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icE1usb fw/gpsdo: Use PID loop instead of ad-hoc algo 

The original algo is just somehting I originally came up with
on the spot and worked and I never got the time to revisit.

Now after some testing, I implemented a PID loop that seems to
present faster lock time and at least as good stability.

Parameters were not thorougly optimized just some 'hand' tuning
trying to simulate the behavior.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: Ie3ea7243aa4234585f1ace222632bb5580caca75
This commit is contained in:
Sylvain Munaut 2022-10-04 14:54:36 +02:00
parent ad0fc21d61
commit b8fc8a6a64
1 changed files with 22 additions and 28 deletions
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50
firmware/ice40-riscv/icE1usb/gpsdo.c
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@ -49,8 +49,8 @@ struct {
/* Fine tracking */
struct {
int acc;
int div;
int acc; /* Accumulated error */
int integral; /* PID Integral term */
} fine;
} g_gpsdo;
@ -60,6 +60,7 @@ struct {
* VCXTO parameters
*
* - iKv is reciprocal sensitivity vs 'coarse' count for fast initial acquisition
* - Kp, Ki, Kd are params for the fine-tracking PID loop
*
* Note that the spec if often a guaranteed minimum range and goes
* from ~0.1V to 3.2V instead of 0-3.3V so actual sensitivity is
@ -67,13 +68,22 @@ struct {
*/
static const struct {
int iKv; /* hi-count / Hz (.8 fixed point) */
int Kp; /* PID proportional term (.12 fixed point) */
int Ki; /* PID integral term (.12 fixed point) */
int Kd; /* PID differential term (.12 fixed point) */
} vctxo_params[] = {
[VCTXO_TAITIEN_VT40] = {
.iKv = 300, /* +- 50 ppm pull range => ~ 0.75 Hz / hi-count (set to 0.85) */
.Kp = 14336,
.Ki = 410,
.Kd = -6144,
},
[VCTXO_SITIME_SIT3808_E] = {
.iKv = 160, /* +- 100 ppm pull range => ~ 1.50 Hz / hi-count (set to 1.6) */
.Kp = 7168,
.Ki = 205,
.Kd = -4096,
},
};
@ -188,7 +198,7 @@ _gpsdo_fine_start(void)
/* Reset the long term error tracking */
g_gpsdo.fine.acc = 0;
g_gpsdo.fine.div = 0;
g_gpsdo.fine.integral = 0;
}
static void
@ -219,7 +229,6 @@ static void
_gpsdo_fine_track(uint32_t tick_diff)
{
int freq_diff = (int)tick_diff - TARGET;
uint16_t tune;
/* Did we deviate too much ? */
if ((freq_diff < -2*MAX_DEV_FINE) || (freq_diff > 2*MAX_DEV_FINE)) {
@ -227,30 +236,16 @@ _gpsdo_fine_track(uint32_t tick_diff)
return;
}
/* Accumulate long term error */
/* Accumulate long term error and integrate it */
g_gpsdo.fine.acc += freq_diff;
g_gpsdo.fine.integral += g_gpsdo.fine.acc;
/* Update fine tuning value */
if (((g_gpsdo.fine.acc >= 0) && (freq_diff > 0)) ||
((g_gpsdo.fine.acc <= 0) && (freq_diff < 0)))
{
g_gpsdo.tune.fine -= freq_diff;
}
if (g_gpsdo.fine.acc) {
if (++g_gpsdo.fine.div > 10) {
g_gpsdo.fine.div = 0;
if (g_gpsdo.fine.acc > 0)
g_gpsdo.tune.fine--;
else if (g_gpsdo.fine.acc < 0)
g_gpsdo.tune.fine++;
}
} else {
g_gpsdo.fine.div = 0;
}
/* Compute value with a bias from long term accumulator */
tune = g_gpsdo.tune.fine - (g_gpsdo.fine.acc / 2);
/* PID */
g_gpsdo.tune.fine = 2048 - ((
vctxo_params[g_gpsdo.vctxo].Kp * g_gpsdo.fine.acc +
vctxo_params[g_gpsdo.vctxo].Ki * g_gpsdo.fine.integral +
vctxo_params[g_gpsdo.vctxo].Kd * freq_diff
) >> 12);
/* If fine tune is getting close to boundary, do our
* best to transfer part of it to coarse tuning */
@ -260,13 +255,12 @@ _gpsdo_fine_track(uint32_t tick_diff)
g_gpsdo.tune.coarse += coarse_adj;
g_gpsdo.tune.fine -= coarse_adj << 6;
tune -= coarse_adj << 6;
pdm_set(PDM_CLK_HI, true, g_gpsdo.tune.coarse, false);
}
/* Apply fine */
pdm_set(PDM_CLK_LO, true, tune, false);
pdm_set(PDM_CLK_LO, true, g_gpsdo.tune.fine, false);
/* Debug */
#ifdef GPSDO_DEBUG