Evaluation Kit for CMX838 Family Radio Processor

21

EV8380 User Manual

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9.

Wait 250

µs

10. Write $60 to SUBAUDIO PROCESSOR GENERAL CONTROL ($93) register.

a.

Task 0 command (to place device normal run mode with IRQs enabled)

NOTE: This command is not required if the device was already in normal run mode

5.3

Audio Filtering Examples

5.3.1

How to Generate “Melody Tone” And Process It with the CMX838

A common feature in FRS-type radios is the generation of ‘melody tones’ for use as call alerts. The CMX838

can generate audio tone square wave signals and filter them to make them suitable to drive the RXOUT (to

an external speaker path) and TXMOD1 and TXMOD2 (to external RF transmitter paths) output pins, as

desired. The CMX838 eliminates the need for external filtering. Simultaneous output to all of RXOUT,

TXMOD1 and TXMOD2 pins supports playing a melody tone at the local loudspeaker while simultaneously

transmitting it via RF.

For example, to configure the CMX838 to generate an audio “melody tone” and perform signal processing on

it, a typical C-BUS sequence could be:

1.

Configure other registers as necessary for proper baseband clock generation, signal path routing,

amplification, etc.

2.

Configure the tone signaling processor to generate the audio tone frequency of interest as described

in sections 5.2.3.1, Set the Audio Tone Frequency, pg. 18, and 5.2.3.2, Internally Connect the AUX

I/O Pin to the Audio Tone Generator, pg. 18.

3.

Write $DE to SETUP ($5F) register:

a. Tx disabled (this is a ‘don’t care’)

b.

Audio signal input supplied from AUX I/O pin

c.

Audio signal passed from HPF output to Limiter/LPF input

d. Bit 0 is unused and must be set to logic 1.

4.

Write $50 to AUDIO CONTROL ($81) register:

a.

First stage of audio filtering configured as LPF

b.

Audio signal routed through HPF

c.

Audio level (gain stage between pre-emphasis network and HPF) set to 0.0dB

5.

Write $97 to RX AUDIO LEVEL CONTROL ($82) register:

a.

Deviation limiter bypassed.

b.

Post-deviation limiter LPF not bypassed

c.

De-emphasis network is not bypassed

d.

Rx Audio Level (gain stage downstream of de-emphasis network) set to 0.0dB

6.

Connect speaker to connector J3 of the EV8380.

The desired tone should be audible at the speaker.

5.3.2

How to Use Pre-Emphasis/De-Emphasis

For the transmit path, pre-emphasis can be realized either through an appropriate combination of external

components between the microphone and the CMX838 microphone input pin, or by activating the CMX838

internal pre-emphasis filter. The CMX838 internal pre-emphasis filter can be activated by setting bits 7 and 6

internal pre-emphasis filter should not be utilized in the signal flow path.

De-emphasis can be realized by activating the CMX838 internal de-emphasis filter; this is done by setting bit

5 of the RX VOLUME CONTROL register ($82) to 0.