Build environment:

summon-arm-toolchain - https://github.com/esden/summon-arm-toolchain.git
perl on PC with the perl modules "Inline::C" and "Moose"

use "make" to build the STM32F4 code.  Flash with st-link utility (I've had the best success with their windows tool)


Hardware setup:

STM32F4 discovery board
Fastrax UP501 GPS
some 3.3V power source
20x4 LCD display connected via i2c gpio expander (I used sainsmart's)

wire GPS PPS pin to PA1 on the STM32F4 board
wire GPS serial to STM32F4's UART: GPS RX=STM32F4 PA2, GPS TX=STM32F4 PA3
wire power of GPS to a 3.3V supply (STM32F4 "3V" is 2.9V, and will not work)
connect STM32F4's USB port to PC
power STM32F4 via its debug USB port or via the 5V pin
wire LCD display to PA8=scl, PC9=sda and use external pull-up resistors to 5V

NTP setup:

The STM32F4 will show up as /dev/ttyACM0 (or some number other than 0) on the linux host.  Check dmesg to see:
[697103.547376] usb 2-1.1: Product: STM32 Virtual ComPort in FS Mode
[697103.574835] usb 2-1.1: Manufacturer: STMicroelectronics
[697103.601224] usb 2-1.1: SerialNumber: 00000000050C
[697103.627691] usb 2-1.1: adding 2-1.1:1.0 (config #1, interface 0)
[697103.627726] cdc_acm 2-1.1:1.0: This device cannot do calls on its own. It is not a modem.
[697103.657570] cdc_acm 2-1.1:1.0: ttyACM0: USB ACM device

Add to /etc/ntp.conf:
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 0.000 refid GPS stratum 0

Run as root (from this directory):
./ntp-shm </dev/ttyACM0

Verify NTP is getting data with "ntpq -p":
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*SHM(0)          .GPS.            0 l    9   16  377    0.000    0.006   0.008

(the reach column should be non-zero, and then "when" column will tell you the time of the last sample)


Source code:

(stm32f4) start by looking through src/main.c, src/pps.c, src/mytimer.c, src/uart.c
(PC) see the file ntp-shm


Results:

see http://dan.drown.org/stm32/