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JTAG cables for Xilinx FPGA dynamic probe
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on Apr 2, 2008
on Apr 2, 2008 by Souhaibe_Forums
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What Xilinx JTAG cables work with FPGA dynamic probe? And Does the FPGA dynamic probe work if I have non-Xilinx devices on the scan chain?
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Apr 2, 2008 2:38 PM
Xilinx Parallel III and IV MultiLINX cables and the platform USB cable. The cable makes a connection between the parallel or USB port on the logic analyzer and the JTAG pins on the FPGA being measured. Xilinx Platform Cable USB is supported on logic analyzers with Version 3.2 or higher software.
For non xilinx devices on the scan chain: The application can handle up to 256 devices in a scan chain. The devices can be Xilinx or non-Xilinx devices. When a non-Xilinx device is found, dynamic probe will ask for the JTAG IR length. This value is readily available from the chip manufacturer. The application saves this value and uses it when communicating with your scan chain.
When the application needs to talk to a selected Xilinx FPGA device, it puts all devices into bypass mode except for the selected device. BYPASS mode is a JTAG required function in which the device simply adds a flop delay between TDI data to TDO. When in bypass the data is ignored by the BYPASS-ed device and it operates as it normally does (for example a microprocessor keeps running user code).
One thing to keep in mind is that Xilinxâ€™s parallel programmer hardware may not support driving 256 devices in one scan chain. In particular the TCK and TMS lines are heavily loaded when JTAG is configured as a chain. The reason is that the cable will drive TCK and TMS to all devices in a scan chain. For this reason you should consider buffering these signals on you board if you have more than six devices in a scan chain (simple rule of thumb). One other thing you can do if you have more than six devices or are simply having problems connecting to your FPGA is simply lower the JTAG TCK speed.
With dynamic probe you can select from various speeds down to 200KHz as a TCK rate. By slowing down TCK you will increase the setup/hold margin for TMS on all devices and TDI on the first device.
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