Spectrum M2p.59 Series, Hardware Manual, Page: 96 / 190

96 (c) Spectrum Instrumentation GmbH
External reference clock Clock generation
Termination of the clock input
If the external connector is used as an input, either for feeding in an external reference clock or for external clocking you can enable a
50 Ohm termination on the board. If the termination is disabled, the impedance is high. Please make sure that your source is capable of
driving that current and that it still fulfills the clock input specification as given in the technical data section.
Clock threshold level
The external clock input of the M2p cards allows to set a threshold level in the range of ± 5V to adopt the input to different logic standards
(such as 1.5V LVTTL, 3.3V LVTTL, 5VTTL etc) as well as to allow to detect an externally AC-coupled clock by setting the level to 0 mV.
The threshold levels for the external clock is to be programmed in mV:
Example for setting the external clock threshold level:
Register Value Direction Description
SPC_CLOCK50OHM 20120 read/write A „1“ enables the 50 Ohm termination at the external clock connector. Only possible, when using
the external connector as an input.
Table 53: Spectrum API: clock threshold software registers and available range therefore
Register Value Direction Description Range
SPC_CLOCK_AVAILTHRESHOLD_MIN 42423 read returns the minimum clock threshold level to be programmed in mV
SPC_CLOCK_AVAILTHRESHOLD_MAX 42424 read returns the maximum clock threshold level to be programmed in mV
SPC_CLOCK_AVAILTHRESHOLD_STEP 42425 read returns the step size of clock threshold level to be programmed in mV
SPC_CLOCK_THRESHOLD 42410 read/write Threshold level for clock input -5000 mV to +5000 mV
spcm_dwSetParam_i32 (hDrv,
SPC_CLOCK_THRESHOLD
, 1500); // set threshold to 1.5V, suitable for 3.3V LVCMOS clock