This GPSDO is based on the Lars Walenius design posted on eevblog.com:
https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/
The code has been translated from the Arduino to PIC, a PIC18F27Q84. Unlike the original design external ICs are not used to divide the 10 MHz input or measure the phase difference between the 1 PPS signal and the input. Instead, PIC Configurable Logic Cell (CLC) units are used to perform these functions. The only active devices besides the PIC are an op amp to buffer the PWM output controlling the OCXO, and an op amp buffer and inverter gate to buffer the sine wave OCXO output.
- Time interval Counter (TIC) with range of 1000 ns and 0.24 ns/bit resolution (12 bit ADC)
- 16 bit PWM output for controlling the OCXO
- Direct frequency count of 10 MHz input to monitor frequency
- serial connections for computer and GPS
- LCD display showing UTC time, 6 character grid square, PWM value, lock status, number of satellites, and frequency counter error
The OCXO is a UCT 108663 with sine wave output. This OCXO requires a 12 V supply. The voltage on the EFC pin for mine is about 5 V when tuned for 10 MHZ. RV2 is adjusted to first set the frequency of the OCXO close to 10 MHz. The OCXO is buffered with an ADA4851 op amp and sent to a 74LVC1G14 inverter before going to the PIC input. The PIC is a PIC18F27Q84. A PIC18F26Q84 should also work.
The PIC PWM output is buffered by two op amp stages using an LM358. The PIC PWM output is set up as a single 16 bit PWM, but a second pin is available for combining two PWM outputs. A voltage regulator on the board generates 5 V for the PIC and the GPS module. Resistor R17 is not used. The value of R12/R13 depend on whether current limiting is needed for the LCD backlight; mine did not need it, so I used a zero ohm jumper. R23 is also a zero ohm jumper.
MPLab 6.05; XC8 2.40
RA0 - RA7 : (outputs) data for 2x16 LCD in 8 bit mode
RB0 : LCD RS (output)
RB1 : ADC input; reads TIC voltage
RB2 : (input) 10 MHz in
RB3 : (output) TIC to 3.9 k resistor
RB4 : button 1 (input)
RB5 : button 2 (input)
RB6 : ICSP clock
RB7 : ICSP data
RC0 : (input) PPS in
RC1 : LCD E (output)
RC2 : (input) UART2 RX (GPS)
RC3 : (output) UART2 TX (GPS)
RC4 : unused; can be used as second PWM output combined with RC5
RC5 : (output) 16 bit PWM output
RC6 : (output) UART1 TX (computer)
RC7 : (input) UART1 RX (computer)
The PIC Signal Measurement Timer (SMT) is used as a frequency counter to directly count the 10 MHz input signal. The SMT is used in Counter mode; the signal comes from SMT1SIGPPS connected to RB2 (10 MHz); the window comes from SMT1WINPPS connected to RC0 (PPS). Period is set to 0xFFFFFF, pulse width acquisition interrupt is enabled.
In this mode, the interrupt function SMT1_PW_ACQ_ISR is called at
every PPS pulse. This function updates the total count and the error
compared to 10 MHz.
In the original design, an external IC is used to divide the 10 MHz down to 1 MHz to drive the TIC. This function is instead done using PIC CLC gates, one D-type flip-flop (CLC2) configured as divide-by-two followed by three JK flip-flops (CLC3, CLC5, CLC6) configured as divide-by-five. CLCIN2 is connected to RB2, 10 MHz in. The output of CLC6 is the 1.0 MHz signal.
####CLC2 (divide by 2)
####CLC3, CLC5, CLC6 (divide by 5)
####CLC3
####CLC5
####CLC6
####CLC1 and CLC7 (2 input D flip-flops with R)
Configured as the phase detector of the TIC. The inputs to CLC1 are from CLC6 (1.0 MHz) and CLCIN3, which is mapped to RC0 (PPS). The inputs to CLC7 are also PPS, and the output of CLC1. The output of CLC7 drives the diode of the TIC circuit (RB3).
CLC1
CLC7
The reference voltage of the ADC is set to 1.024 V using the FVR
voltage reference. The ADC is set to trigger on the output of
CLC7. The ADC interrupt is enabled; adcInterrupt() is the interrupt
function which reads the value of the TIC. With a 12-bit ADC this is
in the range 0-4095.
The two input buttons are debounced using CLC8 and timer 6. CLCIN6 and
CLCIN7 are mapped to the two button pins, RB4 and RB5. Timer 6
is set up in monostable mode with a period of 15 ms, with the output
of CLC8 as the reset source. The timer interrupt function is
buttonInterrupt(), which checks to see which button was pressed.
####CLC8
####2x16 LCD
2x16 LCD library adopted from