Avidyne IFD540 Pilot's Manual page 27

Emergency Com Frequency
121.5 kHz can be quickly put into the Active com slot by pressing
and holding the bezel frequency swap button for approximately 3
seconds.
Stuck Mic
If the IFD determines that there is a stuck mic situation (defined
as 30 seconds or more of continuous transmission), a "Stuck Mic"
CAS message (Blue Advisory message) will be displayed.
Remote Tuning Control and Frequency Swapping
The IFD supports a host of remote tuning and frequency
swapping capability.
A "Com Presets" LSK will be present on the AUDIO tab of the
AUX page and up to 16 com frequencies can be preset. Some
aircraft installations will also include a dedicated Remote Tuning
control (e.g. dedicated button on the yoke) which will step through
the list of preset com frequencies loading each into the #1
Standby slot. Each time that control is activated, the display will
pop up a dialog box next to the #1 Standby slot indicating which
item in the preset com list has been selected. In addition, a cyan
arrowhead will be displayed next to the currently selected
frequency on the Com Preset page.
Another capability that can be added during installation is a
remote com frequency swapping function that, when activated,
performs the same action as pressing the bezel Frequency Swap
button. In this case, your hands do not need to leave the controls
to command a frequency swap. Typical installations that support
this feature include a dedicated Frequency Swap control (e.g.
dedicated button on the yoke). Pressing and holding the
dedicated Frequency Swap control button for approximately 3
seconds will insert 121.5 kHz into the Active com slot.
Other remote tuning and frequency swapping capability is present
when dedicated third-party radio control display units are wired
into the aircraft. Usually in these cases, when the radio control
display unit is active, the Active and #1 Standby frequencies are
only displayed on that external control display unit and not on the
IFD, even though the actual radio is still housed inside the IFD.
System Overview 1-21