TDC-GPX datasheet(41/54 Pages) AMSCO

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TDC-GPX

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Start-Offset

For several reasons a programmable offset is added

to the stop time. One reason is to compensate for

different internal delays from the input buffers to the

TDC unit in the Start and Stop paths. The other reason

is to allo the ALU to look "into the past" without han-

dling negative values (which the ALU could not do). It

allows handling Start-Stop intervals down to 0 and

even less. The offset value ‘StartOff1’ is set in register

5 in multiples of 3 x BIN and 18 bits wide. Internally

the start offset is added to the time measurement

result and has to be subtracted from the value read

from the TDC.

Adjusting the Start-Offset

The Start-Offset register of the TDC-GPX allows the

compensation for the offset due to the different inter-

nal delays. It allows to do measurements down to 0 ns

time intervals between Start and Stop. The correct

setting should be done by experiment.

Procedure for single start applications:

1. Set StartOff1 = 0

2. Apply Start and Stop signals with a short delay (e.g.

12ns)

3. Step down the interval and look at the output data.

They are getting smaller and smaller until you pass

the internal Start time stamp. The output data then

jumps to a very high value.

4. Take the time interval tcross (from your generator)

where this happens

5. Calculate StartOff1 = tcross / 3 x BIN and write this

value into register 5, StartOff1. In case you expect

negative values add an additional amount X to Star-

tOff1 and substract this value later on from your

output data

With start retrigger the value StartOff1 should be set

StartOff1 = 1,000 (

≈ 81 ns).

The reason is that the ALU is internally stopped for

about 50 ns during a start retrigger to add the new

start time stamp. If there is a hit during this period the

ALU will need

25 ns + 50 ns = 75 ns to transfer those data to the

IFIFO. The ALU cannot handle negative values. There-

fore the start offset is added so that the ALU can

handle the data being collected during the break. In

case the input data rate is higher (bursts) it might be

necessary to increase also the offset value , e.g. to

10,000.

The start retrigger adds some indeterminacy – due to

the 25ns reference – to whether a stop refers to the

old start or the new one. This is not an uncertainty.

Each time stamp that is negative after offset substrac-

tion can be remapped to the old start by adding the

start period.

Internal Data Processing

The raw values of the stop events are stored in 32-

stage Hit FIFOs. This Hit FIFO can be filled with data at

a peak rate of 182 MHz.

The following pipelined post-processing unit is respon-

sible for compression, Start selection and correct

Stop-Start subtraction. Subsequently a collection unit

transfers the data to the Interface FIFOs, which are

256 stages deep. Each channel has its own interface

FIFO. The maximum rate for transfer into the Interface

FIFO is 40 MHz.

Finally a data multiplexer adds data from both Inter-

face FIFOs to the data bus. The data bus is 28 Bits

wide and capable of 40 MHz transfer rate. The data

bus can be switched to 16 Bit width writing

0x0000010 into address 14. A LOW at pin ‘Output

enable’ forces the bidirectional bus drivers to perma-

nent output state. This is helpful for fast data read out

routines.

Each Interface FIFO has an empty flag (EF) and a load-

level flag (LF). All flags are HIGH active. At low data

rates it is recommended to check the EF to see

whether there are data available for read out. It is not

allowed to read from an empty Interface FIFO. The LF

is helpful at high data rates. The load level threshold

can be set in ‘Fill’ in register 6 and is the same for

both FIFOs. As soon as the set number of data is avail-

able this can be read from the FIFO as a block without

the need of checking the EF.

Note: the load-level flags are not synchronized. The

load-level flag for a FIFO is valid only if it is not read

from this FIFO. Otherwise there might be spikes.

Quiet Mode

TDC-GPX offers two options for the post-processing:

Quiet Mode

Non-quiet Mode

In Quiet Mode the post-processing and calculation

does not start automatically after each single event,

but after a dedicated trigger. The trigger can be given

externally by a rising slope at pin ALUTRIGGER or by

software setting a dedicated ALU-Trigger Bit. This

mode is introduced to reduce the noise during a

measurement and to allow the small values for pulse-

pair and pulse-width resolution.

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