Discussion on ARM JTAG Emulator Circuit

Introduction: The following is some of my experience in practice. I found this because I encountered problems when trying to use the programming board circuit suitable for SAMSUNG S3C44B0 JTAG to program another ARM9 core MPU S3C2440 of SAMSUNG. After consulting some information, I finally solved it. I hope this will be helpful to those friends who have difficulties in making their own ARM JTAG programmer.

1. The essence of JTAG emulator

JTAG (Joint Test Action Group) programming and debugging actually utilizes the tracking and debugging function built into the MCU/MPU chip (MCU/MPU hardware support is required). One end of the JTAG programming board is connected to the parallel port of the PC, and the other end is connected to the target board. Since the operating voltage of the MCU/MPU is usually between 1.8V and 3.6V, and the level logic output by the PC parallel port is 5V, a level conversion is required. Usually a buffer/driver (such as: 74××244/74××541) is used for isolation, and the voltage is limited by resistors to divide the voltage. Because the PC parallel port has no voltage output, the IC on the programming board must be powered by the target board, that is: the VCC pin in the JTAG interface must be properly connected.

2. Pin definition of JTAG interface

The mainstream JTAG interface has two types: 14-pin and 20-pin. The pin assignment is shown in Figure 1.

The 14-pin JTAG interface is an old-fashioned interface.

The definition of non-ground pins in JTAG is shown in Figure 2.

3. The first line sequence JTAG programming board circuit

The JTAG programming board circuit in the SAMSUNG ARM7 S3C44B0 development kit I used is shown in the figure

However, after connecting the programming board to S3C2440, the program cannot be loaded correctly.

According to the input and output relationship of 244, the corresponding relationship between the PC parallel port and the JTAG interface pins can be sorted out as follows:

PC parallel port pinout

2 3 8 4

JTAG Pinouts

TCK TMS TDI nSRST

4. The second line sequence JTAG programming board circuit

After consulting the programming board circuit in the USER'S GUIDE of the official JTAG programming board SJF2440 of S3C2440, the corresponding relationship between the PC parallel port and the JTAG interface pins is sorted out as follows:

PC parallel port pinout

2 3 4 11

JTAG Pinouts

TCK TDI TMS TDO

Based on this, it can be inferred that the failure of JTAG debugging of S3C2440 using S3C44B0 JTAG programming board is due to the difference in the allocation of PC parallel port control pins. That is, the parallel port pins used by S3C2440 JTAG debugging agent to simulate JTAG timing are different from those used by S3C44B0 debugging agent.

Based on this, we flexibly flew wires on the S3C44B0 JTAG programming board and successfully debugged the S3C2440 using the programming board again.

5. Some speculation

In the JTAG related search, I found a JTAG programming board circuit given by netizen Li Jie, as shown in Figure 5.

The correspondence between the JTAG pins and the PC parallel port pins in this circuit is the same as that in the S3C2440 JTAG programming board, and the author claims that this circuit is applicable to all ARM chips. Based on this, it is speculated that most companies develop ARM debugging software that supports two different line sequences of parallel port simulation JTAG debugging, and can automatically detect the programming board hardware connection and implement control when debugging starts. The official S3C2440 JTAG debugging software only supports one line sequence.