Analysis of the role and principle of JTAG

JTAG is one of the most basic communication protocols. You can understand that it is the same as RX TX or USB. It is just a communication method, but it is very different from RX TX and USB. That is, this JTAG protocol is the most Underlying, to put it plainly, generally speaking, in the mobile phone, the CPU is the boss, right? But in front of JTAG, he is not the boss, the JTAG protocol is used to control the CPU, and the CPU becomes good in front of JTAG. Too. The general agreement is to ask the CPU to read and write the program of the font library, but JTAG can read and write the program of the CPU. The command allows the CPU to do everything. The thief first captures the king. JTAG is the dragon knife.

All mobile phone CPUs support the JTAG protocol, and must also support the JTAG protocol, because the data in the CPU can only be written through the JTAG function. When the internal data is messy or other important data is messy, the CPU may ignore RX TX Or USB, at this time, use the JTAG protocol function to force the mobile phone CPU to work obediently. Therefore, all mobile phones are said to be bricks. The real technical reason is that the basic data necessary for the mobile phone to respond to RX TX or USB operation is lost, and the mobile phone CPU ignores RX TX or USB. Because the CPU is the boss at this time, he said that it would be impossible for ordinary people to do it; the only solution is to use the JTAG function to make the CPU into a horse, forcing him to restore these basic data, and the bricks into mobile phones. .

JTAG signal, the most basic should have 5 lines, TCK TMS, TDI, TDO, reset signal.

There are some reset signals, some and many. His role is to suppress the CPU, let him stop all work, wait for the JTAG signal to control it, or let some parts of it work and some parts suspend work.

TCK signal, the clock signal of the host computer, the data sent to the mobile phone CPU on the rising edge, and the data returned from the mobile phone CPU on the falling edge.

TDI signal, the data signal of the host computer.

The TMS signal, another data signal of the host computer, is actually a signal for command indication and differentiation.

TDO signal, the data signal from the mobile phone CPU to the host computer, let the host computer know whether the current state of the mobile phone CPU is very good or not, and then control according to the state of the CPU.

To make it simpler, let everyone understand that when the CPU is not online due to software reasons, JTAG is the only tool that can restore this part of the software. JTAG can direct the CPU to do anything. In front of JTAG, the CPU is just a horse.