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Hello,

I'm from PLTW and have seen several (at least 4) CMOD S6 devices fail this year in my class.  I've used over 20 in my classes for the past 2 years and this is the first year that I've seen so many fail.  The red led that normally lights when plugged into the USB is out.  In probing the device, I measure about 4.5V at the input of IC1 (the 3.3V regulator) but only about .5V at the output (on the VCC3V3 line).  Are there any known issues with these boards as it relates to blowing the regulators?  Or any other common failures that is occurring where the board is operating fine but then the power led (red led) stops lighting and the device stops working?  Is there any way to get these repaired/replaced?  Thank you

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HI @Tony Mauro,

I am not aware of any common/known failures with the Cmod S6. Have you probed all 4 of the Cmod S6 with the same result? Prior to the board failures how are you powering the Cmod S6, through the USB UARTt or the VU pin 24? If through the VU what voltage were you powering the board at?

thank you,

Jon

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Hi Tony,

According to the datasheet the forward drop for the input diode (B240A-13-F) is 0.5V at 2 amps and figure 1 suggests that the drop will be 0.35V or less at 500mA or less. If you are applying 5V to the VU pin and seeing 4.5V at the input of the regulator (IC1) then that suggests that the regulator is either fried or that there is a short at the output of the regulator. The regulator claims to have short-circuit protection but does not specify which method it uses. I assume that when it detects a short it will blank the output for some amount of time. Since there is an LC filter at the output of the regulator a multi meter will measure a non-zero average voltage that's significantly lower than the target output voltage. Do you have an oscilloscope that you could use to probe the VCC3V3 rail (regulator output)? If so, I suspect you will see the voltage spikes up and then quickly drops off to something close to zero and that this process repeats at some periodic rate.

The Spartan 6 FPGA pins are rated for a maximum input voltage of 4.1 volts and the recommended operating range is -0.5V to 4.0V. Are you students applying voltages above this to the PIO pins? If so, then it's likely that repeated application of higher voltages has caused the pins to short internally, and that's why the power LED doesn't come on. The board wasn't designed to be 5 volt tolerant and applying voltages above 3.3V to the pins will result in random failures. Once this happens there is no way to repair the board without replacing the FPGA, which is not cost effective.

Thanks,
Michael

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Thank you for the detailed response, Michael!

Do you have a reference document on the voltage specs?  Though we don't typically interface to off-CMOD logic, there may have been input signals (e.g. switch inputs) that exceeded 4.0 volts.  I do not have any documentation that specifies the recommended operating range and would like to add this to my library for future classes and teachers.   Thanks also for the debugging tips, I will probe the VCC3V3 rail to see if there are voltage spikes as you suggest.

Best regards,

Tony

 

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