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Figure 6.8 3.5 mm Push-to-Talk Handset Audio Jack Connection Diagram
Push-to-Talk Switch Detection
Push-to-talk handsets provide a switch to the user which will enable the microphone when pushed,
otherwise leaving the microphone deactivated. The PTT Switch Detection will generate an active
low signal when the switch is pressed. In the reference design, this is accomplished with a
comparator. When the switch is not pressed, the microphone is not connected and the DC voltage
applied at the microphone matches the 3.0 V biasing voltage. When the switch is pressed, the
microphone will draw some current, lowering the applied bias voltage below 3.0 V. A comparator
with hysteresis is used to toggle the PTT_DETECT signal.
Table 6.3 Comparator Thresholds and Hysteresis for PTT_DETECT
Condition
Minimum (V)
PTT_DETECT
Falling Edge
PTT_DETECT
Rising Edge
Figure 6.9 Push-to-Talk Handset Switch Operation
Comparator Input Thresholds
Typical (V)
2.77
2.81
2.70
2.75
Chapter 6: Omnii Expansion Ports and Connectors
Audio Reference Designs
Maximum (V)
2.86
2.80
Psion Teklogix Omnii HDK User Manual
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