As we see increased usage of mixed low voltage JTAG interfaces (3.3V, 2.5V,
1.8V, and even lower) on single PCB, it becomes important to consider different
methods of managing multiple voltages. The solution usually comes down to two
possible methods, or a combination of both: Separate chains by voltage, or use
voltage translators on the chain.
Multiple Single-voltage Chains
Corelis multi-TAP controllers include individually configurable TAP voltages to
accommodate boards with JTAG chains of different voltages. Simply select the
appropriate voltage for each TAP in the controller settings. The figure below
depicts an example configuration using 3 chains.
Figure 1: A NetUSB-1149.1/E operating with 3 JTAG chains
In cases where it is ideal to include a single JTAG chain on the board, voltage
translators may be used to interface between the different voltages. Analog
Devices have an excellent article about Architecting a Multi-Voltage JTAG Chain
available on their Analog Dialogue site.
Boards with multiple JTAG TAP voltages are very common and should be
considered early in the JTAG chain design. Note that voltage translation methods
aren’t limited to single chain designs and may also be employed on a multi-TAP
board. For example, you may elect to keep the CPU on its own TAP for use with a
JTAG debugger, but include all other devices—including multiple voltage
interfaces—on a different chain using voltage translators.