Universal JTAG Cable

Every electronic device around has some microcontroller or other programmable device in it. Even kitchen appliances or toy has some kind of electronics which controls important functions. In some cases this electronic device has predefined functionality which can not be changed. But in most cases this electronics contains a processor that uses flash memory to store code. And if a device is programmable then you can change the program it runs at any time.

Of course, in many cases this programming is not needed since the electronics performs what is supposed to. But in some cases it makes sense to allow end users to update the device with the latest firmware. Therefore, it makes sense to have a universal interface that can be used in factory and at home to program electronic devices with simple additional hardware. This universal interface is JTAG. This is a standard that defines a serial interface to transfer data between a device and computer (or between devices). The JTAG interface needs only four basic signals. This makes it easy to use PC parallel port as JTAG interface. While this works many modern JTAG programmers use USB port to connect to the PC.

Another beauty of the JTAG interface is the possibility to connect many devices in chain. This way you can program all the JTAG enables devices on the board with only one interface. In any case you need a JTAG cable to connect to the device. Each device may have own JTAG pinout but this is not a problem since JTAG cables can be made (almost) universal--at least for few device types. Unfortunately, JTAG cable is not enough. You need also a suitable software that will program the target device.

JTAG is not used only for commercial electronics. It is also a standard interface for all embedded systems including FPGAs, microcontrolelrs and various memories. Each manufacturer has some kind of JTAG cable to program their devices. Unfortunately, these cables use dedicated software and can not be used for other purposes.

One of the very popular uses of the JTAG interface in consumer electronics is to reprogram some router models with new firmware. There are many cheap JTAG cables that can be used to bring a new life for an old router. Changing original firmware is very popular not just for routers but also for other highly popular electronic devices. The Xbox is such example where a simple JTAG hack can change some built-in functions.

There are many JTAG cable supplies around. Before you decide for specific cable you need to check the JTAG pinout and software support. Usually, cheap cables are made only for specific devices and use the parallel port signals to program the device. More advanced JTAG cables use USB interface or at least a buffer to boost signals from parallel port.