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XDS Connector Design Checklist

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This checklist may be applied to any XDS connector for both JTAG and Trace applications.

JTAG Signals

See the Emulation Header Signal Definitions for detailed description of each pin.

Table 1. JTAG 1149.1 Port Description
PIN XDS Signal
Type(1)
Target Signal Type NAME CHECKLIST ITEMS
TRST- O I Test Logic Reset TRST- should normally be pulled down on the target card. This holds the device's JTAG/debug logic in reset when an XDS cable is not present. For all cases this is the normal operating mode of the device. If a device does not have a TRST- pin then the pin on the connector can be left disconnected. TI recommends a 4.7K pull-down but common values in the 4.7K to 10K range are acceptable.
TCK O I Test Clock The requirements for TCK are dependent on the number of devices in the JTAG serial chain and on the routing distance of the TCK signal.
  • Single device case AND TCK routing distance less than 6 inches - In this case the only requirement is a series termination resistor. Normally a value of 22 ohms is sufficient. The termination resistor should be placed physically near the XDS connector and its location noted in the schematics.
  • Multiple device case OR single device case and TCK routing greater than 6 inches - In this case a buffer is required to drive TCK from the connector to the device (or all devices in the multiple device JTAG serial chain). The buffer should be series terminated near the buffer output, the size of which is dependent on the buffer selected. The buffer input should be AC terminated per the following link and pulled up to eliminate switching if the XDS is disconnected. TI recommends a 4.7K pull-up but common values in the 4.7K to 10K range are acceptable.
TMS O I Test Mode Select The requirements for TMS are dependent on the number of devices in the JTAG serial chain and on the TMS signal routing distance.
  • Single device case AND TMS routing distance less than 6 inches - This signal is a direct connect between the XDS connector and the device.
  • Multiple device case OR single device case and TMS routing greater than 6 inches - A buffer is required with a pull-up on the input. TI recommends a 4.7K pull-up but common values in the 4.7K to 10K range are acceptable. The buffered output is routed to all devices in the serial scan chain.
TDI O I Test Data Input The requirements for TDI are dependent on the number of devices in the JTAG serial chain and on the TDI signal routing distance.
  • Single device case AND TDI routing distance less than 6 inches - This signal is a direct connect between the XDS connector and the device.
  • Multiple device case OR single device case and TDI routing greater than 6 inches - This signal is buffered from the XDS connector to the device. The buffer requires a pull-up on the input. TI recommends a 4.7K pull-up but common values in the 4.7K to 10K range are acceptable. In the single device case the output of the buffer is connected to the devices TDI pin. In the Multiple device JTAG serial chain case the buffered output is routed to the first device in the serial scan chain. TDO of each device is connected to TDI of the next device (except in the case of the last device). In any cases where the routing between devices (TDO to TDI) is greater than six inches a buffer (with a pull-up on the input) is required.
TDO I O Test Data Output The requirements for TDO are dependent on the number of devices in the JTAG serial chain and on the TDO signal routing distance.
  • Single/Multiple device case AND TDO routing distance less than 6 inches - This signal is a direct connect between the device (in the multiple device case the last device in the scan chain) and the XDS connector and requires a series termination resistor near the device. Normally a 22 ohm termination resistor is sufficient, but ideally matching the input impedance of the XDS cable (normally 50 ohms) will provide the most robust connection (see Non-buffered JTAG Signal Termination for details).
  • Single/Multiple device AND TDI routing greater than 6 inches - This signal requires a buffer between the device (in the multiple device case the last device in the scan chain) and the XDS connector. The buffer requires a pull-up on the input. TI recommends a 4.7K pull-up but common values in the 4.7K to 10K range are acceptable.
RTCK I O TCK Return The requirements for TCK are dependent on the number of devices in the JTAG serial chain, on the routing distance of the TCK signal, and if the device supports a RTCK pin.
  • Single device AND the device has no RTCK pin AND TCK is not buffered - In this case a simple loop from the TCK pin to the RTCK pin of the XDS connector with a series termination resistor is all that is required. Normally a 22 ohm termination resistor is sufficient, but ideally matching the input impedance of the XDS cable (normally 50 ohms) will provide the most robust connection (see Non-buffered JTAG Signal Termination for details).
  • Single/Multiple devices AND none of the devices in the JTAG scan chain have a RTCK pin AND TCK is buffered - If TCK is buffered, RTCK should normally be buffered to provide the same delay to the XDS as the device (or first device) will see through the TCK buffer. When buffering RTCK a series termination resistor should be utilized. Normally a 22 ohm termination resistor is sufficient, but ideally matching the input impedance of the XDS cable (normally 50 ohms) will provide the most robust connection (see Non-buffered JTAG Signal Termination for details). The RTCK buffer input should be pulled-up by the same pull-up resistor that is used to pull-up TCK's buffer input.
  • Single device AND the device has a RTCK pin AND TCK is not buffered - This signal is a direct connect between the device and the XDS connector and requires a series termination resistor near the device. Normally a 22 ohm termination resistor is sufficient, but ideally matching the input impedance of the XDS cable (normally 50 ohms) will provide the most robust connection (see Non-buffered JTAG Signal Termination for details).
  • Single device AND the device has a RTCK pin AND TCK is buffered - In this case RTCK is buffered from the device to the XDS connector. A series termination resistor is required on the output of the buffer. Normally a 22 ohm termination resistor is sufficient, but ideally matching the input impedance of the XDS cable (normally 50 ohms) will provide the most robust connection (see Non-buffered JTAG Signal Termination for details).
  • Multiple devices AND the device has a RTCK pin - See Multi-device Adaptive Clocking for details. When working with these topographies if any signal routing exceeds six inches, it's recommend you buffer the signals (with pull-ups on the inputs) and on clock signals adding series termination resistors.

