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/* AFC (Automatic Frequency Correction) Implementation */
/* (C) 2010 by Harald Welte <laforge@gnumonks.org>
*
* All Rights Reserved
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*
*/
#include <stdint.h>
#include <stdio.h>
#include <debug.h>
#include <osmocore/gsm_utils.h>
#include <layer1/afc.h>
#include <layer1/avg.h>
#include <calypso/dsp.h>
#define AFC_INITIAL_DAC_VALUE -700
/* Over how many TDMA frames do we want to average? (this may change in dedicated mode) */
#define AFC_PERIOD 40
/* How many of our measurements have to be valid? */
#define AFC_MIN_MUN_VALID 8
/* The actual AFC code */
struct afc_state {
struct running_avg ravg; /* running average */
int16_t dac_value; /* current DAC output value */
uint16_t arfcn;
};
static void afc_ravg_output(struct running_avg *ravg, int32_t avg);
static struct afc_state afc_state = {
.ravg = {
.outfn = &afc_ravg_output,
.period = AFC_PERIOD,
.min_valid = AFC_MIN_MUN_VALID,
},
.dac_value = AFC_INITIAL_DAC_VALUE,
};
/* The AFC DAC in the ABB has to be configured as follows:
* DAC = 1MHz / 947MHz * FreqErr(Hz) / AFCslop(ppm/LSB)
* where:
* 947 MHz is the center of EGSM
* AFCslope is coded F1.15, thus a normalization factor of 2^15 aplpies
*/
#define AFC_NORM_FACTOR_GSM ((1<<15) / 947)
#define AFC_NORM_FACTOR_DCS ((1<<15) / 1894)
/* we assume 8.769ppb per LSB, equals 0.008769 * 32768 == 287 */
//#define AFC_SLOPE 320
#define AFC_SLOPE 287
/* The DSP can measure the frequency error in the following ranges:
* FB_MODE0: +/- 20 kHz
* FB_MODE1: +/- 4 kHz
* Sync Burst: +/- 1 kHz
* Normal Burst: +/- 400 Hz
*/
/* Update the AFC with a frequency error, bypassing averaging */
void afc_correct(int16_t freq_error, uint16_t arfcn)
{
int32_t afc_norm_factor;
int16_t delta;
switch (gsm_arfcn2band(arfcn)) {
case GSM_BAND_900:
case GSM_BAND_850:
afc_norm_factor = AFC_NORM_FACTOR_GSM;
break;
default:
afc_norm_factor = AFC_NORM_FACTOR_DCS;
}
delta = (int16_t) ((afc_norm_factor * (int32_t)freq_error) / AFC_SLOPE);
printd("afc_correct(error=%dHz, arfcn=%u): delta=%d, afc_dac(old=%d,new=%d)\n",
freq_error, arfcn, delta, afc_state.dac_value, afc_state.dac_value+delta);
afc_state.dac_value += delta;
/* The AFC DAC has only 13 bits */
if (afc_state.dac_value > 4095)
afc_state.dac_value = 4095;
else if (afc_state.dac_value < -4096)
afc_state.dac_value = -4096;
}
void afc_reset(void)
{
afc_state.dac_value = AFC_INITIAL_DAC_VALUE;
}
void afc_input(int32_t freq_error, uint16_t arfcn, int valid)
{
afc_state.arfcn = arfcn;
runavg_input(&afc_state.ravg, freq_error, valid);
runavg_check_output(&afc_state.ravg);
}
/* callback function for runavg */
static void afc_ravg_output(struct running_avg *ravg, int32_t avg)
{
afc_correct(avg, afc_state.arfcn);
}
/* Update DSP with new AFC DAC value to be used for next TDMA frame */
void afc_load_dsp(void)
{
dsp_api.db_w->d_afc = afc_state.dac_value;
dsp_api.db_w->d_ctrl_abb |= (1 << B_AFC);
}
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