Design for Test Guidelines and Considerations

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Boundary-Scan Chain

Successful testing and ISP of your design depends on a fully functional boundary-scan chain.  Maximum test coverage is achieved by testing all JTAG devices simultaneously.

Outline

• Boundary-Scan Chain

• JTAG TAP Connector

Scan-Chain

• Place all JTAG devices into a single scan-chain and add test points for debug access.

  • All JTAG devices are tested simultaneously in the serial chain.

  

   

• Multiple scan-chains are acceptable but should be merged externally; in some cases this is the preferred method:

  • Use a multi-TAP JTAG controller to externally combine multiple JTAG scan-chains into a single chain. Each JTAG TAP is actively buffered and can interface to different voltage levels.

  • Alternatively combine multiple chains with a fixture or cable. Lower reliability and susceptible to noise.

  
   

Scan Chain Debug Access & Test Points

• Check devices for full JTAG compliance

  • Some devices support emulation only or ISP only and cannot be utilized for interconnect tests. Check datasheets, device errata and BSDL files.

  • Place devices in BYPASS that do not include a boundary-scan register or do not support the EXTEST instruction

• Group components with similar voltage levels

  • Utilize a multi-JTAG TAP controller for programmable voltage interfacing or add level shifting components to the design

• Treat TCK as a critical high speed clock signal during layout

• Always consider signal loading on the common signals TCK and TMS and proper routing for good signal quality

  • Add TCK and TMS buffering on target when driving a large number of JTAG devices (more than 5)

• Dedicate a schematic page for a block diagram of the JTAG scan-chain

JTAG Interface Connector (TAP)

• Recommend Corelis JTAG TAP pin-out for robust one-to-one connection between the JTAG controller and the target

  • Every other pin is ground on cable providing noise immunity

• A solid ground is very important

• JTAG signal termination

• Multiple JTAG TAP connectors

  • Group by IC vendor

  • Group by voltage

  • Group by maximum device

  • Group by devices that requires

  • TMS, TCK, TRST_N signal

  • Chain isolation debug

  

   

• Connector types

  • Although different pinouts, most vendors use standard
    0.1” x 0.1” pitch headers

  • Shrouded header recommended to prevent incorrect insertion

  • 2mm

  • 0.050” x 0.050”

  • Test points for fixture probe access

  • Edge connectors

  • Backplane

• Keep JTAG TAP cables as short as possible

  • Ideally under 12” in length, shorter is better

• When using custom cable adapters, ensure adequate grounding between the JTAG controller and the target

• 3rd party JTAG tools may require a separate JTAG TAP connection for an individual component

Jumper example to isolate a single device

Logic example to isolate a single device

 

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