CDC readout requirements

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Aim to establish how many bits are needed for each quantity, then later fit the quantities into the words.

First word has (header +) 15 bits available for data Second word has 31 bits for data


11 bits, 0-2047, firm (minimum)

I expect max drift time of 155 samples. To record time in tenths of samples, I need max value of 1550 which requires 11 bits, 0-2047.

(Garfield calcs for 2.24T, 2.1kV, 47% Ar 53% CO2, 10mm radius straws give max drift time 1.19us = 150 x 8ns from max radius 9.5mm. Garfield calcs for 2.0T, 2.1kV, 47% Ar 53% CO2, 10mm radius straws give max drift time 1.20us = 150 x 8ns from max radius 9.6mm and 1.17us from 9.5mm.)

I am using units of sample/10 because the upsampling works in units of sample/5 and it is straightforward to find the threshold crossing and then interpolate using units of sample/10. Multiplying this x 1.25 to give ns is possible but it makes the output number larger without adding precision, and it uses more clock cycles (ie the calculation takes longer), I think it is better to keep the firmware to the minimum and then convert to ns later on somewhere else. If the FDC needs better precision than 0.8ns then I could have it interpolate further but this takes yet more clock cycles.

QF time

1 bit (firm)

1 bit to indicate that less accurate time is being returned

QF overflow

3 bits, 0-7

Count up to 6 (and indicate 7 or more) overflow samples; this info might also be deduced from the integral if it maxes out.


14 bits, 0-16383

Scale integral x 1/16 to fit into 14 bits. Need to cover min ~80 to max ~ 200,000 Output max value if it is exceeded.


8 bits, 0-255

No scaling; output 255 for 255 and higher. Expect pedestal width < 20. Set pedestal height at 4sigma=80 and expect it to vary between 1sigma=20 and 7sigma=140.

Max amplitude

Scale full range 0-4095 x 1/8 to fit into 9 bits MIP gives a handle on gain shifts; also amplitude might be useful for dedx.