User Guide

RH-26
2 - System Module CCS Technical Documentation
Page 34 ©Nokia Corporation Issue 1 02/04
that the comparator level is 1.9V. The HeadInt signal does not change the state, even if
the HF output is biased to 0.8V. When the plug is inserted, the switch is opened and the
HeadInt signal is pulled up by the internal pull-up. The 1.9V threshold level is reached
and the comparator output changes to low state causing an interrupt.
The hook signal is generated by creating a short circuit between the headset microphone
signals. In this case, an LP-filter is needed on the HookInt input to filter the audio signal.
In this mode, the earpiece signal on the HF and HFCM pins is in opposite phase. The ear-
piece is driven differentially. When no accessory is present, the HookInt signal is pulled
up by the UEMK resistor. When the accessory is inserted and the microphone path is
biased, the HookInt signal decreases to 1.8V as a result of the microphone bias current
flowing through the resistor. When a button is pressed, the microphone signals are con-
nected together, and the HookInt input gets half of the MicBias DC value 1.1 V. This
change in DC level causes the HookInt comparator output to change state, in this case
from 0 to 1. The button can be used for answering incoming calls but not for initiating
outgoing calls.
Figure 15: RH-26 Audio connections (all components are not shown)
HeadInt comparator reference level is 1.90 V +-0.15 V. HookInt comparator reference is
selected, level is 1.35 V.