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64 lines
2.6 KiB
Plaintext
64 lines
2.6 KiB
Plaintext
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jtag-boot (for Milkymist One)
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=============================
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This collection of scripts talks via JTAG to the FPGA of a Milkymist One.
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It makes the FPGA load a bitstream (FPGA configuration) from NOR.
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Elements involved
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-----------------
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The mkboot script generates a very short bitstream that instructs the
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configuration subsystem of the FPGA to perform the boot operation.
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A bitstream is a sequence of instructions for the configuration subsystem.
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Bitstreams are used for various purposes, for example, the configuration
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data of the FPGA is also contained in a (much larger) bitstream.
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mkboot also adds the minimum set of headers of the ".bit" format to the
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bitstream. This allows the bitstream to be transferred to the FPGA via
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JTAG, using the UrJTAG utility.
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UrJTAG is controlled via the boot.jtag script, which performs the basic
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initialization of the JTAG system, defines a few chip-specific constants,
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and then loads and executes the bitstream generated by mkboot.
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Bitstream content and format
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----------------------------
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The bitstream instructs the configuration subsystem of the to erase the
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present FPGA configuration, load a new one from NOR, and then let the
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FPGA "run" it.
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The structure of this kind of bitstream is described on Table 7-1 on page
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126, section "IPROG Reconfiguration", of [1]. The format of the bitstream
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is described in the section "Configuration packets" starting at page 88
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of [1].
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The NOR address from where the configuration bitstream will be loaded was
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obtained from the M1 standby bitstream, starting around line 205 of [2].
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Finally, the format of the .bit file was simply taken from the function
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xlx_bitstream_load_bit of [3]. This is the same function in UrJTAG that
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loads the file with the "pld load" command.
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[1] Xilinx. "Spartan-6 FPGA Configuration User Guide"
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http://www.xilinx.com/support/documentation/user_guides/ug380.pdf
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[2] https://github.com/milkymist/milkymist/blob/master/boards/milkymist-one/standby/standby.v
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[3] http://urjtag.git.sourceforge.net/git/gitweb.cgi?p=urjtag/urjtag;a=blob_plain;f=urjtag/src/pld/xilinx_bitstream.c;hb=HEAD
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System state and behaviour
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--------------------------
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This process differs from a regular boot in that it should be able to
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boot the FPGA into the "regular" bitstream (or the "rescue" bitstream)
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from any state, including after a failed attempt to load the "standby"
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bitstream.
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The successful booting of FlickerNoise with jtag-boot does therefore not
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mean that the M1 would be able to boot regularly. E.g., if the standby
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bitstream is corrupted, the M1's FPGA initialization - without using
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JTAG - would fail at this first step.
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