Chapter 12: IEEE 1149.1 (JTAG) Boundary-Scan Testing for the Cyclone III Device Family
12–5
I/O Voltage Support in a JTAG Chain
■
Only the three mandatory JTAG 1149.1 JTAG instructions (BYPASS
,
SAMPLE/PRELOAD
,
EXTEST) and the FACTORYprivate instruction are supported from
the JTAG pins upon power up. The FACTORYinstruction (instruction code:
10 1000 0001) must be issued before the device starts loading the core
configuration data to enable access to all other JTAG instructions. This instruction
also clears the device configuration data and AES volatile key.
■
■
IDCODEinstruction is not supported upon power-up, prior to issuing the FACTORY
instruction. However, it is the default instruction when the TAP controller is in the
reset state. Without loading any instructions, you can go to the Shift_DRstate and
shift out the JTAG Device ID.
IDCODE, CONFIG_IO, ACTIVE_DISENGAGE, HIGHZ, CLAMP, USERCODEand PULSE_NCONFIG
instructions are supported, provided that the FACTORYinstruction is executed.
I/O Voltage Support in a JTAG Chain
A JTAG chain can contain several different devices. However, you must 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 III device
family, the TDOpin is powered by the VCCIO power supply. Because 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 III device family can support the input levels of VCCIO of bank 1A.
1
For multiple devices in a JTAG chain with 3.0-V or 3.3-V I/O standard, you must
connect a 25- series resistor on a TDOpin driving a TDIpin.
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 must
be built in such a way 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.
December 2011 Altera Corporation
Cyclone III Device Handbook
Volume 1
ALTERA [ ALTERA CORPORATION ]
