JTAG Speed Limitations

JTAG speed limitations

JTAG is a serial protocol, and a JTAG chain can contain a large number of bits. One significant limitation inherent in JTAG operation is that the signal sampling rate is directly related to the rate at which the JTAG chain's TCK (clock) can be toggled.

For example, if the maximum rate is 33 MHz and the boundary scan chain contains 1000 cells, each signal corresponding to a cell in the scan chain is only sampled at a rate of 33 kHz.

To counter this, when speed is necessary, it is important to capture information using as short a scan chain as possible. This can be achieved by placing devices into CLAMP or BYPASS mode, in order to shorten the effective JTAG chain length.

Overclocking

Although XJTAG does not recommend doing so as a routine measure in a production test system, it has been observed that many silicon manufacturers appear to write a default TCK speed of 10 MHz into all of their BSDL files, in spite of the fact that their chips can actually run at significantly greater TCK speeds, which can also improve capture performance.