Add JTAG Primer to doxygen manual, contributed by Strontium.

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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.
+
+ */