TDC-GPX acam messelectronic gmbh, TDC-GPX Datasheet - Page 40

• Manufacturer Part Number: TDC-GPX
• Description: Precision Time Interval Measurement
• Manufacturer: acam messelectronic gmbh

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.
3. Step down the interval and look at the output data.
4. Take the time interval t
5. Calculate StartOff1 = t
With start retrigger the value StartOff1 should be set
to
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
12ns)
They are getting smaller and smaller until you pass
the internal Start time stamp. The output data then
jumps to a very high value.
where this happens
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
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
StartOff1 = 1,000 ( 81 ns).
cross
cross
/ 3 x BIN and write this
(from your generator)
40
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:
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.
Quiet Mode
Non-quiet Mode
TDC-GPX