7-22
Theory of Operation
The allowable internal alert tones are 304, 608, 911, and 1823Hz. In this case a
code contained within the SPI BUS load to the ASFIC sets up the path and
determines the tone frequency, and at what volume level to generate the tone. (It
does not have to be related to the voice volume setting).
For external alert tones, the
µ
P can generate any tone within the 100-3000Hz
audio band. This is accomplished by the
µ
P generating a square wave which enters
the ASFIC at U0201-C3.
Inside the ASFIC, this signal is routed to the alert tone generator; the output of the
generator is summed into the audio chain just after the RX audio de-emphasis
block. Inside U0201 the tone is amplified and filtered, then passed through the 8-
bit digital volume attenuator, which is typically loaded with a special value for
alert tone audio. The tone exits at U0201-J4 and is routed to the audio PA like
receive audio.
