CML Microcircuits CMX7031 Application Note page 5

Application Note
3. The user can compose the text message by one of the two following methods:
•
Preset text messages. The user can scroll through a list of preprogrammed text
messages stored on the host microcontroller and highlight the desired message.
•
User typed text messages. The user enters the desired text through the radio's
keypad.
4. Once these steps are performed and the "Send" button is pressed, the host microcontroller
and CMX7031 can generate an FFSK / MSK data burst containing both the recipient's
address and the actual message data.
If a received message has errors, the radio can use its CMX7031 to transmit a "retransmission
request" via an FFSK / MSK data burst to the remote radio.
3.2 Receive Functions
In this application, either voice or FFSK / MSK data can be received.
The FRS radio, configured to receive on FRS channel 1 (462.5625MHz), will normally be in "sleep"
mode waiting to receive a call. The CMX7031's ADC1 monitors the RSSI signal from the RF
transceiver. An incoming transmission will cause the RSSI signal to exceed the ADC's "high"
threshold and cause an interrupt to be generated. The host microcontroller should then read the
STATUS ($C6) and AUXADC1 DATA ($A9) registers to ensure that the source of the interrupt was a
rising RSSI signal. Once the IRQ source has been verified as RSSI, the CMX7031 should be
configured for both voice and FFSK / MSK reception.
The next IRQ should indicate the type of incoming call:
•
If STATUS ($C6) b11 = 1, CTCSS detection has occurred and a voice call is imminent.
•
If STATUS ($C6) b4 = 1, 2400 bps FFSK / MSK has been detected and a data burst is
imminent.
If the incoming transmission is a voice call, the CMX7031 will:
•
Generate a ring tone to alert the user that a call is about to be heard.
•
Process the recovered voice signal for presentation to the external speaker driver.
If the incoming transmission is FFSK / MSK data, the host microcontroller will interpret the received
FFSK / MSK message and proceed accordingly.
The user can press the radio's "Monitor" button to listen for all activity on a channel. When this
happens:
1. An audible alerting tone will be generated by the CMX7031 so the user knows a button was
pressed.
2. The CMX7031 will activate its audio output amplifier to allow any received signal to be passed
to the radio's speaker or headset.
© 2007 CML Microsystems PLC
CMX7031
: An Example Core Design for a Complete Family Radio
5 of 28 AN/2WR/7031/FRS/3 July 2007