To validate connectivity with a specially enabled audio interface, the CD326x provides an authentication sequence upon start-up. After enabling tone mode (applying V_MIC > 2.35V typical), the specially enabled audio interface waits to receive an acknowledgment from the CD326x. This acknowledgment is a tone sequence similar to a button press when operating as tone mode. When the CD326x recognizes tone mode upon power-up, the CD326x sends an acknowledgment (ACK) tone sequence to the specially enabled audio interface. The tone frequency is identical to the S1 button press tone for the CD3268 and to the S2 button press tone for the CD3269. However, the ACK tone sequence generates the second tone to be 3.7ms longer compared to the standard tone sequence for button presses.

The specially enabled audio interface reads the tone sequence as shown in Figure 7-6. The specially enabled audio interface takes three samples: the first to determine calibration frequency, the second to determine the button/ACK frequency, and the third to differentiate between an authentication sequence and a button press. The third sample is taken 2.89ms after the second sample. If the specially enabled audio interface reads the ACK frequency during the third sample, the interface acknowledges a proper authentication sequence. If the tone frequency is not present during the third sample, the specially enabled audio interface recognizes the tone sequence as a button tone. When a button tone is recognized any time after the timeout period, the specially enabled audio interface records a button press event.