Description: Xilinx Virtual Cable (XVC) is a TCP/IP-based protocol that acts like a JTAG cable and provides a means to access and debug your FPGA or SoC design without using a physical cable.
This capability helps facilitate hardware debug for designs that:
- Have the FPGA in a hard-to-access location, where a "lab-PC" is not close by
- Do not have direct access to the FPGA pins – e.g. the JTAG pins are only accessible via a local processor interface
- Need to efficiently debug Xilinx FGPA or SoC systems deployed in the field to save on costly or impractical travel and reduce the time it takes to debug a remotely located system.
- Ability to debug a system over an internal network, or even the internet.
- Debug via Vivado Logic Analyzer IDE exactly as if directly connected to design via standard JTAG or parallel cable
- Zynq®-7000 demonstration with Application Note and Reference Designs available in XAPP1251 - Xilinx Virtual Cable Running on Zynq-7000 Using the PetaLinux Tools
- Extensible to allow for safe, secure connections
XVC 1.0 Protocol
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Xilinx Virtual Cable (XVC) is a TCP/IP-based communication protocol that acts like a JTAG cable and provides a means to access and debug your FPGA or SoC design without using a physical cable. In this document are the general details of this XVC 1.0 protocol. For source code examples of this protocol please visit:
https://github.com/Xilinx/XilinxVirtualCable
The XVC protocol is used by a TCP/IP client and server to transfer low level JTAG vectors from a high level application to a device. The client, for instance Xilinx's EDA tools, will connect to an XVC server using standard TCP/IP client/server connection methods. Once connected, the client will issue an XVC message (getinfo:) to the server requesting the server version. After the client validates the protocol version, a new message is sent to the XVC server to set the JTAG tck rate for future shift operations.
The shift message is the main command used between the client and the server to transfer low level JTAG vectors. The client will issue shift operations to determine the JTAG chain composition and then perform various JTAG instructions for instance driving pins or programming a device.
In summary, XVC 1.0 protocol defines a simple JTAG communication method with sufficient capabilities for high level clients, like Xilinx Vivado and SDK tools, to perform complex functions like programming and debug of devices. In this document is a basic description of the protocol. The intent of this document is to provide a blueprint for users of the XVC 1.0 protocol to create their own custom clients and servers.
The XVC 1.0 communication protocol consists of the following three messages:
getinfo:
settck:<period in ns>
shift:<num bits><tms vector><tdi vector>
For each message the client is expected to send the message and wait for a response from the server. The server needs to process each message in the order recieved and promptly provide a response. Note that for the XVC 1.0 protocol only one connection is assumed so as to avoid interleaving locking and interleaving issues that may occur with concurrent client communication.
The primary use of "getinfo:" message is to get the XVC server version. The server version provides a client a way of determining the protocol capabilites of the server.
Syntax
Client Sends:
"getinfo:"
Server Returns:
“xvcServer_v1.0:<xvc_vector_len>\n”
Where:
<xvc_vector_len> is the max width of the vector that can be shifted
into the server
The "settck:" message configures the server TCK period. When sending JTAG vectors the TCK rate may need to be varied to accomodate cable and board signal integrity conditions. This command is used by clients to adjust the TCK rate in order to slow down or speed up the shifting of JTAG vectors.
Syntax:
Client Sends:
"settck:<set period>"
Server Returns:
“<current period>”
Where:
<set period> is TCK period specified in ns. This value is a little-endian
integer value.
<current period> is the value set on the server by the settck command. If
the server cannot set the value then it will return the
current value.
The "shift:" message is used to shift JTAG vectors in and out of a device. The number of bits to shift is specified as the first shift command parameter followed by the TMS and TDI data vectors. The TMS and TDI vectors are sized according to the number of bits to shift, rouneded to the nearest byte. For instance if shifting in 13 bits the byte vectors will be rounded to 2 bytes. Upon completion of the JTAG shift operation the server will return a byte sized vector containing the sampled target TDO value for each shifted TCK clock.
Syntax:
Client Sends:
"shift:<num bits><tms vector><tdi vector>"
Server Returns:
“<tdo vector>”
Where:
<num bits> : is a integer in little-endian mode. This represents the number
of TCK clk toggles needed to shift the vectors out
<tms vector> : is a byte sized vector with all the TMS shift in bits Bit 0 in
Byte 0 of this vector is shifted out first. The vector is
num_bits and rounds up to the nearest byte.
<tdi vector> : is a byte sized vector with all the TDI shift in bits Bit 0 in
Byte 0 of this vector is shifted out first. The vector is
num_bits and rounds up to the nearest byte.
<tdo vector> : is a byte sized vector with all the TDO shift out bits Bit 0 in
Byte 0 of this vector is shifted out first. The vector is
num_bits and rounds up to the nearest byte.