Improve auto-calibration of TDC

[samiBendou]

TESTED CONFIGURATION 3 COARSES - 1 FINE - 32 BIT DATA PATH - 200 MHz

current calibration :

• minimum of the sampling line register by changing the delay register (coarse and fine settings)
• leads to average output of TDC around 0x00ffffff

possible calibration :

• minimum distance to 0x0000ffff (whats done in the original project)
• minimum in the data register but > 0x0000fffff (less accurate but faster)

note it is impossible to calibrate the sensor in order to have a data register containing 0x00000000 it seems that at least the first bits are ones (eg . 0x0000000ff)

A reason can be because the clock signal still crosses part of the sampling line whatever the delay tuning is.

[samiBendou]

When switching to 4 coarse delays, the TDC cannot be calibrated and always outputs zeros 0x00000000

[samiBendou]

Calibration in d6ccb4bb9201d607753f02083365e59cd7244548

if (OP_hamming_distance(0xffff, value) < OP_hamming_distance(0xffff, best_value))
{
    best_delay = delay;
    best_value = value;
}

[samiBendou]

Working on multiple TDC simultaneous calibration :

• 1 coarse and fine per TDC
• seeking for either a 0x0000ffff or a 0xffff0000 value in each TDC sensor

[samiBendou]

Chosen calibration to be reviewed

if (abs(16 * iterations - value) < abs(16 * iterations - best_value))
{
    best_value = value;
    best_fine = fine;
    best_coarse = coarse;
}