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210 lines
6.6 KiB
Plaintext
210 lines
6.6 KiB
Plaintext
JTAG with UBB
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=============
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JTAG [1] typically uses 4 to 6 signals between the host and the
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target, making it perfectly suitable for UBB.
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UBB can be either used with just a cable or with minimum circuitry
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if level shifting is needed. If the level difference is small
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enough, simple resistive dividers that can be soldered directly to
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UBB will be sufficient.
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If using the "gpio" cable driver of UrTAG, we don't even need new
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code for this.
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[1] http://en.wikipedia.org/wiki/Jtag
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[2] http://urjtag.org/
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Overview
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--------
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This is an example of using UBB to communicate via JTAG with the
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Xilinx XC6SLX45 FPGA of a Milkymist One. We first describe the
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hardware setup, then configuration and installation of UrJTAG,
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and finally changes to the kernel configuration.
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The whole setup looks like this:
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http://downloads.qi-hardware.com/people/werner/ubb/jtag/ubb-jtag-m1-all.jpg
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UBB-JTAG-M1 hardware: 2.5 V level adaptation
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--------------------------------------------
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Note that this is target-specifc. No level adaptation is necessary
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if the target's JTAG interface runs at 3.3 V.
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Milkymist One uses 2.5 V for its JTAG interface while the Ben's
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IO runs at 3.3 V. The difference is small enough that we need no
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special provisions for going from 2.5 V to 3.3 V, but we have to
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adapt the level in the opposite direction.
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Since we will run the interface at fairly low speeds, simple
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resistive dividers will be sufficient. A 220/680 Ohm divider
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reduces the Ben's nominal 3.3 V to a nominal 2.5 V. Small
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through-hole resistors can be soldered directly to UBB.
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ubb-jtag-m1.sch shows the configuration. Since CLK is used as
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input and has no pull-up inside the Ben, no resistor is needed.
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However, it may be advisable to add a 220 Ohm series resistor
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as protection in case of misconfiguration.
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This is what UBB with voltage dividers (without series resistor
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on CLK) looks like:
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http://downloads.qi-hardware.com/people/werner/ubb/jtag/ubb-jtag-m1-card.jpg
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UBB-JTAG-M1 hardware: connector
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-------------------------------
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Milkymist One uses a shrouded 14 pin male connector with a 2 mm
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pitch for JTAG. While suitable connectors for ribbon cables are
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available, e.g., Molex 087568-1493, one can also improvise with
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female contacts designed for the ubiquitous 2.54 mm headers.
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If building a UBB-JTAG board with individual wires, the labels
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in m1-labels.fig can be used to identify them. Each label has
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the name of the JTAG signal and the position in the J6 connector
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of the Milkymist One.
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Building UrJTAG
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---------------
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Prequisite: the OpenWRT cross-compilation toolchain for the Ben
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has to be installed. I.e., mipsel-openwrt-linux-gcc and friends
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must be in PATH.
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git clone git://urjtag.git.sourceforge.net/gitroot/urjtag/urjtag
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cd urjtag/urjtag
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./autogen.sh --host=mipsel-openwrt-linux --enable-cable=gpio \
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--enable-lowlevel= --without-libusb --without-libftdi
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make
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Installing UrJTAG
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-----------------
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Prerequisites: the Ben must be reachable via TCP/IP. We assume we
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can reach it under the name "ben". ldconfig must be installed (*).
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(*) E.g., with
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wget http://downloads.qi-hardware.com/software/packages/NanoNote/Ben/2012-10-24/ldconfig_0.9.33.2-1_xburst.ipk
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opkg install ldconfig_0.9.33.2-1_xburst.ipk
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scp src/apps/jtag/.libs/jtag ben:/usr/bin/
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scp src/.libs/liburjtag.so.0.0.0 ben:/usr/lib/
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ssh ben mkdir -p /usr/local/share/urjtag
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scp -r data/* ben:/usr/local/share/urjtag/
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ssh ben ldconfig
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Enabling the /sys/class/gpio/... interface
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------------------------------------------
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For simplicity, we use the "gpio" cable driver of UrTAG. This
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driver requires the Ben kernel to be compiled with
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CONFIG_GPIO_SYSFS (this option is not available as a module).
