PCB connections

The following figure shows a typical JTAG connection scheme:

Figure 7. Typical PCB connections

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  • The signals TDI, TMS, TCK, RTCK and TDO are typically pulled up on the target board to keep them stable when the debug equipment is not connected.

  • DBGRQ and DBGACK are typically pulled down on the target.

  • If there is no RTCK signal provided on the processor, it can either be pulled to a fixed logic level or connected to the TCK signal to provide a direct loop-back.

  • All pull-up and pull-down resistors must be in the range 1K-100KΩ.

  • The VTREF signal is typically connected directly to the VDD rail. If a series resistor is used to protect against short-circuits, it must have a value no greater than 100Ω.

  • To improve signal integrity, it is good practice to provide an impedance matching resistor on the TDO and RTCK outputs of the processor. The value of these resistors, added to the impedance of the driver must be approximately equal to 50Ω.

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