One PMOD GPS to provide 1PPS signal for several FPGA

[jsobas]

Hello,

I am using PMOD GPS and exactly 1PPS signal as time reference for 5 FPGAs (XCZU3EG). I am supplying PMOD GPS with 3.3V from one FPGA, and the ground  is connected bewteen all the five FPGAs and the PMOD GPS. I want to distribute the 1PPS signal from a GPS PMOD to 5 FPGAs. By measuring with the oscilloscope I see that the PPS signal at the output of the PMOD is at 2.2V (blue curve in the attach image). Why is the PMOD output not 3.3V ? If I put a 3V cell battery, it will improve the output voltage ?

Thank you

Justin

[JColvin]

Hi @jsobas,

I was surprised to hear that the PPS signal wasn't outputting at a 3.3 V logic level, so I took a look at the GlobalTop datasheet for the GPS Antenna (available on the right hand side of the Pmod GPS Resource Center under Documentation, https://digilent.com/reference/pmod/pmodgps/start) and according to section 2.4 Pin Assignment (page 13), the 1PPS only outputs at a 2.8 V CMOS level, despite the module operating a 3.3 V. Adding the external battery would not change this voltage.

In fact, the Rx and Tx operate lines at 2.8 V logic (based on section 2.8) though the receive line can handle up to Vcc.

I don't know what drive strength the GlobalTop module runs its pins at, but if it's running at 6 mA, that would account for the 0.6 V drop across the 100 Ohm resistor down to the 2.2 V you observe.
However, I believe this should still register as a logic high signal, at least for devices operating at 3.3 V LVCMOS logic, which has a logic minimum high input of 2.0 V (which should be true for the Zynq you referenced based on DS925 from Xilinx, https://docs.xilinx.com/r/en-US/ds925-zynq-ultrascale-plus/PS-I/O-Levels)

Let me know if you have any questions.

Thanks,
JColvin

Edited September 9, 2022 by JColvin
added link to DS925