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+/** @page primerjtag OpenOCD JTAG Primer
+
+JTAG is unnecessarily confusing, because JTAG is often confused with
+boundary scan, which is just one of its possible functions.
+
+JTAG is simply a communication interface designed to allow communication
+to functions contained on devices, for the designed purposes of
+initialisation, programming, testing, debugging, and anything else you
+want to use it for (as a chip designer).
+
+Think of JTAG as I2C for testing. It doesn't define what it can do,
+just a logical interface that allows a uniform channel for communication.
+
+See:
+
+ http://en.wikipedia.org/wiki/Joint_Test_Action_Group
+
+and
+
+ http://www.inaccessnetworks.com/projects/ianjtag/jtag-intro/jtag-state-machine-large.png
+
+The first page (among other things) shows a logical representation
+describing how multiple devices are wired up using JTAG. JTAG does not
+specify, data rates or interface levels (3.3V/1.8V, etc) each device can
+support different data rates/interface logic levels. How to wire them
+in a compatible way is an exercise for an engineer.
+
+Basically TMS controls which shift register is placed on the device,
+between TDI and TDO. The second diagram shows the state transitions on
+TMS which will select different shift registers.
+
+The first thing you need to do is reset the state machine, because when
+you connect to a chip you dont know what state the jtag is in,you need
+to clock TMS as 1, at least 7 times. This will put you into "Test Logic
+Reset" State. Knowing this, you can, once reset, then track what each
+transition on TMS will do, and hence know what state the jtag state
+machine is in.
+
+There are 2 "types" of shift registers. The Instruction shift register
+and the data shift register. The sizes of these are undefined, and can
+change from chip to chip. The Instruction register is used to select
+which Data register/data register function is used, and the data
+register is used to read data from that function or write data to it.
+
+Each of the states control what happens to either the data register or
+instruction register.
+
+For example, one of the data registers will be known as "bypass" this is
+(usually) a single bit which has no function and is used to bypass the
+chip. Eg, assume we have 3 identical chips, wired up like the picture
+and each has a 3 bit instruction register, and there are 2 known
+instructions (110 = bypass, 010 = some other function) if we want to use
+"some other function", on the second chip in the line, and not change
+the other chips we would do the following transitions.
+
+From Test Logic Reset, TMS goes:
+
+ 0 1 1 0 0
+
+which puts every chip in the chain into the "Shift IR state"
+Then (while holding TMS as 0) TDI goes:
+
+ 0 1 1 0 1 0 0 1 1
+
+which puts the following values in the instruction shift register for
+each chip [110] [010] [110]
+
+The order is reversed, because we shift out the least significant bit
+first. Then we transition TMS:
+
+ 1 1 1 1 0 0
+
+which puts us in the "Shift DR state".
+
+Now when we clock data onto TDI (again while holding TMS to 0) , the
+data shifts through the data registers, and because of the instruction
+registers we selected (some other function has 8 bits in its data
+register), our total data register in the chain looks like this:
+
+ 0 00000000 0
+
+The first and last bit are in the "bypassed" chips, so values read from
+them are irrelevant and data written to them is ignored. But we need to
+write bits for those registers, because they are in the chain.
+
+If we wanted to write 0xF5 to the data register we would clock out of
+TDI (holding TMS to 0):
+
+ 0 1 0 1 0 1 1 1 1 0
+
+Again, we are clocking the lsbit first. Then we would clock TMS:
+
+ 1 1 0
+
+which updates the selected data register with the value 0xF5 and returns
+us to run test idle.
+
+If we needed to read the data register before over-writing it with F5,
+no sweat, that's already done, because the TDI/TDO are set up as a
+circular shift register, so if you write enough bits to fill the shift
+register, you will receive the "captured" contents of the data registers
+simultaneously on TDO.
+
+That's JTAG in a nutshell. On top of this, you need to get specs for
+target chips and work out what the various instruction registers/data
+registers do, so you can actually do something useful. That's where it
+gets interesting. But in and of itself, JTAG is actually very simple.
+
+ */