1.1 JTAG signals
Most Arm-based devices are physically equipped with several pins that are dedicated to debug and test
purposes. Four of these pins make up the IEEE 1149.1 interface, also known as the JTAG interface. This
interface is often used for boundary-scan testing during the manufacture of printed circuit boards. The
interface also provides a useful way to access one or more cores and other components in a device, while
running its application software.
Test Data In (TDI)
The TDI signal is an input to the target device which provides a stream of serial data from the debug
unit.
The TDI signal must be pulled HIGH on the target to keep the signal inactive when no debug unit is
connected.
Test Mode Select (TMS)
The TMS signal is an input to the target device which controls its JTAG state-machine.
The TMS signal must be pulled HIGH on the target to keep the signal inactive when no debug unit is
connected.
Test Clock (TCK)
The TCK signal is an input to the target device which synchronizes its JTAG state-machine. On each
rising edge of the TCK signal, the target samples the TDI, and TMS signals.
Consider TCK as a strobe signal, rather than a clock signal, because it is typically non-continuous and
only becomes active during debug communications.
TCK can be pulled HIGH on the target, however, to maintain full compatibility with other JTAG
equipment, Arm recommends you pull TCK LOW.
Test Data Out (TDO)
The TDO signal is an output from the target device which returns a stream of serial data to the debug
unit.
TDO can be left floating on the target, however, to maintain full compatibility with other JTAG
equipment, Arm recommends you pull TDO HIGH.
Basic JTAG connection
In the simplest form (omitting pull-up and pull-down resistors), a connection between the debug unit and
the target device looks like:
Target
Device
Debug
Unit
TDI
TMS
TCK
TDO
TDI
TMS
Figure 1-1 Simple JTAG connection
Note
The naming convention of the TDI (Test Data In) and TDO (Test Data Out) signals is always with
respect to the target device.
1 Debug and trace interface
1.1 JTAG signals
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