From 7626139950a4282564cadbc2cdd349593841e3b8 Mon Sep 17 00:00:00 2001 From: zwelch Date: Wed, 13 May 2009 11:10:35 +0000 Subject: Add JTAG Primer to doxygen manual, contributed by Strontium. git-svn-id: svn://svn.berlios.de/openocd/trunk@1776 b42882b7-edfa-0310-969c-e2dbd0fdcd60 --- doc/manual/primer/jtag.txt | 109 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 109 insertions(+) create mode 100644 doc/manual/primer/jtag.txt (limited to 'doc/manual/primer') diff --git a/doc/manual/primer/jtag.txt b/doc/manual/primer/jtag.txt new file mode 100644 index 00000000..95637290 --- /dev/null +++ b/doc/manual/primer/jtag.txt @@ -0,0 +1,109 @@ +/** @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. + + */ -- cgit v1.2.3