Digilent Pmod Interface Specification 1.2.0

Last July, we announced that the original Digilent Pmod Interface Specification version 1.0.0 was revised and republished as version 1.1.0. This was a long time coming, given that the original spec was written in 2011. Now that Pmod has become a global standard, and continues to grow in popularity, you can expect to see updates to the spec more frequently than once in six years. When we do update the spec, you will find the most current and previous versions here on the Digilent Wiki.

The update from 1.0.0 to 1.1.0 was mostly an update to the communication protocol types supported by Pmods. Most noteworthy was the addition of a 6-pin I2C type, which replaced the old mechanically incompatible 2×4 I2C type, as well as the addition of a formal I2S type and H-bridge type. If you’d like to read more about those changes, check out the blog post announcement.

The update from 1.1.0 to 1.2.0 is an update to the power section. The difference between the two lies in the first paragraph. The first paragraph of the 1.1.0 power section reads:

The power pins of the interface provide power from the host to the peripheral. The complete interface requires that the host provide the ability to switch the voltage on the power pins between 5.0 V and 3.3 V. A reduced functionality subset of the specification allows the host to provide only 3.3 V at the power supply pins, with no ability to switch. On the twelve-pin version of the interface, both power supply pins switch together and always supply the same voltage. These pins may be shorted together at either the host end or the peripheral end.

The first paragraph of the 1.2.0 power section reads:

The power pins of the interface provide power from the host to the peripheral. The interface requires that the host provide 3.3 V at the power supply pins. Digilent system boards use exclusively 3.3 V at the power supply pins; however, the host is allowed to provide the ability to switch the voltage on the power pins between 5.0 V and 3.3 V. On the twelve-pin version of the interface, both power supply pins switch together and always supply the same voltage, if switching is provided as an option. These pins may be shorted together at either the host end or the peripheral end.

The difference between the two is subtle but important. Previously, the spec required that the host provide the ability to switch between 5.0 V and 3.3 V at the Pmod ports. This is problematic now as the input pins on recent Digilent FPGA boards will be damaged by 5 V. By the current spec, switching is allowed but no longer required. Only 3.3 V is required to be compliant.

If you have any questions regarding Pmod compliance or how to design your own, take a look at the most current spec, or visit the Digilent Forum!

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