The following signals may also be present on your XDS Target Cable and are common to many XDS products.

Table 2. Additional Common Emulation Header Signal Descriptions
PIN XDS Signal Type Target Signal Type NAME DESCRIPTION
TVRef(2) I O Target Voltage Reference Must be tied to the I/O voltage of the target device used for JTAG through a 100 ohm current limiting resistor. Used by the XDS to detect if power is active and to set JTAG signal voltage level translators (if supported by the XDS(1).
TDIS I O Target Disconnect If your XDS connector supports this signal it must be tied directly to the target's ground plane. Connecting it through a pull-down resistor may cause the XDS to NOT detect the target system and prevent normal operation.
EMU[0:n] I/O I/O Emulation Port EMU0 and EMU1 must be pulled-up on the target card. For EMU0 and EMU1 we do not recommend relying on the internal pull-ups normally provided in most devices on the EMU pins. It's recommended to connect all the XDS pins supported by your device to the XDS connector. In cases where you are connecting more than EMU0 and EMU1 follow the instructions in Emulation and Trace Headers Technical Reference Manual. In cases where the device supports more than two EMU pins, these devices normally support high speed trace (either Core Trace, System Trace or both) in which case it's a requirement to utilize termination resistors per Emulation and Trace Headers Technical Reference Manual. In cases where the device has more EMU pins than the XDS supports, the general rule is to connect as many EMU signals as possible to the XDS connector.

For some device families, due to packaging size limitations, EMU signals are being multiplexed with other device functional signals on a single pin. If you are not using the functional signal then simply connect the pin to the XDS connector per the instructions in the previous paragraph. If you need both the functional and EMU (trace) capabilities then TI recommends you isolate the XDS header and functional circuit from each other using a FET mux.

nRESET(3) O I Target Reset This is an optional signal that if integrated into your application's power-up-reset circuit may be used to remotely reset the target board. This signal driven from the XDS (if supported) is open-drain and therefore requires a pull-up.
  1. To confirm compatibility with your XDS for signal types voltage levels, check your specific device’s data sheet.
  2. Other TI documents refer to TVRef as TVD, VREF_DEBUG, and VTRef (ARM’s naming convention).
  3. Other TI documents refer to nRESET as nSYSRST or nTGTRST.

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