| 摘要 |
The average frequency of a low-power timer oscillator 2, such as a capacitor-based relaxation oscillator or a ring oscillator, may be subject to fine adjustment by switching an additional capacitor 4 into circuit during some oscillator cycles. The switching may be controlled by a sigma-delta modulator (figure 4). The oscillator 2 may be calibrated against an accurate but high power crystal oscillator 11, using a counter 8 and logic 9, when the crystal oscillator and a microcontroller are briefly enabled by a sleep timer to respond to TDM radio packets. The timer receives a clock derived from the crystal oscillator via divider 12 while the oscillator 2 is being calibrated. An initial coarse calibration may be achieved by variation of a current source (IREF, figure 6). The oscillator may operate at around 32768 Hz. |
