Repository file and dirs reorgniazation / OLED fix and improvements

README.md

@@ -1,27 +1,94 @@
+[![License: GPL v3](https://img.shields.io/github/license/AndrewBCN/STM32-GPSDO)](https://www.gnu.org/licenses/gpl-3.0) [![License: CC BY-SA 3.0](https://img.shields.io/badge/License-CC_BY--SA_3.0-lightgrey.svg)](https://creativecommons.org/licenses/by-sa/3.0/)
+
 # An STM32 GPSDO
-Designing, assembling, programming and testing an ultra-low-cost GPS-disciplined oven controlled crystal oscillator / lab clock based on an STM32 32-bit ARM microcontroller
 
-Keywords: STM32, GPS, GPSDO, crystal oscillator, frequency standard, atomic clock
+Designing, assembling, programming and testing an ultra-low-cost GPS-disciplined oven controlled crystal oscillator / lab clock based on an 100 MHz STM32 32-bit ARM microcontroller
+
+**Keywords**: STM32, GPS, GPSDO, OCXO, 10MHz, crystal oscillator, frequency standard, atomic clock
+
+## Introduction
+
+I have been following the thread by the late Lars about his inspirational DIY Arduino GPSDO project. And I checked the dozen or so GPSDO projects published on the net.
+Then I decided to join the party and design and build my own.
+
+I didn't quite know what I was getting into, but after a couple of months waiting for the parts to come from China, and then another couple of months writing the firmware, I have a working, blinking, timekeeping and happily and precisely oscillating GPSDO.
+
+Like with Lars' design, any OCXO, DOCXO or even rubidium source can be used, but the recommended oscillator is an inexpensive used square wave 10MHz 5V OCXO, which is what I am using in the breadboard prototype. These OCXOs are available from various sources on the net, recycled from decommissioned telecom equipment and sometimes still soldered onto a piece of PCB, for around 10€ or less (< US $12). The OCXO is the most expensive part in this GPSDO.
 
-André Balsa, April 2021
+For Lars' design check following links:
+- Original Lars' EEVBlog forum thread: https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/
+- GitHub project combaning all information in one place: https://github.com/AndrewBCN/Lars-DIY-GPSDO
 
-The Project
-===========
+The second most expensive part is the GPS receiver module. I strongly recommend a u-blox Neo-M8 GPS module with an SMA antenna connector. With a u-blox Neo-M8, I am still getting 5~9 satellites even indoors in my basement lab. The much cheaper Neo-M6 struggles to get a fix in the same conditions.
 
-I decided to build my own version of a 10MHz GPSDO with the following features:
-- Very low cost i.e. < 30 euros ($35), short and easily purchased BOM.
-- Acceptable performance (+/- 1ppb) "out of the box", can be fine tuned over time.
-- Uses an STM32F411CEU6 "Black Pill" microcontroller module, programmed in C/C++ using the Arduino IDE (STM32duino).
-- Requires only a 5V @ 1A power supply, supplied through a USB C cable.
+## Features
+
+I have decided to build my own version of a **10MHz GPSDO** with the following features:
+- **Very low cost i.e**. < 30 EUR ($35), short and easily purchased BOM.
+- **Acceptable performance (+/- 1ppb)** "out of the box", can be fine tuned over time.
+- Uses an **100 MHz STM32F411CEU6 "Black Pill"** microcontroller module, programmed in C/C++ using the Arduino IDE ([STM32duino](https://github.com/stm32duino/Arduino_Core_STM32)).
+- Requires only a **5V @ 1A** power supply, supplied through a USB C cable.
 - Compact and portable.
 - Optionally battery powered (allows taking it outdoors for optimal satellite signal reception).
-- An optional suite of environmental sensors (temperature, humidity, air pressure, voltages, OCXO current/power consumption).
-- An optional Bluetooth serial interface for wireless communication with a PC.
+- **Optional modules**
+  - **BMP280** for atmospheric pressure and temperature monitoring,
+  - **AHT10**  for temperature and humidity monitoring,
+  - **INA219** for OCXO power consumption monitoring,
+  - **HC-06** for wireless communication with a PC/mobile using Bluetooth UART.
 - Extensive logging of various operating parameters to allow further software tuning.
 - An optional small OLED displaying room temperature, UTC time, uptime, operating status, measured frequency.
+- **New!** Optional UTC-aligned 1PPS output using a **picDIV**.
+- It uses a digital **FLL (Frequency Locked Loop)**,
+  - **New!** It can also use a **PLL (Phase Locked Loop)**.
+- **OXCO Voltage control** using a **16-bit DAC** provided by a **2kHz PWM** output pin on the STM32F411CEU6
+  - [A quick discussion of noise, DAC resolution and OCXO frequency control](docs/DAC_resolution_discussion.md)
+  - [Notes on using a software 16-bit PWM DAC instead of an external 12-bit I2C DAC](docs/DAC_STM32_16bit_PWM_notes.md)
+
+## Resolution of 10MHz
+
+|   10 MHz |    |        | 1 ppm (10^-6) |       | 1 ppb (10^-9) |
+|---------:|----|-------:|---------------|------:|---------------|
+| 10000000 | Hz | 1E+06  | ppm           | 1E+09 | ppb           |
+|       10 | Hz |      **1** | **ppm**           |  1000 | ppb           |
+|        1 | Hz |    0,1 | ppm           |   100 | ppb           |
+|      0,1 | Hz |   0,01 | ppm           |    10 | ppb           |
+|     0,01 | Hz |  0,001 | ppm           |     **1** | **ppb**           |
+|    0,001 | Hz | 0,0001 | ppm           |   0,1 | ppb           |
+
+## Schematics
+
+Latest (0.6.2 revision) schematics are available here: [EDA/Schematics-STM32-GPSDO-0.6.2.pdf](EDA/Schematics-STM32-GPSDO-0.6.2.pdf)
+
+## The first breadboard prototype
 
