Duty Cycle Distortion
(T/2 – D1) / T (the low percentage boundary)
(T/2 + D2) / T (the high percentage boundary)
DCD Measurement Techniques
DCD is measured at an FPGA output pin driven by registers inside the
corresponding I/O element (IOE) block. When the output is a single data
rate signal (non-DDIO), only one edge of the register input clock (positive
or negative) triggers output transitions (Figure 5–9). Therefore, any DCD
present on the input clock signal, or caused by the clock input buffer, or
different input I/O standard, does not transfer to the output signal.
Figure 5–9. DCD Measurement Technique for Non-DDIO (Single-Data Rate) Outputs
IOE
DFF
output
D
Q
clk
However, when the output is a double data rate input/output (DDIO)
signal, both edges of the input clock signal (positive and negative) trigger
output transitions (Figure 5–10). Therefore, any distortion on the input
clock and the input clock buffer affect the output DCD.
5–68
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2008
ALTERA [ ALTERA CORPORATION ]
