
Altera Corporation
5–79
April 2011
Stratix II Device Handbook, Volume 1
DC & Switching Characteristics
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 5–9). Therefore, any distortion on the input
clock and the input clock buffer affect the output DCD.
Figure 5–9. DCD Measurement Technique for DDIO (Double-Data Rate) Outputs
When an FPGA PLL generates the internal clock, the PLL output clocks
the IOE block. As the PLL only monitors the positive edge of the reference
clock input and internally re-creates the output clock signal, any DCD
present on the reference clock is filtered out. Therefore, the DCD for a
DDIO output with PLL in the clock path is better than the DCD for a
DDIO output without PLL in the clock path.
derivation for different I/O standards on Stratix II devices. Examples are
also provided that show how to calculate DCD as a percentage.
DQ
inst2
PRN
CLRN
inst8
DFF
INPUT
VCC
clk
NOT
OUTPUT
output
IOE
DQ
inst3
PRN
CLRN
DFF
VCC
GND
Table 5–80. Maximum DCD for Non-DDIO Output on Row I/O Pins (Part 1
of 2)
Row I/O Output
Standard
Maximum DCD for Non-DDIO Output
-3 Devices
-4 & -5 Devices
Unit
3.3-V LVTTTL
245
275
ps
3.3-V LVCMOS
125
155
ps
2.5 V
105
135
ps