TDC-GPX acam messelectronic gmbh, TDC-GPX Datasheet - Page 33

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

Start-Offset
For several reasons a mathematical offset is added to
the stop time. One reason is to compensate for differ-
ent 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 handling
negative values (which the ALU could not do). It allows
handling Start-Stop intervals down to 0 and even less.
In G-Mode the TDC-GPX creates a second pulse on the
Start channel by its own. Therefore two offset values
have to be set. ‘StartOff1’ in register 5 and StartOff2
in register 6, both in multiples of BIN and 18 bits wide.
Internally the start offset is added to the time meas-
urement 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:
1. Set StartOff1= StartOff2 = 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
6. Continue stepping down the time interval until you
7. Take the time interval t
8. Write this value into register 6
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
12ns)
They are getting smaller and smaller until you pass
the first internal Start time stamp. The output data
then jumps to a very high value.
where this happens
value into register 5, StartOff1
see the same effect as in 3
and calculate StartOff2 = t
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
cross1
cross1
cross2
cross2
/ BIN and write this
(from your generator)
(from your generator)
/ BIN
33
FIFO is 40 MHz, 20 MHz for rising edge data and 20
MHz for falling edge data.
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.
In the Non-quiet Mode, the post-processing starts
immediately after the first hit arrived in a raw FIFO.
The post-processing doesn’t start before there is a
START signal and at least one STOP signal.
The time needed from the start of the post-processing
until first data is available in the interface FIFO is typ.
200 ns.
3.4 Data structure and readout
The output data are integers with a LSB width defined
by the setting of the resolution adjust unit.
Quiet Mode
Non-quiet Mode
TDC-GPX