Clock accuracy considerations

The crystal oscillator is active during normal operation, and is the clock reference for the RF synthesizer, the Analog-to-Digital Converter (ADC)/Digital-to-Analog Converter (DAC) sample clocks, and the baseband logic. The Real-time Clock (RTC) is active during sleep state, and is used to run the wakeup timer used as part of Wi-Fi® Power Save. The crystal oscillator is inactive during sleep.

The IEEE 802.11 specification defines the accuracy of the Wi-Fi carrier frequency to be within ± 25 ppm, which in turn defines the required accuracy of the external crystal. There is provision to trim the crystal oscillator through a value programmed into the One Time Programmable (OTP) memory on the nRF7001 device (or any other available non-volatile memory). This trimming will compensate for the combined frequency offset resulting from the crystal itself as well as any crystal oscillator variation at room temperature. The crystal and crystal oscillator will both exhibit frequency drift across temperature, and the crystal will also be subject to aging. The combination of these temperature and aging effects, along with the trimming accuracy, will consume the majority of the ± 25 ppm allowance, assuming a crystal with ± 10 ppm stability over temperature. The crystal tolerance (that is, accuracy at room temperature) is less important since this will be trimmed out by the crystal oscillator trim function (up to ± 20 ppm). The crystal oscillator/crystal is typically trimmed by transmitting Wi-Fi packets through the antenna connector and using a Vector Signal Analyzer (VSA) to measure the frequency offset. Alternatively, a generic spectrum analyser can be used to measure the frequency offset on a transmitted Carrier Wave (CW).

The RTC is automatically calibrated against the trimmed 40 MHz crystal oscillator reference at runtime, and as such nothing needs to be done on the production line. During sleep, the RTC clocked wakeup timer is used to time wakeup intervals (since the last Delivery Traffic Indication Message (DTIM) beacon in regular Wi-Fi Power Save), and as such any residual inaccuracy is not accumulated.