Platform Cable USB II
X-Ref Target - Figure 28
Propagation delay from A to B (26 ns) captured directly at
the target represents 70% of the total propagation delay
seen by the cable (Figure 25).
TDO Sampling Point
TCK
TDO
TDO Sampling Point
DS593_28_021408
Figure 28: TDO Sampling Example at 12 MHz (Analog Signals on Target System)
Signal Integrity
Platform Cable USB II uses high-slew-rate buffers to drive its output pins. Each buffer has a 30.1Ω series termination
resistor. Users should pay close attention to PCB layout to avoid transmission line effects. Visit the Xilinx Signal Integrity
Central website, and see XAPP361, Planning for High Speed XC9500XV Designs for detailed signal integrity assistance.
If the target system has only one programmable device, the 2-mm connector should be located as close as possible to the
target device. If there are multiple devices in a JTAG or slave-serial single chain on the target system, users should consider
buffering TCK_CCLK_SCK. Differential driver/receiver pairs provide excellent signal quality when the rules identified in
Figure 29 are followed. Buffering is essential if target devices are distributed over a large PCB area.
X-Ref Target - Figure 29
Route A & B traces for each differential
pair in parallel with equal length and
consistent spacing
SN65LVDS105
Four Differential
Drivers
SN65LVDS2 (2)
Series Termination Resistor
(20Ω −30Ω)
TCK_CCLK_SCK
1
4
TCK_CCLK_SCK1
Four
Buffered
Clocks
TCK_CCLK_SCK4
Locate one receiver adacent
to each target device
Locate driver package adjacent to 2-mm connector
DS593_29_021408
Figure 29: Differential Clock Buffer Example
Each differential driver and/or receiver pair contributes approximately 5 ns of propagation delay. This delay is insignificant
when using 12 MHz or slower clock speeds.
DS593 (v1.2.1) March 17, 2011
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