This past November, Elliot Williams of Hackaday.com wrote a quick review of the Digilent Pmod™ Standard. Yay for product feedback!
Elliot sets the stage by noting how difficult is it to navigate the range of connectivity options between main boards and add-on modules, or as he calls it, “the connector zoo.” We feel his pain, which is why the Pmod™ Standard was born in the first place! We at Digilent are constantly doing our best to provide consistency via the Pmod™ Standard while also allowing for flexibility, which is no easy task. Not to imply that there isn’t always room for improvement, but it’s great to hear we’re having success in this department. Here are some highlights from the article.
“There are a few aspects of Pmod that we think are particularly clever. First, the number of pins involved is “just right” at six, and it’s easily expandable. They use standard 0.1” pitch pins and headers. Two lines carry power and ground, leaving four free pins for SPI, UART, or whatever else. The specification is that all power and signal voltages are 3.3 V because they’re designed for FPGAs after all. You can mix and match if you know what you’re doing, but they won’t let you call it Pmod ™.”
A quick aside, Pmod™ boards work with microcontrollers as well! Also, many modules can operate at 5 V and this is not forbidden by the Pmod™ Interface Specification. However, the primary intention is operation at 3.3 V. Now where were we…
“If you need more than four signals, there’s a twelve-pin version which is just two six-pin Pmods stacked into a double-row header. The extra power and ground are redundant, but it makes a twelve-pin output very flexible, because nothing stops it from being used as two sixes. The standard also says that the twelve-pin headers are to be spaced at 0.9” center-to-center, so you can even connect two of them together when you need sixteen board-to-board signal connections. We like the modularity and expandability.”
“Pmod connectors are multi-protocol, but for each protocol there is a single pinout. So there’s an SPI Pmod and I2C Pmod, and the pins are always in the same place. There isn’t a Pmod standard for every conceivable application, of course, so there’s a GPIO pinout that gives you free rein over what goes where. We think that it would be nice if some additional notable protocols (I2S? one-wire? Servos? Analog stereo audio?) were included in the specs, but the community can also handle these lower-level details.”
We agree! As the Pmod™ product manager, I can say that a revision of the Pmod™ Interface Specification is already in the works and will include an updated 6-pin I2C spec and I2S.
To sum up, Elliot says:
“In our eyes, Pmod is nearly perfect. It uses cheap hardware, is easily expandable, and the smallest incarnations are small enough to fit on all four sides of a one-inch-square board. If you’re willing to pay the brand-name premium, Digilent makes an incredible range of modules. We want to see more hackers outside of the FPGA scene get on it.”
Thank you Elliot and Hackaday for the review! We welcome all feedback and suggestions for improvement so if you have any on the Pmod™ line, please post to the add-on board section of the Digilent Forum or comment below. If you’re interested in making your own Pmod™ board, take a look at the Pmod™ Interface Specification and keep your eyes peeled for the updated version in a few weeks.