PMOD TMP3 on a Zybo

[Sol Arnu]

Hi All

Newbie question here. I'm new to PMODs and got the PMOD pack with a ZYBO. I'm pretty familiar with FPGAs, but trying to get more familiar with the ZYNQ family.

How should I connect my TMP3 PMOD to the zybo's connectors? Since the main connector is 4 pins on the PMOD vs 6 on the PMOD connector on the ZYBO, and the j4/j5 jumpers are right next to the I2C interface, power, and ground, it seems like I can't plug this right into the ZYBO PMOD connector. Am I missing something?  I must be missing something...

Thanks

Sol

[JColvin]

Hi Sol (and Dan),

I guess the short answer (I'll give the longer one too) is that this is (clearly) an oversight on Digilent's part; there is no convenient way to plug in the Pmod TMP3 into the Zybo without a DTE cable and a gender-changer pin set (link for visual reference). I'll speak to our Pmod Product Manager so we can get this amended by either including the cable and pin header with the Pmod Pack or changing the Pmod TMP3 for a different one.

The longer answer is that this is a leftover artifact from what the I2C protocol based Pmods used to be. They merited (and still do to some extent) their own hardware style since you did not need a full 6 pins (like SPI based Pmods do) and they also need to have the pull-up resistors on both their clock and data lines. I2C (as you likely know) can be nicely "daisy-chained" together so you can have multiple I2C based chips on that single bus, so I2C based Pmods were designed to have a matching "top" and "bottom" pin header so you could daisy-chain them with other Pmods or system boards. But then the orientation of the pins becomes kinda funky; if the long portion of the pins are parallel to the board and you plug them into a Pmod port (like every other Pmod) you lose that daisy-chaining capability as well as running into the problem of whether or not your Pmod had pull-up resistors built in or not. If you used cables with that orientation, the next Pmod or chip in line (so-to-speak) would physically have to be sitting on top of the host board, which isn't really the best plan when you have contact points on the top and bottom of the dev boards that could easily short out if not handled with care. So, it was decided to put the I2C pins perpendicular to the PCB base, much like the Pmod TMP3 is oriented. It was then presumed not to be a problem; if you used the I2C option on the Pmod ACL, you could just plug the I2C portion into the Pmod host port, making sure the power and ground pins appropriately lined up, and then went on with your life (albeit with a tilted system board thanks to the 12 pin connector). But not all of the I2C based Pmods account for that, and the Pmod TMP3 is not the only culprit. 

That being said, I have heard (I don't tend to participate in those conversations much anymore) that Digilent is working an alternate I2C standard, hopefully one that will address these issues, but I don't know when that will be implemented.

Regardless, I'll speak with the Pmod Product Manager and see what we can do for you in a reasonable time frame.

Thanks,
JColvin

[D@n]

You should be able to plug it directly into any PMod port.  The wires of the PMod are then connected to generic I/O ports on the Zynq.  How you map those I/O wires to PMods is up to you.

I might recommend, though, that you work slowly and methodically as with any hardware project: you want to make certain before plugging it in that you are connecting inputs to inputs, outputs to outputs, etc.  Short circuits are never good for electronics ...

Dan

[Sol Arnu]

Hi D@n,

Thanks for your response. But I'm confused - how to plug it in? Looking at the photo below I don't think I can plug it right into the PMOD connector on the ZYBO

https://reference.digilentinc.com/reference/pmod/pmodtmp3/reference-manual

Googling around a bit more, is the intention that I need to buy a DTE cable?

I guess I assumed that the ZYBO PMOD pack would come with everything needed, but maybe I was supposed to buy connection cables too?

Sol

[D@n]

Yeah, okay, now I get your problem.  It took me a bit, and the picture really helped.  Let's see if we can ask @JColvin if he has any ideas on this.

Dan

[JColvin]

Hello again,

I spoke with the Pmod Product Manager and learned that for the Zybo Pmod Pack demo they ran into the same problem and just ended up using female right angle header (visual reference) they had lying around to get the Pmod TMP3 attached. She (the product manager) is going to look into getting a cable and header or something similar incorporated into the Pmod Packs (as needed per pack type). 

As a side note, I also learned that the new I2C style will be a 6-pin header (or 12-pin) with the I2C lines and the ground/power so that all of the I2C based Pmods can just plug into Pmod Host ports without any fuss. The function of the two extra pins will probably vary from Pmod to Pmod as necessary and how the pull-up resistors will be implemented needs to be ironed out, but I'm at least personally encouraged that oddball group of the Pmod family is being more formally accepted.

Thanks,
JColvin

[Sol Arnu]

Thanks, @JColvin for your detailed response. I understand much better the process and how to resolve my issue!

Best

Solarnu