The ultra-minature ECX-34RR is a very compact SMD Tuning Fork Crystal with a Typical ESR of 20K Ω. Low ESR part recommended by STMicroelectronics for the STM32 Microcontrollers. ECX-34RR SMD CRYSTAL Request a Sample ECX-34R SMD CRYSTAL Low Power RTC Applications such as STM32 AN2867 20K Typ ESR 3.2 x 1.5 mm Footprint Extended Temp. Range RoHS Compliant DIMENSIONS (mm) OPERATING CONDITIONS / ELECTRICAL CHARACTERISTICS PARAMETERS Frequency Frequency Tolerance Load.