“Weak Driver” Bidirectional Point-To-Point Topography - Xilinx Zynq-7000 Design Manual

X-Ref Target - Figure 4-9
R = R
O VRN
LVDCI_15
Figure 4-9: DCI Controlled Impedance Bidirectional Point-to-Point Topography
X-Ref Target - Figure 4-10
R = 0.5 x R = 0.5 x R
O VRN
LVDCI_DIV2_15
V
REF
Figure 4-10: HSLVDCI Controlled Impedance Driver Bidirectional Point-to-Point
X-Ref Target - Figure 4-11
Figure 4-11: "Weak Driver" Bidirectional Point-to-Point Topography
In general, parallel resistive termination (R
impedance Z of the transmission line it is terminating. Some interfaces, such as DDR2
0
memory interfaces, use 75Ω termination resistors instead of 50Ω in an effort to open the
data eye. In this case, the trade-off is eye height against a small amount of signal reflection
from the impedance discontinuity. Controlled-impedance drivers are typically tuned such
that the driver output impedance (R
transmission line it is terminating.
Assuming transmission lines with 50Ω characteristic impedance and a driver output
impedance of 25Ω , 50Ω parallel terminations are appropriate
Controlled-impedance drivers, whether implemented with DCI or with weak LVCMOS
drivers, should be sized to have an output impedance (R
of a controlled-impedance driver would be the LVDCI_15 I/O standard. By using 50Ω
Zynq-7000 PCB Design Guide
UG933 (v1.8) November 7, 2014
≈ Z
= R = 50Ω
VRP 0
Z = 50Ω
0
≈ Z
= 50Ω
VRP 0
Z = 50Ω
0
V
Topography
≈ Z
R = 50Ω
O 0
LVCMOS (DRIVE = 6, SLEW = FAST)
Z = 50Ω
0
) is equal to the characteristic impedance (Z
O
www.xilinx.com
R = R = R
O VRN VRP
LVDCI_15
REF
R = 0.5 x R = 0.5 x R
O VRN
LVDCI_DIV2_15
UG933_c4_10_032411
≈ Z
R = 50Ω
O 0
LVCMOS_6F
UG933_c4_11_031711
) has a value equal to the characteristic
P
) of 50Ω . An example of the use
O
Chapter 4: SelectIO Signaling
≈ Z
= 50Ω
0
UG933_c4_09_032411
≈ Z
= 50Ω
VRP 0
) of the
0
(Figure 4-7).
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