forked from sdr/rtl-sdr
r82xx: improve tuner precision
Improve tuner precision by calculating the VCO divisor at full precision, not at kHz resolution. Also replace the manual divison loop with a simpler fixed-point calculation.
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@ -429,12 +429,11 @@ static int r82xx_set_pll(struct r82xx_priv *priv, uint32_t freq)
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int rc, i;
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unsigned sleep_time = 10000;
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uint64_t vco_freq;
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uint32_t vco_fra; /* VCO contribution by SDM (kHz) */
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uint32_t vco_min = 1770000;
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uint32_t vco_max = vco_min * 2;
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uint32_t freq_khz, pll_ref, pll_ref_khz;
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uint16_t n_sdm = 2;
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uint16_t sdm = 0;
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uint64_t vco_div;
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uint32_t vco_min = 1770000; /* kHz */
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uint32_t vco_max = vco_min * 2; /* kHz */
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uint32_t freq_khz, pll_ref;
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uint32_t sdm = 0;
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uint8_t mix_div = 2;
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uint8_t div_buf = 0;
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uint8_t div_num = 0;
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@ -446,7 +445,6 @@ static int r82xx_set_pll(struct r82xx_priv *priv, uint32_t freq)
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/* Frequency in kHz */
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freq_khz = (freq + 500) / 1000;
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pll_ref = priv->cfg->xtal;
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pll_ref_khz = (priv->cfg->xtal + 500) / 1000;
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rc = r82xx_write_reg_mask(priv, 0x10, refdiv2, 0x10);
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if (rc < 0)
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@ -495,8 +493,24 @@ static int r82xx_set_pll(struct r82xx_priv *priv, uint32_t freq)
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return rc;
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vco_freq = (uint64_t)freq * (uint64_t)mix_div;
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nint = vco_freq / (2 * pll_ref);
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vco_fra = (vco_freq - 2 * pll_ref * nint) / 1000;
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/* We want to approximate:
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* vco_freq / (2 * pll_ref)
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*
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* in the form:
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* nint + sdm/65536
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*
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* where nint,sdm are integers and 0 < nint, 0 <= sdm < 65536
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*
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* Scaling to fixed point and rounding:
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*
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* vco_div = 65536*(nint + sdm/65536) = int( 0.5 + 65536 * vco_freq / (2 * pll_ref) )
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* vco_div = 65536*nint + sdm = int( (pll_ref + 65536 * vco_freq) / (2 * pll_ref) )
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*/
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vco_div = (pll_ref + 65536 * vco_freq) / (2 * pll_ref);
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nint = (uint32_t) (vco_div / 65536);
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sdm = (uint32_t) (vco_div % 65536);
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if (nint > ((128 / vco_power_ref) - 1)) {
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fprintf(stderr, "[R82XX] No valid PLL values for %u Hz!\n", freq);
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@ -511,7 +525,7 @@ static int r82xx_set_pll(struct r82xx_priv *priv, uint32_t freq)
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return rc;
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/* pw_sdm */
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if (!vco_fra)
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if (sdm == 0)
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val = 0x08;
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else
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val = 0x00;
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@ -520,17 +534,6 @@ static int r82xx_set_pll(struct r82xx_priv *priv, uint32_t freq)
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if (rc < 0)
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return rc;
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/* sdm calculator */
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while (vco_fra > 1) {
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if (vco_fra > (2 * pll_ref_khz / n_sdm)) {
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sdm = sdm + 32768 / (n_sdm / 2);
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vco_fra = vco_fra - 2 * pll_ref_khz / n_sdm;
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if (n_sdm >= 0x8000)
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break;
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}
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n_sdm <<= 1;
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}
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rc = r82xx_write_reg(priv, 0x16, sdm >> 8);
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if (rc < 0)
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return rc;
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