The ECS-.327-12.5-12-TR: The 12.5 pF load capacitance and ±20 ppm frequency tolerance at room temperature make this the standard fit for real-time clock circuits in microcontrollers that include an integrated oscillator — the tolerance holds well within the ±1.5 minute/day budget most RTC functions require.
ESR and oscillator start-up in low-power designs
The equivalent series resistance is rated at 90 kΩ — a typical value for tuning fork crystals, but it is the parameter that most limits reliable start-up in oscillator circuits with weak on-chip driver strength. When selecting or swapping this crystal, check the microcontroller datasheet's crystal drive-level specification: a part whose oscillator can only supply 0.5 µW may start inconsistently with a 90 kΩ crystal at cold temperature, where ESR rises further.
Industrial temperature grade and frequency stability
In this window the crystal's frequency shift with temperature follows the characteristic cubic curve of a tuning fork: flat near 25°C, drifting as temperature moves away from the turnover point, typically around 25°C for a standard cut.