Protecting MityDSP module during development
Added by Udi Fuchs over 7 years ago
We just fried a MityDSP L138F module, and I'm not sure why. I'm suspecting that the problem was that we had an ADC module connected to it (via FPGA) while the L138F module was off. I'm measuring 1.8V on some of the ADC output pins. Cans such voltage damage the MityDSP? Should I add resistors (what value) to prevent it from happening again?
When I try to power the L138F (though the evaluation board), the LEDs flicker for a fraction of a second and the DSP and FPGA chips get hot to the touch. Is there a way to salvage the module?
RE: Protecting MityDSP module during development - Added by Adam Dziedzic over 7 years ago
Yes the FPGA I/Os can be damaged by sinking too much current - powered or off. The I/Os have protection diodes to the VCCO and GND rails that can withstand a max of 10mA per pin. Adding series resistors to the I/O pins that can be powered beyond the VCCO rail will keep it safe using a design limit of 8mA. When the VCCO is powered down, the current limit must still be met. This is basically the same design methods noted for making a 5V tolerant IOB.
For an example of your case, choose a series resistor that will be near 8mA. Assuming the ADC has +3.3V I/Os...
Hope this helps. Unfortunately, once the oxide is stressed, there is no way to recover the damaged chip. Some ADC devices can be held in a safe reset state until the VCCO rail is up - this may be an easier way to rework you carrier board.