Choosing RTC Timekeeping Accuracy: Plain Crystal vs TCXO / DTCXO (ppm, Drift & Seconds per Month)
Same 32.768kHz, yet accuracy can differ tens-fold over temperature. This guide converts ppm into seconds-per-month, explains the tuning-fork parabolic drift, and uses Micro Crystal’s CM8V-T1A (bare crystal), RV-3028-C7 (room-temp-accurate RTC) and RV-8803-C7 (DTCXO temperature-compensated RTC) as three tiers to pick by your accuracy need.
Converting ppm into seconds per month
Timekeeping accuracy is given in ppm (parts per million), and the conversion is simple: 1 ppm × 86,400 s/day = 0.0864 s/day ≈ 2.6 s/month ≈ 31.5 s/year. So a ±3 ppm RTC drifts at most about 8 s/month, while a ±20 ppm plain crystal can already be off ~52 s/month at room temperature — and worse over temperature. (This matches Micro Crystal’s datasheets, which label ±3 ppm directly as 0.26 s/day.) So the first step in choosing an RTC is to decide how many seconds per month or year you can tolerate, then work back to the ppm you need.
Why the same crystal drifts when temperature changes
A 32.768kHz tuning-fork quartz crystal’s frequency follows a parabola versus temperature: ΔF/F = −0.035 ppm/°C² × (T − 25°C)², peaking at the turnover temperature near 25°C and falling off on both sides. The further from room temperature, the error grows with the square — about −3.5 ppm at 10°C away, about −31 ppm at 30°C away (~2.7 s/day), and up to a few hundred ppm at −40°C or +85°C. That is before adding the crystal’s own ±20 ppm initial tolerance and the extra error from the MCU oscillator circuit and load-capacitance matching. The value of a TCXO / DTCXO is to measure the present temperature and compensate that parabola back toward flat.
Three tiers: bare crystal / room-temp RTC / DTCXO RTC
These map to three Micro Crystal tiers: (a) the bare 32.768kHz tuning-fork crystal CM8V-T1A — ±20 ppm @25°C, no temperature compensation, the cheapest and smallest paired with an MCU’s on-chip RTC but you accept the drift; (b) RV-3028-C7 — an integrated I²C RTC with room-temperature accuracy as tight as ±1 ppm @25°C and ultra-low power (45 nA typ), with a built-in calendar, alarm and UNIX counter; (c) RV-8803-C7 — a DTCXO digitally-temperature-compensated integrated RTC with an internal temperature sensor compensating in real time, accuracy ±1.5 ppm (0–50°C) / ±3 ppm (−40~+85°C), 240 nA typ. All three offer an AEC-Q200 automotive option, so you can step between tiers by ppm need within one vendor’s family.
⚠️ Important: the RV-3028-C7 is not a TCXO
A common and important misconception: the RV-3028-C7’s “±1 ppm” does not mean it is more accurate than the RV-8803-C7’s “±3 ppm” — the key is temperature. The RV-3028-C7’s ±1 ppm holds only at 25°C; it has no continuous temperature compensation and drifts along the same −0.035 ppm/°C² parabola away from room temperature — its real strength is “high room-temperature accuracy plus extreme low power (45 nA).” The RV-8803-C7 is the DTCXO, and its ±3 ppm is a guaranteed figure across the full −40~+85°C industrial range, far steadier over wide temperature. So never judge on a single ppm number alone — check the temperature range over which that ppm is specified.
How to choose
The rule: set the tolerable error, work back to ppm, then check the temperature range. About ±20 ppm, room temperature, cost-first, and able to re-sync periodically (NTP/GPS/manual) → bare CM8V-T1A crystal + MCU RTC (cheapest, but you accept drift and loss of time on power-down). Room-temperature accuracy with wearable/IoT ultra-long battery life → RV-3028-C7. Single-digit ppm across the whole industrial/automotive temperature range, for data-logging timestamps or smart metering that must keep stable time without re-sync → RV-8803-C7. Tighter still (sub-1-ppm over temperature, or holdover / telecom-grade) is beyond these three and calls for an OCXO or a GNSS-disciplined clock. An integrated RTC module gives predictable accuracy plus battery switchover and a calendar for fast development; a bare crystal is lowest cost and smallest but its accuracy also depends on the MCU oscillator. JLink Technology is an authorized Micro Crystal distributor supplying CM8V / RV-3028 / RV-8803 and more; tell us your accuracy requirement and temperature range and we will help with selection and reply with samples, datasheets and pricing.
Products mentioned
CM8V-T1A 32.768kHz Tuning-Fork SMD Crystal
The CM8V-T1A is a 32.768kHz tuning-fork SMD quartz crystal from Micro Crystal in a tiny 2.0×1.2×0.6mm package. With a ±20ppm frequency tolerance (±10ppm option), a 70kΩ max ESR, and multiple load-capacitance options (CL 4.0/7.0/9.0/12.5pF) to match the host oscillator, over −40~+85°C, it serves as the clock source for an RTC/MCU low-speed external oscillator (LSE) — for wearables, IoT, and metering. Note: this is a bare crystal, a different category from our RV-xxxx RTC modules (which embed their own crystal).
RV-3028-C7 Extreme Low Power I²C RTC Module
The RV-3028-C7 is one of the industry's most power-efficient RTC modules, drawing typically just ~45 nA. It integrates a tuned crystal, offers an I²C interface, and includes alarm, timer, and event logging — ideal for coin-cell powered wearables, meters, and IoT applications needing long battery life.
RV-8803-C7 High-Accuracy I²C RTC Module
The RV-8803-C7 is a high-accuracy I²C real-time-clock module from Micro Crystal with a built-in 32.768kHz DTCXO, ±3ppm accuracy (−40~85°C), ~240nA timekeeping at 3V, a wide 1.5~5.5V supply, plus alarm, timer, interrupt output, and a time-stamp event input.
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