
LatticeECP/EC and LatticeXP
Lattice Semiconductor
sysIO Usage Guide
8-6
The programmable drive feature also allows the user to match to the impedance of the transmission line.
Table 8-4 shows the drive current setting required to match 50 transmission line with 50 and 200 termina-
tions.
Table 8-4. Impedance Matching Using Programmable Drive Strength
The actual impedance matching may vary on the transmission line design and the load. To find the best matching,
it is recommended to drive the transmission line with different combinations of I/O standards and drive strengths
that best match the line impedance. Lattice provides IBIS buffer models for the users to further analyze the imped-
ance matching.
The figure below shows how this impedance matching is done for a 50 transmission line with 200 termination
using LVCMOS18 I/O buffers programmed to drive 16mA, 12mA, 8mA and 4mA. From this experiment it is empiri-
cal that the best matching is achieved with the 8mA drive setting.
Figure 8-2. Impedance Matching for a 50 Transmission Line with 200 Termination
50 Transmission
Line Termination
()
I/O Standard
Drive Strength (mA)
200
LVCMOS18
8
LVCMOS33
12
50
LVCMOS18
16
LVCMOS33
20
LVCMOS18 16mA
LVCMOS18 12mA