J-Link / J-Trace User Guide (UM08001) - Microcontrollers

11.3 Signal analysis
The following screenshots show the data flow of the startup and ID communication
between J-Link / J-Trace and the target device.
11.3.1 Start sequence
This is the signal sequence output by J-Link / J-Trace at start of JLink.exe. It should
be used as reference when tracing potential J-Link / J-Trace related hardware problems.
The sequence consists of the following sections:
• 5 clocks: TDI low, TMS high. Brings TAP controller into RESET state
• 1 clock: TDI low, TMS low: Brings TAP controller into IDLE state
• 2 clocks: TDI low, TMS high: Brings TAP controller into IR-SCAN state
• 2 clocks: TDI low, TMS low: Brings TAP controller into SHIFT-IR state
• 32 clocks: TMS low, TDI: 0x05253000 (lsb first): J-Link Signature as IR data
• 240 clocks: TMS low, last clock high, TDI high: Bypass command
• 1 clock: TDI low, TMS high: Brings TAP controller into UPDATE-IR state.
J-Link / J-Trace checks the output of the device (output on TDO) for the signature to
measure the IR length. For ARM7 / ARM9 chips, the IR length is 4, which means TDO
shifts out the data shifted in on TDI with 4 clock cycles delay. If you compare the
screenshot with your own measurements, the signals of TCK, TMS, TDI, and TDO
should be identical.
Note that the TDO signal is undefined for the first 10 clocks, since the output is usually
tristated and the signal level depends on external components connected to TDO,
such as pull-up or pull-down.
Zoom-in
The next screenshot shows the first 6 clock cycles of the screenshot above. For the
first 5 clock cycles, TMS is high (Resulting in a TAP reset). TMS changes to low with
the falling edge of TCK. At this time the TDI signal is low. Your signals should be
identical. Signal rise and fall times should be shorter than 100ns.
11.3.2 Troubleshooting
If your measurements of TCK, TMS and TDI (the signals output by J-Link / J-Trace)
differ from the results shown, disconnect your target hardware and test the output of
TCK, TMS and TDI without a connection to a target, just supplying voltage to J-
Link’s/J-Trace’s JTAG connector: VCC at pin 1; GND at pin 4.
J-Link / J-Trace (UM08001) © 2004-2010 SEGGER Microcontroller GmbH & Co. KG
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