AX5043 Sending FM Tones (ZVEI)

APPLICATION NOTE AND9316/D
AX5043
Sending FM tones (ZVEI)
Revision 2
2
Table of Contents
1. Introduction................................................................................................ 3
2. The AX5043_TX_FM_tones_(ZVEI) Project ................................................. 4
2.1. Routines ....................................................................................................... 4
void init_ax5043(void).................................................................................... 4
void send_tone_prepare(void) ......................................................................... 5
void send_tone(uint8_t n) ............................................................................... 5
void send_tone_end() .................................................................................... 5
void timer0_interrupt(void) ............................................................................. 5
2.2. Variables ...................................................................................................... 6
uint8_t freq_tbl_sin[256][4]............................................................................ 6
uint8_t tone_rclk_div ..................................................................................... 6
uint16_t tone_t0period ................................................................................... 6
const uint16_t tone_tbl[] ................................................................................ 6
const uint16_t tone_len .................................................................................. 6
3. Conclusion .................................................................................................. 7
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Introduction
1. Introduction
This application note describes how to use the AX5043 for transmitting FM tones as they
are used for selective calling (ZVEI and similar schemes).
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The AX5043_TX_FM_tones_(ZVEI) Project
2. The AX5043_TX_FM_tones_(ZVEI) Project
The MASTER/main.c file of the AX5043_TX_FM_tones_(ZVEI) project demonstrates the
transmission of FM tones. Upon pushing button “D” on the DVK-2 mainboard the tones
encoding the digits ‘A’, ‘5’, ‘0’, ‘4’, ‘3’ in the ZVEI scheme are transmitted.
The carrier frequency is 868.3MHz, +/-3kHz FM deviation are used and an 48MHz TCXO
is assumed.
The FM tones are generated as follows: The AX5043 is set up transmitting CW. Using a
TIMER0 interrupt the frequency register is updated periodically in order to generate the
desired tone. This is done for a defined number of sampling points. The procedure can be
repeated for generating further tones. Finally the AX5043 is put into power-down mode
and the MCU goes into sleep mode.
Note: This project was derived from an AX-RadioLAB project. However, due to the nature
of the task, the dependence of the AXRadioV2API has been removed and communication
with the AX5043 radio chip is done directly in the code or via LibMF routines. In order to
work with this project MASTER/MASTER.cbp should be opened using the AXCode::Blocks
IDE.
The files axradiolabstate.xml and AX_Radio_Lab_output/* have been kept. Thus it is still
possible to open the project using AX-RadioLAB in order to compute register values when
attempting to change parameters. However, the files AX_Radio_Lab_output/* are not
used by the code. Changed register values have to be copied manually from
AX_Radio_Lab_output/config.c into the init_ax5043() in main.c (see below).
When using the F143-MINI-DVK rather than the DVK-2 it is necessary to comment in the
line “//#define MINI_KIT” and to comment out “#define USE_LCD” at the top of main.c
2.1.
Routines
void init_ax5043(void)
This routine resets the AX5043, configures the registers necessary for transmit and
ranges the PLL for the frequency configured in the FREQA register. In order to change
parameters as the carrier frequency, internal vs external VCO, XTAL or TCXO frequency
etc. It is easiest to open the project with with AX-RadioLAB, change the corresponding
parameters and update the register values in init_ax5043_regs from the values found in
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The AX5043_TX_FM_tones_(ZVEI) Project
AX_Radio_Lab_output/config.c (the carrier frequency is not assigned to the FREQA
register in config.c, but assigned to the axradio_phy_chanfreq variable).
When cahnging the carrier frequency
freq_tbl_sin[][] table (see below).
it
is
further
necessary
to
update
the
void send_tone_prepare(void)
This routine powers up the transmitter in CW mode. Further TIMER0 is set up to be
clocked at fradio_xtal / 2^tone_rclk_div and to generate interrupts at fs = fradio_xtal /
2^tone_rclk_div / tone_t0period. The frequency fs is the sampling frequency at which
the AX5043 frequency register is updated.
void send_tone(uint8_t n)
This routine sets the frequency of the generated tone by writing the tone_tbl. It further
resets tone_sample_cnt, which measures the length of the generated tone in samples.
The routine returns after transmitting the tone.
void send_tone_end()
This routine puts the AX5043 into power-down and disables the TIMER0.
void timer0_interrupt(void)
This routine toggles the FREQSEL bit in AX5043_PLLLOOP, which switches between the
AX5043_FREQA and AX5043_FREQB registers. It then increments the phase accumulator
tone_phase by tone_freq and writes the frequency of the next sampling point to the
currently unused frequency register. Further tone_cample_cnt is incremented. This
counter measures the length of the tone in samples.
Note: The execution time of the timer0_interrrupt ISR is roughly 10us. Modifications of
this routine, e.g. in order to get rid of the hard-coded carrier frequency, may increase
the execution time and thus require lower fs. (Increase tone_rclk_div or tone_t0period.)
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The AX5043_TX_FM_tones_(ZVEI) Project
2.2.
Variables
uint8_t freq_tbl_sin[256][4]
This array contains the frequency register values sampling one period of the modulation
with 256 points, thus
f carrier  f deviation  sin(2  n / 256) 
2 24
f radio_ xtal
.
For each sampling point the 32bit frequency register value is stored as 4 bytes in the
order {FREQA0, FREQA1, FREQA2, FREQA3}. Note that the PLL requires that bit 0 of the
FREQA0 is always set in order to prevent tonal behaviour of the Sigma-Delta Modulator.
Note that the full frequency is tabulated, not just the deviation from f carrier. This avoids
32bit addition on each frequncy update, allowing maximum sampling speed. The
drawback is the lost flexibility in changing fcarrier.
The file MASTER/print_tab.c contains a simple routine for computing the above table on
the AX8052 MCU and printing it on the DebugLink interface.
uint8_t tone_rclk_div
TIMER0 is clocked at fradio_xtal / 2^tone_rclk_div
Allowed values are [0, 11].
uint16_t tone_t0period
TIMER0 interrupts for updating the frequency register are generated at the sampling
frequency fs = fradio_xtal / 2^tone_rclk_div/ tone_t0period.
Higher fs give higher quality representation of the tones. The default used in the example
project is fs = 48MHz / 2^10 / 1 = 46.875kHz, which is on the high side. Note: The
execution time of the timer0_interrrupt ISR, which handles the frequency adjustment, is
roughly 10us. Modifications of that routine, e.g. in order to get rid of the hard-coded
carrier frequency, may increase the execution time and thus require lower fs.
const uint16_t tone_tbl[]
This array lists the tone frequencies used. Default is ZVEI. The units are (f tone/fs)*2^16,
where fs is the sampling frequency.
const uint16_t tone_len
This variable defines the tone length in samples.
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Conclusion
3. Conclusion
It is possible to transmit analog FM tones as used in various selective calling schemes
using the AX5043.
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