7.4 Device Functional Modes

7.4.1 Button Mode

When the CD326x detects a MIC pin voltage below 2.35V (typ), the CD326x shorts the MIC and REM pins

together and disables all other inputs and outputs. When a button press event occurs, the DC voltage on the

microphone bias line changes according to the button being pressed. Table 7-1 shows the corresponding DC

voltage for a given button press when using the resistor values in Figure 7-4. This DC level can then be detected

by a specially enabled audio interface connected to the microphone bias line (MIC pin).

Switch S0 is a unique switch (typically controlled by a specially enabled audio interface) that shorts the MIC

pin to ground. When Switch S0 shorts the MIC pin to ground, the specially enabled audio interface removes

power from the CD326x. When power recovers, the CD326x enters button mode or tone mode depending on the

voltage detected at the MIC pin.

Table 7-1. Typical Switch Voltage Values (Button mode)

Switch Closed

MIC Pin Voltage (V) (1)

S0

0

S1

1.510

S2

1.603

S3

1.690

S4

1.779

(1)

VMICBIAS is the voltage on the MIC pin: 2.0V (typ)

REM

R1

6.81k

S0

S1

R2

2.61k

S2

S3

S4

Phone host

audio interface

MICBIAS

GND

R3

3.57k

R4

6.81k

CD326x

MIC

≤1% tolerance resistors recommended

Figure 7-4. Recommended Button Mode Resistor Values

7.4.2 Tone Mode

When the CD326x detects a value higher than the rising tone mode threshold voltage (2.35V typical) on the

MIC pin, the CD326x enters tone mode. When the device transitions to tone mode while supplying voltages

below the tone mode threshold voltage, the overshoot voltage from the supply temporarily pushes the MIC pin

voltage above the threshold. Decreasing the ramp of the supply or reducing the supply voltage are both options

to reduce the inrush current leading to this functionality.

Figure 7-5 shows the start-up sequence for when the CD326x enters tone mode. Upon entering tone more,

the device opens the switch that connects the MIC and REM pins (Switch B). After a predetermined delay, the

device shorts the VSHUNT pin and the MICPWR pin (Switch A). This functionality aids in preventing unwanted

noise from the internal circuitry being mixed onto the microphone bias line. Following another predetermined

delay, the CD326x sends an acknowledge tone sequence as described in Section 7.3.2.

When a microphone is biased and in use, the switch-resistor network used for button mode causes large DC

level shifts in the bias voltage. This level shift can result in unwanted, audible clicks, pops, or de-biasing of the

microphone. To prevent these results, when the CD326x enters tone mode, the CD326x disconnects the switch-

resistor network from the microphone bias line, provides power to an external microphone through a shunt

regulator, and engages a tone generation circuit. The TONE pin AC couples the tones onto the microphone bias

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