Series-Terminated Unidirectional, Point-To-Point Topography; “Weak Driver” Unidirectional, Point-To-Point Topography - Xilinx Zynq-7000 Design Manual

X-Ref Target - Figure 4-2
Figure 4-2: Series-Terminated Unidirectional, Point-to-Point Topography
X-Ref Target - Figure 4-3
Figure 4-3: DCI-Controlled Impedance Driver Unidirectional, Point-to-Point
X-Ref Target - Figure 4-4
Figure 4-4: "Weak Driver" Unidirectional, Point-to-Point Topography
In general, parallel resistive termination (R
impedance (Z ) of the transmission line it is terminating. Series resistive terminations (R
0
have a value equal to the characteristic impedance of the transmission line (Z
output impedance of the driver (R
drivers are tuned such that the driver output impedance (R
impedance (Z ) of the transmission line it is terminating.
0
Assuming transmission lines with 50Ω characteristic impedance and a driver output
impedance (R
O
(Figure 4-1) is appropriate. Controlled-impedance drivers, whether implemented with DCI
or with weak LVCMOS drivers, should be sized to have an output impedance (R
This corresponds to VRN and VRP resistors equal to 50Ω for DCI. Weak LVCMOS drivers of
6 mA to 8 mA drive strength have an output impedance approximately equal to
50Ω (Figure 4-3).
Typically, parallel terminations have best performance when V
connected to the parallel termination resistor) is equal to half of the signaling voltage. For
2.5V signals (V
CCO
it is possible to use a Thevenin parallel termination. Thevenin parallel termination consists
of a voltage divider with a parallel equivalent resistance (R
impedance of the transmission line (50Ω in most cases). The divided voltage point is
designed to be at V
2.5V V , made up of two 100Ω resistors, resulting in a V
CCO
equivalent resistance (R
Zynq-7000 PCB Design Guide
UG933 (v1.8) November 7, 2014
R = 25Ω
O
R = Z – R = 25Ω
S 0 0
LVDCI
> Z
R = R = R
0 =
O VRN VRP
Z = 50Ω
0
Topography
LVCMOS (DRIVE = 6, SLEW = FAST)
O ≈
R Z ~ 50Ω
0
Z
0
) to which they are connected. Controlled-impedance
O
) of 25Ω , a 25Ω series termination
= 2.5V), V is ideally 1.25V. In cases where this voltage is not available,
TT
. Figure 4-5 illustrates a Thevenin parallel termination powered from
TT
) of 50Ω .
PEQ
www.xilinx.com
Z = 50Ω
0
UG933_c4_02_031711
50Ω
UG933_c4_03_031711
= 50Ω
UG933_c4_04_031711
) has a value equal to the characteristic
P
O
(Figure 4-2) or a 50Ω parallel termination
PEQ
TT
Chapter 4: SelectIO Signaling
) minus the
0
) is equal to the characteristic
) of 50Ω .
O
(the voltage source
TT
) equal to the characteristic
of 1.25V and a parallel
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)
S
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