XRM(2)-DAC-D4/1G User Guide
V2.2 - Mar 8, 2018
driven by the BUFR signal derived from the BUFIO clock to maintain timing alignment. The global clock signal
which drives the data generation hardware (FABRCLK) runs at the same speed as this. The DCM aligns the
global clock with these clocks and a constraint on the path lengths when crossing the clock domains ensures that
data has the required setup and hold.
The BUFR's and BUFIO's required for each FPGA type are automatically instantiated by the clock mapping code;
in most cases only two sets of BUFR's and BUFIO's are used.
Note that the clock(s) produced by the DCM do not drive the DAC directly, ensuring that any clock jitter added by
the DCM is not transferred to the DAC.
Clock alignment is controlled by a few state machine components. The "ClockTest" block samples the image of
the incoming clock signal via a DDR register over a number of cycles and compares the register output with the
value expected for alignment, setting a pass or fail flag accordingly. The "PhaseAdjust" block shifts the DCM
phase under the control of the "AlignControl" block.
When triggered via the local bus, the "AlignControl" block samples the pass/fail flag from "Clocktest" for the full
range of phase shifts, determines the optimum setting and then implements this phase offset using
"PhaseAdjust". Once completed, AlignControl signals back to the user application the result of the alignment
operation.
This scheme also ensures that all clocks are constrained within the limits imposed by the various components in
the FPGA for all speed grades of the FPGA. The DCM clock runs at half the rate of the DCLK rate. Hence for
1GHz DAC operation, the BUFIO clock (DCLK) runs at 500 MHz maximum whilst the DCM clock (FABRCLK)
runs at 250 MHz maximum.
The BUFR clocks are used to provide diagnostic confirmation of the presence of the clock signals, in the same
way as the spare clock signal noted above. In these cases the "divide-by-2" attribute must be set since the
frequency limit for BUFR signals is 300 MHz maximum.
2.11.1 Low Frequency Operation
The above scheme is suitable for operation down to roughly 100 MHz. Below this frequency the use of the DCM
becomes more problematic (e.g. lower frequency limit of 30 MHz). Interpolation techniques could be used to
maintain the clock above the DCM limits whilst generating data at a much lower speed, but a simpler clocking
scheme (shown in Figure Low frequency clocking scheme) can be used, which also has the benefit of
simplifying the data generation requirements.
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Hardware
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