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.
Version 1.1.0 primarily updated 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.
First off, it’s important to point out that version 1.1.0 did not “break” any of the older Pmods. This means all Pmods following version 1.0.0 still work with all Digilent system boards. The main change was an update of the I2C connection standard to bring enhanced compatibility to Pmods that use the I2C communication protocol.
If you’re familiar with Digilent Pmod boards, you’ll know that they are mostly 6-pin or 12-pin add-on modules designed to plug directly into 12-pin female host ports on our system boards. They use SPI, UART, I2C, and GPIO to communicate. The I2C mechanical standard had always been somewhat of the odd one out, as it used a 2×4-pin header which awkwardly did not plug directly into host boards as nicely as the others. You could do it, but it was admittedly a bit of a hack. The new I2C standard is 6-pin and solves this pesky issue. See the table below for the pinout of the 1.1.0 I2C interface.
Pmod I2C Signal Specification
| Pin # | Signal | Direction | Alternate Signal |
| 1 | INT* | In | NC |
| 2 | RESET | Out | NC |
| 3 | SCL | In/Out | – |
| 4 | SDA | In/Out | – |
| 5 | GND | ||
| 6 | VCC |
SCL – Serial Clock SDA – Serial Data NC – Not Connected *INT – Open-drain interrupt signal from slave to master
Now I2C is no longer the odd one out of the Pmod family! We’ve already started designing new Pmods to the above specification. For example, see the Pmod HYGRO below and the Pmod CMPS2. I2C Pmods are great for daisy chaining and maintain the ability to do so with the female 6-pin connector on the back, pictured below.
We have also updated the other interface pinout tables, such as SPI, UART, etc., creating more clarity, and added specifications for the I2S protocol and H-bridge. And for even further clarity, we’ve updated the mechanical drawings so designing your own Pmod should be easier.
The update from 1.1.0 to 1.2.0 is an update to the power section. This change is subtle but important. 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. Supporting 3.3 V alone is sufficient for compliance.
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!
For example, the Pmod PIR Passive Infrared Motion Sensor is a great Pmod to consider if you’re looking to add motion detection capabilities to your designs. If you’re looking for reliable connectors for power and ground connections, check out the Pmod CON1 Connectors to ensure secure and efficient connections in your Pmod designs.
