IEEE 1149.1 (JTAG) Boundary-Scan Testing for Cyclone II Devices
If you are testing the device after configuring it, the programmable weak
pull-up resister or the bus hold feature overrides the CLAMPvalue (the
value stored in the update register of the boundary-scan cell) at the pin.
HIGHZ Instruction Mode
The HIGHZinstruction mode is used to set all of the user I/O pins to an
inactive drive state. These pins are tri-stated until a new JTAG instruction
is executed. When this instruction is loaded into the instruction register,
the bypass register is connected between the TDIand TDOports.
If you are testing the device after configuring it, the programmable weak
pull-up resistor or the bus hold feature overrides the HIGHZvalue at the
pin.
I/O Voltage Support in JTAG Chain
A JTAG chain can contain several different devices. However, you should
be cautious if the chain contains devices that have different VCCIO levels.
The output voltage level of the TDOpin must meet the specifications of
the TDIpin it drives. For Cyclone II devices, the TDOpin is powered by
the VCCIO power supply. Since the VCCIO supply is 3.3 V, the TDOpin
drives out 3.3 V.
Devices can interface with each other although they might have different
VCCIO levels. For example, a device with a 3.3-V TDOpin can drive to a
device with a 5.0-V TDIpin because 3.3 V meets the minimum TTL-level
VIH for the 5.0-V TDIpin. JTAG pins on Cyclone II devices can support
2.5- or 3.3-V input levels.
f
For more information on MultiVolt I/O support, see the Cyclone II
Architecture chapter in Volume 1 of the Cyclone II Device Handbook.
You can also interface the TDIand TDOlines of the devices that have
different VCCIO levels by inserting a level shifter between the devices. If
possible, the JTAG chain should be built such that a device with a higher
VCCIO level drives to a device with an equal or lower VCCIO level. This
way, a level shifter may be required only to shift the TDOlevel to a level
acceptable to the JTAG tester. Figure 14–13 shows the JTAG chain of
mixed voltages and how a level shifter is inserted in the chain.
Altera Corporation
February 2007
14–15
Cyclone II Device Handbook, Volume 1
ALTERA [ ALTERA CORPORATION ]
