3 Replies Latest reply on Mar 4, 2017 4:00 AM by brsi_1319346

    Xres being driven low... but not by me... ?


      Hi I have a strange issue. 


      I have a fairly simple psoc4 (cy8c4248 44 tqfp) design that is adapted from the motor drive reference (cy8ckit-042) and it's been working OK until under load. Under load, there have been a few tearful moments, exploding FETs etc, and I've not been able to put my finger on how and why. 


      One things that is consistently happening (other than smoke) is the the xres pin drives LOW, like the psoc4 is actively driving it. The parts aren't dead, and if I short xres to 3.3V they seem to work, I can talk to them using I2C and the bridge control software when xres shorted to 3.3V. This has happened on perfectly good units that have not leaked smoke from the h-bridge fets, and on ones that have suffered damage on the power drive stages. It seems to happen after a period of use/testing, never straight up. 


      So I'm thinking that the xres problem, might be causing a chip reset at a bad time, tristate drive, destroying my fets which aren't turning off gracefully. (I'm correcting that on next pcb rev of course but still need to hunt down this problem.) That was different to my initial thought that the even that killed the fets, also killed the psoc and xres low was chip damage. 


      The impedance of the low drive is strong but not short circuit, < 1k, probably less. A short to 3.3V does not make anything hot, I see now permanent issue. When I short xres to 3.3V up to 35mA can run (!) but then decays to 0. The xres pin has a top and tail BAT54 for a little more protection, and no capacitor. 


      Any ideas? POR, BOD? damage to the device? System sees 24V for testing, has been tested for a short period with 42V.


      What kind of failure modes are there that look similar? Any errata? Or is there something like the p1[6] pin xres behaviour on psoc 4000 parts? 




      A lot of thanks in advance for your help.

        • 1. Re: Xres being driven low... but not by me... ?

          Hi Brent,

          The issue seems to be with your system. The problem should be with your PCB. Some how the internal pull up of your XRES has been fused off. You can check with an external pull up (do not short). I am also thinking that the exploding FETs and all are because of this, as you deduced. If you could share your schematic, we can check what is the issue.

          Best Regards,

          • 2. Re: Xres being driven low... but not by me... ?

            Hi VSRS,


            I think that the XRES is being driven. This is because I can use a 1k pull up, and it makes no difference. The impedance to GND is quite low... can't think of another reason except that it's being driven by a source. The absense of a pull up would just give a floating pin right? 


            Or is it possible it's fusing to gnd... ?  Would a negative voltage on that pin latch it down? 


            Sch coming..

            • 3. Re: Xres being driven low... but not by me... ?

              I won't bother putting sch up, found the issue. Looked back through notes found that there was a problem with a dual bat54 package burning out. It was a solder fault that connected 24v to either gnd or 3v3 under the package and the trace blew off.


              This has eigher negative biased the PSOC by connecting gnd to 24V or just over voltaged it 3.3V becoming 24V until the fuse current burnt the short out. The device held on for a little while then the XRES let go... It also seems that it has only happened on one board reused in two systems, hence my though it was two chips failing.


              I'll replace it and see how things go.