How JTAG works. Now that we know how to connect JTAG to different ICs, lets see in details how JTAG works and how to control it from a PC. Control JTAG from your PCYou use a JTAG cable to control a JTAG bus from a PC. The JTAG cable is just a way to control the four JTAG signals from the PC. The JTAG cable might connect to a PCs. Parallel printer port. USB port. Ethernet port. The simplest is a parallel port JTAG cable. USB and Ethernet JTAG cables are good too. They are faster at streaming large amount of data, but more complex to control and with more overhead slower for small amount of data. In VHDL, a design consists at a minimum of an entity which describes the interface and an architecture which contains the actual implementation. My6pE.jpg' alt='32 Bit Register File Vhdl' title='32 Bit Register File Vhdl' />A binary to binarycoded decimal, or BCD for short, is a method storing decimal numbers in binary form. The majority of the time a number in a logic design is stored. FPga projects, Verilog projects, VHDL projects input clk, input rst. Designing with the EZUSB FX3 Slave FIFO Interface www. Document No. 00165974 Rev. N 2 1 Introduction The EZUSB FX3, Cypresss nextgeneration. Using this site ARM Forums and knowledge articles Most popular knowledge articles Frequently asked questions How do I navigate the siteThe parallel port. A parallel port can be regarded as a twelve bits output from the PC, and a five bits input to the PC. JTAG just requires a three bits output and a one bit input, so thats no problem. For example, take a look at the schematic of Xilinxs parallel III cable. Visual Basic 2010 Utorrent My Pc more. Bit Register File Vhdl' title='32 Bit Register File Vhdl' />You can see on the left the PCs parallel signals, one the right the JTAG connector, and in the middle a few 7. HC1. 25 electronic buffers. From the software point of view, a parallel port is ideal since it is very easy to control. For example, heres C code that shows how to toggle the TCK signal for an Altera Byte. Blaster JTAG cable. TCK 0x. 01. void toggleTCK. TCK. outportlptaddr, 0. One minor problem with Windows 2. XPVista is that the outport IO instruction is restricted, but Give. IO and User. Port are free generic drivers that open up the IO space. The JTAG TAP controller. We know that a PC is connected to the JTAG bus as illustrated here So we have 4 signals TDI, TDO, TMS, TCK to take care of. TCKTCK is the JTAG clock signal. The other JTAG signals TDI, TDO, TMS are synchronous to TCK. So TCK has to toggle for anything to happen usually things happen on TCKs rising edge. TMSInside each JTAG IC, there is a JTAG TAP controller. On the figure above, that means that there is a TAP controller in the CPU and another in the FPGA. The TAP controller is mainly a state machine with 1. TMS is the signal that controls the TAP controller. Since TMS is connected to all the JTAG ICs in parallel, all the TAP controllers move together to the same state. Heres the TAP controller state machine The little numbers 0 or 1 close to each arrow are the value of TMS to change state. So for example, if a TAP controller is at state Select DR Scan and TMS is 0 and TCK toggles, the state changes to Capture DR. For JTAG to work properly, the tap controllers of a JTAG chain must always be in the same state. After power up, they may not be in sync, but there is a trick. Look at the state machine and notice that no matter what state you are, if TMS stays at 1 for five clocks, a TAP controller goes back to the state Test Logic Reset. Thats used to synchronize the TAP controllers. So lets say we want to go to Shift IR after power up. JTAGclockTMS. now that everybody is in a known and identical state, we can move together to another state. Shift IR. JTAGclock0. JTAGclockTMS. JTAGclockTMS. JTAGclock0. JTAGclock0. TDI and TDONow that we know how to change state, we can use the two most important JTAG states, namely Shift DR and Shift IR. Shift DR and Shift IR are used in combination with the TDI and TDO lines. Lets start with Shift IR. There is one register into each IC TAP controller called IR, the instruction register. You write a value into this register that corresponds to what you want to do with JTAG each IC has a list of possible instructions. Also each IC IR register has a specific length. For example, lets say that the CPUs IR is 5 bits long, and the FPGAs IR is 1. The IR registers form a chain that is loaded through the TDI and TDO pins. From the PC point of view, the above IR chain is 1. To load IR values, the PC has to make sure the TAP controllers are in the Shift IR state, and send 1. TDI. Once shifted, the first 1. FPGA IR, and the last 5 bits into the CPU IR. If the PC was sending more than 1. TDO. For example, lets send 0. CPUs IR, and 0. 00. FPGAs IR. Because the bits are shifted through in a chain, we must start sending the data for the device that is at the end of the chain. FPGA IR bits first. JTAGclock0. JTAGclock1. JTAGclock0. JTAGclock0. JTAGclock0. JTAGclock0. JTAGclock0. JTAGclock0. JTAGclock0. JTAGclock0. CPU IR bits. JTAGclock0. JTAGclock0. JTAGclock1. JTAGclock0. JTAGclock0 TMS last bit needs to have TMS active to exit shift IR. In our example, the CPUs IR is 5 bits long i. That means it can support up to 3. JTAG instructions. In practice, a CPU probably uses 5 to 1. IR values are unused. Same thing for the FPGA, with a 1. IR, it can hold 1. JTAG has a few mandatory instructions what these instructions do will be illustrated later. BYPASSEXTESTSAMPLEPRELOADIDCODE not mandatory but very often implementedTo get the list of possible IR values supported by a specific IC, check the ICs datasheet or BSDL file more on that later. DR registers. Each TAP controller has only one IR register, but has multiple DR registers. The DR registers are similar to the IR registers they are shifted the same way the IR registers are but using the Shift DR state instead of the Shift IR. Each IR value selects a different DR register. So in our example, the CPU could have up to 3. DR registers if all IR instructions were implemented.