Digilent 210-251P-BOARD - Manual (Page 4)

JTAG-SMT2 Reference Manual
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Page 4 of 11
2-Wire Star Topology
TMSC
TDIC
TCKC
TDOC
Target
System 0
Target
System 1
Target
System N
TMSC
TDIC
TCKC
TDOC
TMSC
TDIC
TCKC
TDOC
TMS
TDI
TCK
TDO
Host
+
JTAG-SMT2
(DTS)
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-SMT2 features weak pull-ups
(100K ohm) on the TMS, TDI, TDO, and TCK signals. Though not required in the specifications, the pull-ups on the
TDI and TCK signals ensure that neither signal floats while another source is not driving them (see Fig. 5).
VREF
Output Pin
(TMS, TDI, TCK)
1
0
0
K JtagEN
VREF
Input Pin
(TDO)
1
0
0
K
Users should place a current limiting resistor between the TMS pin of the SMT2 and the TMSC pin of the TS when
using the JTAG-SMT2 to interface with an 1149.7 compatible TS. If a drive conflict occurs, this resistor should
prevent damage to components by limiting the amount of current flowing between the pins of each device. A 200
ohm resistor will limit the maximum current to 16.5mA when using a 3.3V reference (see Figs. 6 & 7). While this
level of resistance should be sufficient for most applications, the value of the resistor may need to be adjusted to
meet the requirements of the TS.
In most cases users can avoid a drive conflict by having applications that use the SMT2 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 SMT2’s JTAG port.
TMSC
TDIC
TCKC
TDOC
Target
System 0
Target
System 1
Target
System N
4-Wire Star Topology
TMSC
TDIC
TCKC
TDOC
TMSC
TDIC
TCKC
TDOC
TMS
TDI
TCK
TDO
Host
+
JTAG-SMT2
(DTS)
Figure 4. 4-Wire and 2-Wire star topology.
Figure 5. Pull-ups on TMS, TDI, TDO, and TCK signals.