When I touch the SCL pin with my finger the SPI works

[jean_prieur]

Posted on September 11, 2013 at 17:32

Hello,

I programmed a LCD screen with SPI on a prototype and everything was working fine. But I mounted

 the screen on the final PCB and now I have a 

problem: the screen didn't understand the datas I send to it. The screen 

remains white. Datas are correctly send, I know it because I compare signals 

with my working prototype.

When I touch with my finger the clock pin, the screen is able to 

understand datas and works fine.

Do you ever had this problem ? I am sure of the screen connexions and 

I tested it with different SPI speeds and configurations...

Thanks !

[jj2]

Posted on September 13, 2013 at 01:12

Dave: ''Sounds like your clock line could benefit from a close examination with a good scope...''   (Suspect you may be the ''source'' - on sister fora...)

Surely you don't mean to impugn the value/capability of a ''finger'' as test probe... Mating an M4/F4 with such non-standard methods... little to be said...

[dthedens23]

Posted on September 13, 2013 at 17:32

The data sheet specifically mentions attention to line-end reflections and noise on SCL.

Some Arduino users mention using internal pull ups.  Not sure that is a good idea.the finger does add R/C which can effect ringing/overshoot/rise time/....you just have to probe with a good scope.

[jean_prieur]

Posted on September 15, 2013 at 13:07

Hello everybody,

I find the problem: it was another SPI device linked on 

the same CLK and SDA pins who caused troubles. When I remove this device (this is a hard to remove device because of his package, this is why I have been slow to disconnect it), the 

screen works perfectly. 

I hope the screen is able to share the SPI with another devices.

This not resolved the finger mystery, I try to put end line resistors or capacitors but no effects...Anyway thanks for your advices !