-The first breadboard prototype:
-<img src="https://github.com/AndrewBCN/STM32-GPSDO/raw/main/GPSDO_breadboarda.jpg">
+![GPSDO Breadboard Prototype](img/Prototype_GPSDO_breadboard.jpg)
+
+![GPSDO Two Breadboard Prototypes](img/Prototype_TwoGPSDOs.jpg)
 
 In normal operation this is displayed on the small OLED:
-<img src="https://github.com/AndrewBCN/STM32-GPSDO/raw/main/OLEDv002i_expl.jpg">
+![OLED display](img/Prototype_OLEDv002i_expl.jpg)
+
+![Oscilloscope 10MHz Square Wave](img/Prototype_10MHz_cleantrace1a.jpg)
+
+You can see it in action on YouTube video (click image below):
+
+https://www.youtube.com/watch?v=mgoK4KuVDhw
+
+[![A $35 DIY GPSDO - with an STM32F411CEU6 MCU](https://img.youtube.com/vi/mgoK4KuVDhw/0.jpg)](https://www.youtube.com/watch?v=mgoK4KuVDhw)
+
+## Links
+
+- GPS
+  - [Timing and Location Performance of Recent u-blox GNSS Receiver Modules by John Ackermann N8UR](https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf)
+
+## License
+
+- Software: **GPL-3.0**
+  - [![License: GPL v3](https://img.shields.io/github/license/AndrewBCN/STM32-GPSDO)](https://www.gnu.org/licenses/gpl-3.0)
+- Hardware: **CC BY-SA 3.0**
+  - [![License: CC BY-SA 3.0](https://img.shields.io/badge/License-CC_BY--SA_3.0-lightgrey.svg)](https://creativecommons.org/licenses/by-sa/3.0/)
+
+
+## Authors
+
+- André Balsa, April 2021

docs/STM32_16bit_PWM_DAC.txt → docs/DAC_STM32_16bit_PWM_notes.md

@@ -1,24 +1,26 @@
-Notes on using a software 16-bit PWM DAC instead of an external 12-bit I2C DAC
-==============================================================================
+# Notes on using a software 16-bit PWM DAC instead of an external 12-bit I2C DAC
 
 To generate Vctl, the analog voltage that control the OCXO frequency, the STM32 GPSDO
 uses either a $0.50 MCP4725 I2C 12-bit DAC module or a pair of rc filters (2 x r=20k,
 2 x c=10uF) connected to a 16-bit, 2kHz PWM output pin on the STM32F411CEU6.
 
 Only 4 lines of C are required to generate this 2kHz PWM signal:
+
+```c
   // generate a test 2kHz square wave on PB9 PWM pin, using Timer 4 channel 4
   // PB9 is Timer 4 Channel 4 from Arduino_Core_STM32/variants/STM32F4xx/F411C(C-E)(U-Y)/PeripheralPins_BLACKPILL_F411CE.c
   analogWrite(PB9, 127);      // configures PB9 as PWM output pin at default frequency (1kHz) and resolution (8 bits), 50% duty cycle
   analogWriteFrequency(2000); // default PWM frequency is 1kHz, change it to 2kHz
   analogWriteResolution(16);  // set PWM resolution to 16 bits, default is 8 bits
   analogWrite(PB9, 32767);    // 32767 for 16 bits -> 50% duty cycle so a square wave
+```
 
 Initially I was worried that the software 16-bit PWM DAC would not provide a clean Vctl to
 precisely control the OCXO frequency, but after extensive testing I now believe it is a better
 solution than the external MCP4725 12-bit I2C DAC module.
 
