Input frequency measurement

A method of measuring a frequency of an input voltage, comprising:

• detecting a positive crossing of the input voltage;
• outputting and saving an internal count of a measurement circuit as a frequency signal;
• resetting the internal count of the measurement circuit after said outputting and saving the internal count of the measurement circuit;
• detecting a zero crossing in response detecting the input voltage is less than a second threshold voltage for a second threshold number of clock cycles;
• waiting a first threshold number of cycles while said detecting the zero crossing;
• waiting until the input voltage is greater than a first threshold voltage before detecting another positive crossing of the input voltage;
• beginning a timer for a third threshold number of cycles;
• validating a zero crossing in response to no further zero crossings of the input voltage being detected within the third threshold number of cycles;
• and
• invalidating the zero crossing and the previously detected positive crossing in response to another zero crossing of the input voltage being detected within the third threshold number of cycles, and adding a saved value of the frequency signal to the internal count of the measurement circuit.

A power converter, comprising:

• a rectifier coupled to a dimmer circuit coupled to receive a line voltage;
• an energy transfer element having an input coupled to receive an input voltage from the rectifier, wherein an output of the energy transfer element is coupled to an output of the power converter;
• a power switch coupled to the input of the power converter;
• and
• a controller coupled to generate a drive signal coupled to control switching of the power switch to control of transfer of energy through the energy transfer element to the output of the power converter, wherein the controller includes:
• an oscillator circuit coupled to generate a system clock in response to a frequency signal;
• a drive circuit coupled to generate the drive signal in response to a feedback signal representative of an output of the power converter, the system clock, and the frequency signal;
• and
• a frequency determination circuit coupled to generate the frequency signal in response to a voltage sense signal representative of the input voltage, wherein the frequency determination circuit includes:
• a positive crossing sense circuit coupled to sense a positive crossing of the input voltage in response to the input voltage being greater than a first threshold voltage after a first threshold number of cycles has passed after a zero crossing of the input voltage, wherein the zero crossing of the input voltage is detected in response to the input voltage being less than a second threshold voltage for a second threshold number of cycles;
• a validation circuit coupled to the positive crossing sense circuit to validate a previous zero crossing and the positive crossing of the input voltage after the positive crossing of the of the input voltage has occurred, wherein the validation circuit is coupled to sense an invalid previous zero crossing and positive crossing of the input voltage in response to sensing another zero crossing of the input voltage before an end of a third threshold number of cycles;
• and
• a measurement circuit coupled to the positive crossing sense circuit and the validation circuit to count a time between positive crossing pulses of the input voltage, wherein the measurement circuit includes a measurement counter coupled to a measurement register, wherein a measurement count of the measurement counter is coupled to be saved in the measurement register and wherein the measurement counter is coupled to be reset in response to each positive crossing pulse that is received from the positive crossing sense circuit, wherein the frequency signal of the measurement register is representative of a frequency of the input voltage.

A frequency determination circuit, comprising:

• a positive crossing sense circuit coupled to receive an input voltage to sense a positive crossing of an input voltage;
• a validation circuit coupled to the positive crossing sense circuit to validate a previous zero crossing and the positive crossing of the input voltage after the positive crossing of the of the input voltage has occurred;
• and
• a measurement circuit coupled to the positive crossing sense circuit and the validation circuit to count a time between positive crossing pulses of the input voltage, wherein the measurement circuit is coupled to output a frequency signal representative of a frequency of the input voltage in response to the time between the positive crossing pulses of the input voltage.