High Precision Frequency Counter using GPS (Part2)

Following from previous posting I have connected a STM32F429i-DISC1 board to us UBLOX NEO-6M GPS module (see https://www.ebay.co.uk/itm/313603433572) and programmed a 32 bit counter in capture compare mode running at 80MHz.

STD32f429i-Disc1 with NEO-6M GPS as timer
NEO-6M GPS with PPS triggering counter

After the GPS settled I recorded a number of counts varying by +-200 using the inbuilt 8MHz crystal.

This is far too much jitter, at present i t is uncertain if the reason is

  1. Crystal frequency variation due to quality of crystal
  2. Crystal frequency variation due to supply voltage
  3. STM32 PLL stability.

I will first attempt to attach a better crystal. The STM32F429 accepts an external input from 4MHz to 26MHz. From RS components I considered a 24Mhz TCXO https://uk.rs-online.com/web/p/tcxo-oscillators/2499379 but I only wanted one, so ended up buying o e of these from ebay https://www.iqdfrequencyproducts.com/products/pn/LFTCXO075797Reel.pdf.

I’ll update on stability when it arrives.

I will provide the eclipse project for the board firmware. It includes UI, GPS serial communications and timer programming. All running under FreeRTOS.

Author: admin

A research engineer in electronics engineering with over 20 years research and development experience. Specialist in software architechture and design.