HSMC Expansion Voltages

The MitySOM-5CSx Development Kit Baseboard exposes a large number of FPGA IO signals from the MitySOM-5CSx modules on three headers; 2 HSMC headers and 1 Expanded IO header (bottom of board).

All FPGA IO's are set to a 2.5V voltage level.

It is possible to alter the MitySOM-5CSX development board to have all of these IO's set to a 3.3V voltage level.

(!) Performing these steps voids your manufacturer warranty. Please contact Critical Link if you have any questions about the steps below as there is inherit risk in making these changes if done improperly which can result in damage to both the baseboard AND any MitySOM-5CSx module installed in it. The schematics for the baseboard can be found on this wiki page for further reference. (!)

Bank 3A5A5B (Expanded IO) and 4A, 3B and 8A FPGA IO

Including Full HSMC, Partial HSMC and Expansion Header (J300, J301 & J303 headers)

  1. U601 on the development board is a LT3502IDC part whose output voltage can be adjusted through the feedback resistor network tied to it. This IC provides the +2.5V_VIO nets voltage.
  2. The 3 resistors are R603, R613 and R616 that would be altered. Page 21 of the LT3502 datasheet ( has a 3.3V application example including recommended resistor values.
    1. 2.5V Resistor Values (As Shipped from CL)
      1. R603 - 12.1K ohm (0402 size)
      2. R613 - 12.1K ohm (0402 size)
      3. R616 - 11.5K ohm (0402 size)
    2. 3.3V Resistor Values (As recommended by LT datasheet)
      1. R603 - 15.8K ohm (0402 size)
      2. R613 - 15.8K ohm (0402 size)
      3. R616 - 10.0K ohm (0402 size)
  3. We recommend that before installing a module after the above modification that you test the new U601 output voltage, 2.5V_VIO net.
    1. Note that on baseboards before Revision 9 (i.e. Rev 1 to 8) the VIO_2V5_ENABLE (Module Pin 46) tied to the module for power sequencing needs to be temporarily tied "high" to 3.3V through a 10k resistor. Once pulled high this will allow for the confirmation of the 2.5V_VIO net as 3.3V and you can then remove the temporary pull-up and test with a module. Rev 9 or higher already have this pull-up installed.

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