Revision as of 17:14, 28 January 2015

CDC time & integral algorithm

When the trigger signal arrives, the presample buffer contains NP (default value 16) samples for pedestal followed by NW (default 100, possibly up to 155) samples to contain full range of drift time.

Quantities to be returned, if a hit is found

• Leading edge time (11 bits, units of sample/10)
• Time quality factor (1 bit) set to 0 if time is good, 1 if time is rough estimate
• Pedestal immediately before the hit (8 bits, set to 255 if >254)
• Signal integral (n bits, scaled down by factor 2^m, all bits set if greater than range; n,m are 14,4)
• Signal maximum (p bits, scaled down by factor 2^q; p,q approx 8,2) this is the first maximum in the signal after the threshold crossing
• Overflow count (3 bits, set to 7 if >6)

Configuration constants (all values are integers, σ=pedestal width ~20)

• NPED: number of samples in pedestal window (default 16)
• WINDOW_START: sample number of first sample in hit search window, this is immediately after the pedestal window
• WINDOW_END: last sample in hit search window
• HIT_THRES: threshold to identify hit (default 5σ ~100, range 50 to 300)
• NSAMPLES: number of ADC samples in subset to pass to time finding module (default 15)
• XTHR_SAMPLE: position in subset of first sample above hit_threshold (default 9, starts with 0)
• PED_SAMPLE: position in subset of sample to be used as local pedestal (default 5, starts with 0)

Timing algorithm configuration constants

• HIGH_THRESHOLD: High timing threshold (default 4σ ~ 80, range 40 to 270) must be lower than hit threshold
• LOW_THRESHOLD: Low timing threshold (default 1σ ~ 20, range 5 to 30)
• ROUGH_DT: rough time - if timing fails, return hit time of xthr_sample-RT (default 24, units sample/10)
• INT_SAMPLE: if timing fails, start integration with this sample (default 6).
• NUPSAMPLED: number of upsampled values to calculate (default 8)
• LIMIT_PED_MAX: do not calculate accurate time if any of samples 0 to PED_SAMPLE in subset exceeds this (default 511)
• LIMIT_ADC_MAX: do not calculate accurate time if any sample in the subset exceeds this (default 4095)
• SET_ADC_MIN: reduce chances of calculating a negative upsampled value by adding a constant offset to the subset values such that the minimum is equal to SET_ADC_MIN (default 20)
• LIMIT_UPS_ERR: do not calculate accurate time if the sum of the error of the 2 upsampled values exceeds this (default 30)

Hit finding module

1. Wake up on trigger
2. Calculate start_pedestal as mean of 4 samples window_start-5 to window_start-1
3. Search from window_start to window_end for ADC value rising above start_pedestal + hit_threshold. If not found, go back to sleep. If found, continue.
4. Call up time-finding module and send in nsamples ADC values from the hit threshold crossing region, with the threshold-crossing sample x as value number xthr_sample in the sequence (starting at 0)
5. Call up signal maximum module
6. Call up signal integral module
7. Return local_pedestal as value of sample (x + ped_sample - xthr_sample), or 255 if local_pedestal>254
8. Return integral and overflow count from integral module
9. Return q_code from time module
10. Return (x - xthr_sample)*10 + time returned from time module, units are 0.1*sample-period (0.8ns)

Time finding module

This returns an error code q_code and the hit time, relative to the threshold-crossing sample time

1. Calculate thresholds from constants and local_pedestal, which is sent in as in sample number ped_sample
2. Search the samples from ped_sample+1 to nsamples-1 for value >= high_threshold + local_pedestal. If this threshold is not exceeded, return bit q_code=1 and time x*10-rough_dt
3. Search back from high_threshold crossing down toward ped_sample to find sample y where value <= low_threshold + local_pedestal
4. Calculate 7 upsampled values, from y-0.2 to y+1.2
5. Search the upsampled values from y+1.2 down to y-0.2 to find where the value <= low_threshold + local_pedestal
6. Interpolate between the upsampled value just found and the next to find the crossing time in units of sample/10
7. Return crossing time found and q_code=0

Signal maximum module

This returns the signal maximum

1. Inspect ADC values after sample x, find the first value which is less than its predecessor
2. Return value of the predecessor scaled down by 2^p

Signal integral module

This returns the signal integral and overflow count

1. Sum signal-local_pedestal, from sample x-integral_offset to sample end_window, and count the number of samples with overflow bit set
2. Return integral scaled down by 2^m (return all bits set if value exceeds that of bits available)
3. Return overflow count (return 7 if count>6)

Upsampled data

ADC samples in (big circles) and upsampled data out (small blue dots)

ADC samples in (big circles) and upsampled data out (small blue dots)

ADC samples in (big circles) and upsampled data out (small blue dots)