18.6 A 32MHz Crystal Oscillator with Fast Start-up Using Synchronized Signal Injection

Low-power sensor nodes (e.g., Bluetooth Low Energy, BLE) use low duty-cycle transceivers to obtain an overall low power consumption. The system typically spends most of the time in a low-power sleep state and only briefly wakes up to transmit and receive data. Due to the high Q (>100k) of the crystal resonator the start-up time of a typical crystal oscillator is relatively long ($\approx 0.5$ to 5ms) which causes a substantial amount of energy to be consumed by the crystal oscillator in a transmit/receive event. As the system usually is in an active state when the crystal oscillator ramps up (multiple sub-systems are powered up) not only should the start-up energy of the crystal oscillator be optimized, but also the startup time. As the crystal resonator is one of the most costly components in IoT sensor nodes, a wide range of crystal resonators should be supported by the circuit. This work proposes a synchronized signal injection (SSI) start-up mechanism that can drive the crystal up to any amplitude in a minimum amount of time, does not require a high-precision RC oscillator, is insensitive to PTV and is effective for a wide range of crystals: the technique does not require the shunt capacitance or the load capacitance to be small. A programmable capacitor-bank is included to support a range of crystals (load-capacitance) and allow for the customer to calibrate the frequency but is not required for the SSI technique.