 With the MCP4725 12-bit DAC the STM32 GPSDO stabilizes the OCXO frequency within +/- 1ppb
 (that is +/- 0.01Hz).
-But with the 16-bit PWM software DAC, the STM32 GPSDO manages to stabilize the OCXO frequency
-within +/- 0.1ppb (a stable and accurate 10MHz +/- 0.001Hz), which is an order of magnitude
+**But with the 16-bit PWM software DAC, the STM32 GPSDO manages to stabilize the OCXO frequency
+within +/- 0.1ppb (a stable and accurate 10MHz +/- 0.001Hz)**, which is an order of magnitude
 better and rather good performance considering the cost and simplicity of the hardware.

docs/DAC_resolution_discussion.txt → docs/DAC_resolution_discussion.md

@@ -1,5 +1,4 @@
-A quick discussion of noise, DAC resolution and OCXO frequency control
-======================================================================
+# A quick discussion of noise, DAC resolution and OCXO frequency control
 
 The STM32 GPSDO can use either an inexpensive 12-bit I2C DAC (MCP4725) module, or a couple of rc filters on one of the PWM pins of the STM32 MCU, to generate Vctl - the voltage used to control the frequency of the OCXO over a narrow range of 10MHz +/- a few Hz. The exact OCXO frequency provided by varying Vctl over a 0V - 4.4V range depends on the OCXO used as well as a number of different factors, but roughly speaking the precision of the OCXO frequency relative to a precise 10MHz is directly proportional to the precision with which we can set Vctl.
 
@@ -10,8 +9,7 @@ Back to the theory, the GPS receiver when locked with a good fix provides a 1PPS
 
 So in theory if we measure the OCXO frequency based on the 1PPS pulse over 1000 seconds (1000 sequential measurements), the maximum measurement error should be < 0.001Hz, or 10E-10 or 0.1ppb. That's exactly an order of magnitude better than what our 12-bit DAC is capable of.
 
-Using a 16-bit DAC provided by a PWM signal
-===========================================
+## Using a 16-bit DAC provided by a PWM signal
 
 The above reasoning led me to test a 16-bit DAC built by connecting a pair of rc filters to the PWM output of the STM32 MCU. Because the STM32 timers are 16-bit timers, we can with four lines of C obtain a 2kHz 16-bit PWM signal from the MCU, which is converted to a DC value (plus a ton of noise) and hence provides us with a 16-bit DAC, which we can use to control the OCXO Vctl. That's 16x more resolution than our 12-bit DAC can provide, at least in theory.
 
@@ -21,8 +19,7 @@ The anecdotal data I have collected so far seems to indicate that yes, we do. Th
 
 Of course I would need an even better clock to confirm to which degree these readings are correct, and I don't have access to such expensive equipment right now.
 
-Can we do even better?
-======================
+## Can we do even better?
 
 The question here is whether we could use a measurement period of 10,000 seconds and target a precision of 10E-11 or 0.01ppb for our STM32 GPSDO breadboard prototype, by either "dithering" the 16-bit PWM signal or mixing two 16-bit PWM signals with different amplitudes to increase the resolution of our software-based DAC by another 4 bits or so.
 
@@ -32,13 +29,13 @@ As already mentioned, my initial precision target for the STM32 GPSDO project wa
 
 For a DIY home project following the time-proven K.I.S.S. principle, costing less than $40 and assembled on a breadboard, these are more than good enough results.
 
-An anecdotal data point
-=======================
+## An anecdotal data point
 
 This is what the STM32 GPSDO reports either on USB serial or Bluetooth serial (but not both), once per second:
 
 Wait for GPS fix max. 1 second
 
+```text
 $GNGSA,A,3,31,25,12,18,02,29,20,,,,,,2.23,1.37,1.76*1E
 $GNGSA1,,86,49,215,*6B
 $GNGLL,4833.64512,N,00746.91322,E,092423.00,A,A*7F
@@ -72,6 +69,4 @@ Pressure = 1021.0 hPa
 Approx altitude = 48.9 m
 AHT10 Temperature: 22.15 *C
 Humidity: 77.75% rH
-
-
-
+```