JTAG-HS2 Reference Manual
Doc: 502-249
page 4 of 5
When the JTAG-HS2 first receives power the three-state buffers attached to the TMS, TDI, and TCK
signals move into a high-impedance state. They remain in the high-impedance state until an
application enables the HS2’s JTAG or SPI port. Once these ports activate, the buffers actively drive
the TMS, TDI, and TCK signals until the port is disabled.
The IEEE 1149.7-2009 specification requires any device that functions as a Debug and Test System
(DTS) to provide a pull-up bias on the TMS and TDO pins. In order to meet this requirement, the
JTAG-HS2 features weak pull-ups (100K ohm) on the TMS, TDI, TDO, and TCK signals. While not
strictly required, the pull-ups on the TDI and TCK signals ensure that neither signal floats while
another source is not actively driving them.
The JTAG-HS2 can interface scan chains that consist of one or more IEEE 1149-7 compatible
Target Systems (TS). The devices in these chains communicate using the TMS, TDI, TDO, and TCK
signals or they may communicate using only the TMS and TCK signals. Communication using only
the TMS and TCK signals requires both the HS2 and TS to drive the TMS pin. The current scan
format, bit period, and the level of the TCK pin determine which device is allowed to drive the TMS
pin.
A drive conflict may occur when the HS2 and TS disagree on the current scan format setting or bit
period. In the event that a drive conflict occurs, the 100 ohm resistor between the TMS buffer and
output pin will limit the maximum current to 50 mA to prevent any damage from occurring to the
JTAG-HS2. The drive conflict may be resolved by having the JTAG-HS2 perform a reset escape,
which will reset the scan format of the TS to JScan0/JScan1. If the TMS pin of the TS is not capable
of sourcing or sinking VDD (VREF) ÷ 100 amps of current then an additional resistor should be placed
in series with the TMS pin of the TS to further limit current flow.
In most cases a drive conflict can be avoided by having applications that use the HS2 communicate
with the TS in two wire mode. Use the applications to reconfigure the TS to use the JScan0, JScan1,
JScan2, or JScan3 scan format prior to disabling the HS2’s JTAG port.
Absolute Maximum Ratings
Symbol
Parameter
Condition
Min
Max
Unit
VDD (VREF)
I/O reference/supply voltage
-0.5
6
V
VIO
Signal Voltage
-0.5
6
V
I
IK
,I
OK
TMS, TCK, TDI, TDO
DC Input/Output Diode Current
VIO < -0.5V
-50
mA
VIO > 6V
+20
I
OUT
DC Output Current
±50
mA
T
STG
Storage Temperature
-20
+120
ºC
ESD
Human Body Model JESD22-A114
4000
V
Charge Device Model JESD22-C101
2000
V
