The delay between the PPS and the rising edge of the 555 output is 150 ns. This is with the 555 running on 3.3V. I could likely get a faster rise time if I ran the 555 on 5V, though I would then have to level shift the output back to 3.3 for the GPIO pin. Not a big deal, as long as there is a benefit. I have one bare board left so I will cut a couple traces and make up a 5V version.
I was concerned about jitter and noise, and it seems that is a valid concern. I'm not familiar with proper methods for measuring jitter in the ns range. I can say that NTP stats shows a jitter of 4 us after some settling, where as I would normally have 2 us jitter with other GPS modules I have tried. So roughly 100% worse than a GPS module sitting on the RPi with no added circuitry. That gives me a starting point to work from though. Measuring the output on my 53131A it reads a period of 1 second out to 10 zeros, with +/- 1 count of jitter on the last digit. I have attached the schematic. The input for the PPS made sense when using pins 1 and 7 on the connector, but with pins 1 and 6 I am not as clear on how to properly interface the differential signal. Besides the fact that what I have now does properly trigger the 555. So thanks for any suggestions on improving that part and reducing jitter/noise. Pete: If I eventually get to a circuit I'm happy with, I'll share the project on OshPark. And a PTH version would be easy to make up as well. That way you can order some if you like, or just see the layout and make your own. A board this size is only about $5 for 3 using that service. Regards, Dan Watson On Tue, Mar 24, 2015 at 3:42 PM, Attila Kinali <[email protected]> wrote: > On Mon, 23 Mar 2015 19:25:33 -0700 > Chris Albertson <[email protected]> wrote: > > > I'm surprised to can't > > get the Pi to interrupt on the raising edge of the PPS and that you had > to > > make the pulse longer. > > That's because the rpi does not have GPIO's like other SoC or uC. > When i wrote that it's a graphics card with attached usb controller, > i wasn't joking. The processor on the rpi was originally designed > as a test system to verify the graphics core on real workload. > To interface it to the outside world, they decided to use USB. > For unknown reasons they decided to use an arm9 core with a bit > of glue logic as USB controller (the best guess i've heard sofar > is, that they would have had to pay for a real USB controller, > while an old arm core like that costs (almost) nothing if you > buy a big/new one anyways). > > This is the reason why there are no I2C or SPI interfaces, > and everything needs to be bitbanged (aka, you write single > bits to outputs in software and poll whether anything changes > on the inputs). Or that the "USB controller" generates an > interrupt every 125us that _must_ be handled imediatly > (the arm core has to set up the next USB microframe otherwise > USB stops working). > > That is also the reason why Eben Upton got it so cheaply. Apparently, > broadcom had produced quite a few of those (couple thousand, don't ask > me why). And there was no risk that anyone would buy those (for above > reasons, they were never intended to be sold). Also, the mask set > (the most expensive part of chip production) was already there and > amortized by other means, so producing more wouldn't cost much either. > > > Attila Kinali > > -- > < _av500_> phd is easy > < _av500_> getting dsl is hard > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >
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