Open-loop frequency lock methods for fast boot-up time

摘要

A method for a near field communication circuit includes entering a low power mode and subsequently determining to exit the low power mode. The method further includes generating an open loop clock signal and providing the open loop clock signal to circuits of the near field communication circuit during a low power mode exit duration. Subsequently a reference clock signal is received from a host and used to clock the near field communication circuit.

主权项

1. A method comprising:
by a near field communication (NFC) circuit configured for data communication with a host device, entering a low power mode of the NFC circuit; detecting a signal indicative of a challenge requiring exit from the low power mode and response by the NFC circuit within a predetermined maximum response time; generating an open loop clock signal; and providing the open loop clock signal to circuits of the NFC circuit during a low power mode exit duration when a reliable clocking signal from the host device is unavailable; and using the open loop clock signal, processing incoming radio frequency signals at the NFC circuit during the low power mode exit duration to ensure response to the challenge by the NFC circuit within the predetermined maximum response time; calibrating frequency of the open loop clock signal by retrieving a stored data value from memory; using the retrieved data value, generating the open loop clock signal; measuring the frequency of the open loop clock signal generated using the retrieved data value; determining an adjusted frequency value suitable to reduce difference between the measured frequency and frequency of a reference clock signal; and storing to a memory location a new data value corresponding to the adjusted frequency value.