Crystals Calibrating crystals, resonators, RC oscillators using NTP host. Arduino UNO, nano, mega2560, DUE, teensy 3/LC, maple, propeller, RTCs Files: crystals.txt measured ppm frequency offset for various devices hostdrift.c NTP host program to poll MCU drift.ino MCU program to send millis() to NTP host drift.spin propeller program to send micros to NTP host Drift.java java program to send millis to NTP host for ridgesoft robot gpspps.spin propeller program to use GPS pps and cnt ds3231.ino MCU program with RTC to send 1khz ticks to NTP host rtt.ino DUE sketch to measure RTT/RTC crystal frequency gpspps.ino UNO sketch to use GPS pps signal and micros() teensyRTC.pde teensy RTC experiments 32KHz crystal -------------- details -------------------- A linux host running NTP on a good broadband network can adjust its time to within a few milliseconds and discipline its frequency (drift) to within one part-per-million(ppm). I have a good network connection and a stratum 0 CDMA time server as well. A simple linux host program polls the mcu every 10 seconds or so and collects the millis() (or ticks for RTCs) from the MCU and can calculate the difference in frequency between the NTP host and MCU/RTC. The longer the data collection runs, the more accurate the frequency estimate (ppm). Most devices I tested ran within their specs -- except for one bad 32KHz crystal (see crystals.txt). The AVR (arduino) processors can run off an internal 8MHz RC oscillator. The RC circuit is precalibrated at the factory (10%), but you can adjust the RC oscillaor by changing in the OSCCAL value. I measured frequency offset of various 328 chips and adjust OSCCAL. Frequency is affected by temperature and voltage. Two of the 328p's I have had an optimum OSCCAL, one other was improved by changing the OSCCAL value by a few units. A unit of OSCCAL varies by chip, but it's around 4000 ppm though non-linear. See avr328p datasheet figure 29-175, sections 8.6, Table 28-1, and www.atmel.com/Images/doc2555.pdf and http://arduino.cc/en/Tutorial/ArduinoToBreadboard ridgesoft robot has ATMEGA128 with java VM. No bill of materials but we believe oscillator is 30-ppm 14.7456Mhz crystal. The drift of 1736 ppm suggests the Java VM is using a prescaler of 64 (doesn't evenly divide frequency) so millis() error is 400/230000 or 1739 ppm. A different software implementation could correct this error. Using hostdrift -f 1001.73913 results in drift error of only 2.3 ppm. The Maxim DS-series RTC's have a SQuare wave output pin that can be enabled. I connected the SQ output to an input pin counted ticks on FALLING edge of pin interrupts. The frequency of the square wave was set at 1024 ticks per second (4096 for the DS1307). The TCXO RTC's ran within 1 ppm of the NTP host. http://www.maximintegrated.com/datasheet/index.mvp/id/4627?ver=C&utm_expid=50713806-2 I conencted a 32Khz crystal to Maple and to breadboard 328p (with and without 22pf load capacitors). Frequency is affected by load capacitance, temperature, voltage and aging. I connected a 32KHz crystal to teensy 3.0 (no capacitors required). It was accurate to within 4ppm using micros() on teensy and pulse-per-second interrupt from the RTC. The ARM processor RTC crystal on the teensy can be configured with different load capacitance and has calibration registers to adjust the frequency (see rtc_calibrate()); The MCU crystal capacitance can also be adjusted on the teensy ARM processors (OSC0_CS). As a second test, hooked up a GPS module's pps pulse to pin 3 on UNO and used micros() to measure frequency difference. Results matched test with NTP host (sketch gpspps.ino), and Sparkfun GPS https://www.sparkfun.com/products/465 We ran a similar sketch on the Maple, teensy, propeller, pyboard, and DUE with frequency measurements matching the NTP tests. Besides the cut of the crystals, frequency is affected by load capacitance, temperature, voltage, and aging. Here are similar experiments by Joris using the pps from a GPS module at various temperatures. http://jorisvr.nl/arduino_frequency.html and another temperature study http://forum.pjrc.com/threads/24628-Interesting-Temperature-Data crystal frequency vs temperature https://ieee-uffc.org/frequency-control/educational-resources/introduction-to-quartz-frequency-standards-by-john-r-vig/introduction-to-quartz-frequency-standards-static-frequency-versus-temperature-stability/ Teensy 4 GPS PPS plots with Allan deviation https://forum.pjrc.com/threads/61581-Teensy-4-1-NTP-server?p=246706&viewfull=1#post246706 “One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper ( Dec 9 1906 to Jan 1 1992)