange.

The signal coming out of PC13 has a signal level from 0 volts to 2.3 volts.  I checked the 3.3V pin and it is at 3V.  I tried different cables, USB power supplies and none of these will change the voltages on the board.  The 5V pin has 4.72V on it and this does not change as well when swapping out supplies.

So why is the regulator putting out 3V instead of 3.3V?  Since this is the first STM32 board I have worked with I am not sure if the 3V output is typical?

I checked pin PA9 and it will output a 2.9V high instead of the 2.3V on PC13.  So if the regulator was actually putting out 3.3V I am sure this would be close.

I am eventually going to be using all three USART's on board so I was trying to find a pin that would not prevent me from using them.  I found through the schematics and data sheet that pin PC13 is not connected to anything and is not related to any USART pins.

After digging into the data sheets a bit more I found this clue:

PC13, PC14 and PC15 are supplied through the power switch and since the switch only sinks a limited amount of current
(3 mA), the use of GPIOs PC13 to PC15 in output mode is restricted: the speed should not exceed 2 MHz with a maximum
load of 30 pF and these IOs must
not be used as a current source (e.g. to drive an LED).

I am driving a data bus and not a LED but still this probably explains why the voltage is much lower.  The data bus is through a long cable which probably exceeds the rating for this pin.

I am sending data to a 5V device so the 2.3V should be above the minimum but it is so close that this could be why it is not receiving the data correctly.  I have a logic level converter to include if things really get out of hand but I was hoping to avoid this based on other peoples experience using 3.3V devices to send data without it.

So my questions are:
- Is what I found in the data sheet correct: that pin is has a lower voltage output due to the construction of the chip.  It goes through a switch so this could explain the voltage drop.
- why 3V?
- When you do get a development board that is supposed to be 3.3V and it is 3V what can you do?  The only thing I see is firing up Eagle and building a custom board and including a regulator design that correctly puts out 3.3V.  This could be just a one off problem for this board and if I connected another disco it would be closer.  But I guess at the end of the day this should not matter since the 3V still falls within the acceptable range for 3.3V comm and a good design would not require exactly 3.3V....