Square wave from \RESET = bad chip?

[yev]

I just got a DragonFly 9S12C128 DIP from Wytec (http://www.evbplus.com/c32_modules/DIP40.html). I connected it to a clean +5V supply and a known working BDM. Upon reset, the BDM power LED glows but the BDM doesn't send anything over to my terminal.

I connected my scope to the XTAL, EXTAL, and \RESET pins on the DragonFly. Very oddly, both clock pins were flat at ground, and \RESET showed a square wave with a period of about 0.75 ms at 30% duty cycle. This tells me that the chip is resetting itself as soon as it goes through its POST. Since the DragonFly is a convenience chip which takes care of the clock generator and power rail filtering, I should in theory only need to supply the DIP with a steady power source, and leave all the other pins unconnected; this, I have done. I also tried briefly shorting MODC / BKGD to ground, and this brought \RESET up to a stable 2.5 V, but XTAL and EXTAL remained at ground, and the BDM still did not give any output. It seems to me that the DIP is bad -- or I'm just overlooking something obvious. In either case, I'd really appreciate some insight!

[SteveRussell]

No signal on EXTAL indicates no external clock.  The Freescale documentation says that the reset value of the SCME bit in the PLLCTL register is 1, so the MCU will start up in Self Clock Mode, using the on-chip PLL for a clock. 

For the PLL to operate in a stable way, you need to have VDDPLL bypassed to VSSPLL and VSSPLL connected to the other VSS pins and ground.  In addition there needs to be a suitable PLL filter connected to the XFC pin.

Even if the MCU is operating in self clocked mode, you will not be able to get reliable BDM communication, because the clock frequency is unknown and not stable.

In addition to possible bad MCU, could have a problem with the crystal or other components connected to EXTAL and XTAL.  You could also have a bad solder jiont somewhere in the above circuits...

It is also possible that your scope probe is adding enough capacitance to prevent the oscillator from oscillating.  Do you get the same /RESET behavior with nothing on EXTAL and XTAL?

[yev]

Thank you for the suggestions! It turns out that PEBCAK: I had been using the VDD1/VSS1 pins of another HCS12 as my VCC/GND pins on the DragonFly. But those pins are at 2.5V instead of the necessary 5V. Switching to the host's VDDR/VSSR pins, which are at 5V, fixed the problem. And even though the DragonFly includes an 8MHz crystal & associated passives, XTAL and EXTAL are still at ground, so I assume the chip is defaulting to the PLL like you said. Checking PE4 / ECLK, I find a nice steady 4 MHz square wave. So, everything works out. Hopefully this thread will help anyone else who has reset issues, for one reason or another. Cheers!