Scan Modes

XJTAG's JTAG controllers have the ability to run their TAP interfaces at different speeds. Depending on the devices on your PCB and the way in which JTAG chains are arranged in your circuit this can give significant speed advantages.

Even though data can be sent to the JTAG chains at different rates, XJTAG still synchronises the application of signals to the circuit by controlling the TMS signals appropriately to the active devices in the current profile.

Older projects run all the JTAG chains at the clock frequency of the slowest JTAG device in the system. This behaviour will continue for these projects, because it is likely that they have not been tested at higher frequencies and it is safer to continue this behaviour than to risk signal integrity problems on existing projects.

XJTAG therefore has two modes for how JTAG scans operate:

Classic Scans

This mode is applied by default when opening projects created before XJTAG 4.0. All JTAG chains will operate at the same TCK frequency.

Dynamic chains are supported in this mode - but they are not necessary. If no Dynamic Chains profiles exist in the project it will continue to operate as before.

Each JTAG scan will pass around the boundary scan registers of subchains at the project frequency.

Optimised Scans

From XJTAG 4.0, new projects will default to using the new Optimised mode, in which each TAP group can run at a different frequency.

The Optimised scans feature requires the project to have a Dynamic Chains setup - however XJDeveloper will by default create most of this setup automatically for you.

By default, instead of all chains running at the speed of the slowest-rated JTAG device in the circuit, each chain's TCK frequency will be set to the speed of the slowest-rated device in that chain. Data will be sent to all JTAG chains simultaneously at these different TCK frequencies.

It is possible that XJTAG will not be able to run every chain at the exact frequency set, in which case a frequency close to but below the target frequency will be used.

Operations which read from or write to the circuit are automatically synchronised between all JTAG chains, PIO pins and external hardware machines which are active in the current profile.

If a subchain is not required for a JTAG scan it will not be used during that scan operation (its inputs will be kept constant).

Opening a project or XJPack file with Optimised scans enabled requires XJTAG 4.0 or higher to be installed.