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If /sys/class/gpio does not exist, we need to rebuild the
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kernel.
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Prerequisite: you have checked out and set up the openwrt-xburst
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repository from Qi-Hardware.
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make kernel_menuconfig
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Then go to "Device Drivers" > "GPIO Support" and enable
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"/sys/class/gpio/... (sysfs interface)" (GPIO_SYSFS)
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Enabling ZRAM
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-------------
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UrTAG spends memory fairly generously and loading a bitstream for
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the XC6LX45 of Milkymist One is likely to exceed the memory
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available on the Ben.
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Since we can't swap on ubifs and we can't use an SD card for swap
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since the 8:10 card slot is already occupied, the next best choice
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is to use ZRAM.
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Note that this is not necessary if the driver for the respective
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target isn't quite as memory-hungry as the one for Xilinx FPGAs,
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or if the bitstream is smaller.
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make kernel_menuconfig
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Then go to "Device Drivers" and enable "Staging drivers" (STAGING).
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After that, enter "Staging drivers" and enable "Compressed RAM
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block device support" (ZRAM).
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Building and installing the OpenWRT kernel
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------------------------------------------
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make -j5
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scp bin/xburst/openwrt-xburst-qi_lb60-uImage.bin ben:
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ssh ben flash_eraseall /dev/mtd1
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ssh ben nandwrite -p /dev/mtd1 openwrt-xburst-qi_lb60-uImage.bin
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Using UBB-JTAG-M1
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-----------------
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First connect UBB-JTAG-M1 to the Milkymist One (powered off) but
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without inserting UBB into the Ben. This should look like this:
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http://downloads.qi-hardware.com/people/werner/ubb/jtag/ubb-jtag-m1-j6.jpg
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Then boot the Ben and disable the MMC driver with
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echo jz4740-mmc.0 >/sys/bus/platform/drivers/jz4740-mmc/unbind
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Now UBB-JTAG-M1 can be safely connected to the Ben. After connecting
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power to the Milkymist One, we can probe it with
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ben# jtag
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jtag> cable gpio tdi=107 tdo=105 tck=109 tms=108
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jtag> detect
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For the next step, we need fjmem.bit, e.g., available here:
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http://milkymist.org/updates/fjmem.bit.bz2
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We also need to enable swap space on ZRAM. 32 MB should be
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sufficient:
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ben# echo 33554432 >/sys/block/zram0/disksize
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ben# mkswap /dev/zram0
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ben# swapon /dev/zram0
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Now fjmem.bit can be loaded and executed:
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jtag> instruction CFG_OUT 000100 BYPASS
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jtag> instruction CFG_IN 000101 BYPASS
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jtag> pld load "fjmem.bit"
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jtag> initbus fjmem opcode=000010
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jtag> detectflash 0
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Since the sysfs GPIO interface is rather slow and we make things
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worse by swapping to compressed RAM, the loading of fjmem.bit
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takes about four or five minutes.
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Going faster
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------------
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In many applications, UrJTAG will be able to run without compressed
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swap and thus faster. Smaller amounts of data will also make even
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slow transfers more bearable.
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To further increase the speed, a Ben-specific GPIO driver could be
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written that accesses registers directly without the very long
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detour through sysfs.
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Finally, even higher speeds could be reached by using the MMC
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controller to send data, similar to UBB-VGA. For this purpose, CMD
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and DAT0 have been left unconnected. When connecting them to each
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other, they could be used to generate fake start bits, signaling
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the MMC controller to begin a transfer. (See ubb-vga for details.)
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