RTC & timing
Timekeeping accuracy is the spec that matters. A DS1307 drifts minutes per month; a temperature-compensated DS3231 holds about 2ppm. Battery backup and supply voltage settle the rest.
DS3231/DS3232 temperature-compensated RTCs, the budget PCF8523 and DS1307, the Si5351A clock generator (8kHz–160MHz), and bare 16MHz crystals — matched to drift tolerance and supply voltage.
- DS3231 boards (Adafruit's Precision RTC breakout, the ChronoDot) and DFRobot's Fermion DS3232 use a temperature-compensated oscillator for roughly 2ppm drift; the DS1307 Gravity module is 5V-only and drifts far more.
- The PCF8523 breakout is the budget middle ground: a 3.3/5V-friendly I2C RTC with coin-cell backup, the same chip Adafruit ships across its datalogger boards.
- Not everything here keeps calendar time: the Si5351A breakout generates programmable clocks from 8kHz to 160MHz, and the S-35710 is a low-power wake-up timer for cutting deep-sleep current.
- Bare 16MHz quartz crystals clock microcontrollers like the ATmega328 with Arduino bootloader — they need the MCU's oscillator circuit and are not timekeeping parts on their own.
