Hi folks, A customer is inquiring as to whether or not they can connect 11 total C6474 devices on a single JTAG chain. Original inquiry below:
We are designing a system with multiple C6474 processors. One board has 6 DSPs and the other has 5. We would like to design the JTAG chain so that we can connect a single Blackhawk Emulator an see all 33 cores of the 11 DSPS. Or connect two emulators and see all of the cores on either the 6 DSP board or the 5 DSP board. SPRU655G says we can connect up to 30 devices ( I assume that a C6474 counts as one device not three) using the single trace configuration. But it then states "use this configuration if you are only interested in trace data from a single device" .
We would like to design the JTAG chain so that we can connect a single Blackhawk Emulator an see all 33 cores of the 11 DSPS. Or connect two emulators and see all of the cores on either the 6 DSP board or the 5 DSP board.
SPRU655G says we can connect up to 30 devices ( I assume that a C6474 counts as one device not three) using the single trace configuration. But it then states "use this configuration if you are only interested in trace data from a single device" .
-Tim
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I feel confident that they could not maintain the routing rules when connecting from one board to another, so 60-pin connection between boards is not realistic. If they only want to get standard emulation without trace, then board-to-board is more likely, and buffers are probably needed.
In section 18, it says
SPRU655G Sec 18Given the source current capability of the individual devices, a maximum of 30 devices on the JTAG serial scan chain or for parallel routing of EMU0 and EMU1 should not be exceeded. Exceeding this number of devices may severely impact performance. When designing target systems with a large number of devices, always minimize the number of vias and trace lengths, and pay special attention to the lengths of individual trace stubs.
This tells you that the 30-device limitation is a package-count limitation and not a DSP core limitation.
In spite of the caution above on vias and trace lengths, I always recommend that new board designs provide a way to isolate each DSP on the scan chain. This can be done with 0-Ohm resistors that can be removed and replaced with bypass wires for debug, for example.
And I have seen cases where the emulation speed drops significantly with more devices on the scan chain. This may have been an anomaly that was later repaired in a DSP revision, but I do not remember for sure.
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