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zpu: More precision for GPSDO.

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Now we really get to +-1 Hz of the base TCXO frequency precision.

Previously we ignored least significant bits of the frequency due to integer
arithmetics rounding, which meant that we could get up to about 80 ppb of
static frequency error which is not good enough for telco systems.
This commit is contained in:
Alexander Chemeris
2017-07-26 22:03:20 +03:00
parent 7e351e3465
commit dfc98a6900
1 changed files with 10 additions and 4 deletions
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14
zpu/lib/gpsdo.c
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@@ -148,7 +148,11 @@ _gpsdo_pid_step(int32_t val)
/* Driver */
/* ------ */
static int32_t g_val_lpf = PID_TARGET;
/* Extra bits of precision for g_val_lpf */
#define VAL_LPF_PRECISION 3
#define VAL_LPF_INIT_VALUE (PID_TARGET<<VAL_LPF_PRECISION)
static uint32_t g_val_lpf = VAL_LPF_INIT_VALUE;
static void
_gpsdo_irq_handler(unsigned irq)
@@ -156,7 +160,7 @@ _gpsdo_irq_handler(unsigned irq)
if (gpsdo_regs->csr & GPSDO_CSR_RDY)
{
/* Counter value */
int32_t val = gpsdo_regs->cnt;
uint32_t val = gpsdo_regs->cnt;
#ifdef GPSDO_DEBUG
printf("GPSDO Count: %d\n", val);
@@ -173,10 +177,12 @@ _gpsdo_irq_handler(unsigned irq)
if (abs(val - PID_TARGET) < 100000)
{
/* LPF the value */
g_val_lpf = (g_val_lpf * 7 + val + 4) >> 3;
/* Integer overlow warning! */
/* This works for val ~= 52M, but don't try to use it with much larger values - it will overflow */
g_val_lpf = (g_val_lpf * 7 + (val<<VAL_LPF_PRECISION) + 4) >> 3;
/* Update PID */
_gpsdo_pid_step(g_val_lpf);
_gpsdo_pid_step(g_val_lpf>>VAL_LPF_PRECISION);
}
}
}