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Programming problem on Open407I-C with LCD

Question asked by miller.danny on Dec 17, 2012
Latest reply on Dec 18, 2012 by miller.danny
I just started with the STM32F407 Open407I-C board:
http://www.wvshare.com/product/Open407I-C-Standard.htm

It's a pretty excellent demo board.  Very little pin commitment on the board, most are just straight brought-out to 0.1" pin headers for the many cheap accessories to be plugged in.  LCD, camera, ethernet phy, USB phy, etc.

The thing I love is there's straightforward, fully working demo code for EVERYTHING for EVERY board they make, including a Keil UVision project file.  Unfortunately, very little of that can be downloaded from their website, it's on a DVD you get with the motherboard.

Anyhow, my problem is this.  Anytime I have the LCD plugged in, my Segger J-Link fails to program or debug with this:
"No Cortex-M Device found in JTAG chain"
Without the LCD plugged in, it programs fine through the J-Link, but there'd be no way to debug.

So I contacted their support (broken English) and got:
"The RESET pin will be shared, use the SWD mode to avoid it, thanks!!"

Well that's not a good solution.  I'm not clear on what the consequences are of using the SWD, and I need to be able to program without removing the LCD each time!  There's the USB OTG port on the core breakout board containing the STM32, but I've got a Segger J-Link right here.

I'm confused.  Yes, the LCD's Reset pin (active-low AFAIK) was tied to NRST (pin 31 of the 144-pin STM32F407), which doesn't make a lot of sense since the controller doesn't drive NRST AFAIK and it's not under software control.  It's an input to the LCD, and an ohmmeter shows only a 5meg pullup resistance to 3.3v on the LCD hardware itself.  

So, any idea why the LCD would be interfering here?  I don't get it!  The only thing I could think of was maybe the demo code temporarily turned NRST into an output to reset the LCD alone from the STM32 (I'm unfamiliar with these STM32 hardware features, that may not make sense), and if the LCD was installed, maybe the code left it high but DRIVEN high (not high impedance).  Which would be an error.  I found nothing in the code so far that suggests this, but the code's fairly complicated.

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