Design for Test Guidelines and Considerations

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Board-Level Design

Outline

• Schematic Design Considerations

  • JTAG Device Selection / Identification

  • Product Design

  • Memory Cluster Testing

  • Power Supply Loading
     

• FPGA Considerations
   

• Flash Programming Considerations
   

• PCB Layout Considerations

  • General

  • Component Placement

  • Trace Routing

  • Test Points

  • Multiple Scan-Chains

Schematic Design Considerations

• JTAG Device Identification

• Scan-Chain Design (see boundary-scan chain tips)

• TAP Interface Connector

• Board-Level DFT Design

JTAG Device Selection / Identification

• Datasheet indicates IEEE-1149.1, JTAG or boundary-scan compliance

• High pin count BGA or surface mount device
  (FPGA, CPLD, CPU, DSP)

• Pin names TCK, TMS, TDI, TDO and TRST_N

• BSDL file available

• Chip vendors such as Altera, AMCC, Atmel, Broadcom, Cypress, Freescale, IDT, Lattice, Marvell, Micron, National Semiconductor, QLogic, Samsung, ST Microelectronics, TI, Xilinx, and more

Design boundary-scan into the product, not as an afterthought

• Design engineers should think ahead about testing

• Utilize DFT guidelines prior to and during board layout

• Identify up front whether devices have BSDL files available and ensure they have been tested

• Consider the initial power-up or reset state of board

• Ensure scan-chain is operational when power is applied

  • Properly terminate compliance enable pins

  • CPLDs that control power logic or scan-chain paths must remain in BYPASS

  • TRST* pins must be high for JTAG testing

  • Constraints cannot be used if the scan-chain is not working

• Tri-state or disable non-boundary-scan devices

  • Provide boundary-scan control to disable device outputs that will otherwise conflict with nets involved in boundary-scan test (enable pins, test pins, reset signals, power shutdown circuitry)

• Disable these devices by:

  • Connecting a boundary-scan controllable output on the net to control the chip enable of the conflicting device (FIXED_HIGH, FIXED_LOW constraint)

  • Installing a dedicated jumper which put the target into a boundary-scan ready state

  • Connecting a GPIO pin available on Corelis JTAG controllers to the offending one

   

Considerations for memory cluster testing

• Memory cluster tests are performed by controlling the pins on the memory device using surrounding JTAG logic including address pins, data pins, chip select/enable, output enable, RAS (DRAM), CAS (DRAM), clock (synchronous devices), write strobe (FIFO), read strobe (FIFO), and write enable

• The driving boundary-scan devices must have separate control cells for the address, data, and control pins on the memory device

  • Some CPU devices have shared control cells

  • The BSDL file contains this information

• JTAG control of the clock signals must be provided

  • Ensure the clock pin has a JTAG output cell to drive the clock

  • Pins identified in the BSDL file cannot drive test patterns

  • If a PLL is used to drive the clock, ensure the PLL has a bypass feature that can be used during JTAG testing

  • The PLL bypass feature should be controllable using JTAG

    • Note some PLLs enable bypass by grounding the AVDD pin

  • Adding stubs to the clock net may alter the functional operation of the circuit

  • Memories can be functionally tested at-speed if they are connected to a supported EJTAG CPU

  • Utilize ScanExpress JET to test the memory if it is connected to a supported CPU (contact Corelis for a current list)

   

Consider Power Supply Loading

• The UPDATE-DR state on all components can induce large current swings and ground bounce

  • Ensure the power supply and voltage regulators can handle JTAG test current

  • Turn up the current limit. Functional current and boundary-scan test current requirements may widely differ

  • BYPASS can be used to remove components from the test

  • Identify low impedance pull-up and pull-down resistors, especially digitally controlled impedance (DCI) nets

  • Zero ohm resistors should be marked transparent
     

Xilinx and Altera FPGA Considerations

• Xilinx and Altera FPGA devices have different test characteristics depending on if the devices are configured

• Maximum testability on these devices is in the pre-configuration state

• To keep Xilinx parts in pre-configuration mode, the INIT* pin needs to be held low prior to and during power-up of the target

• To keep Altera parts in pre-configuration mode, the NCONFIG* pin needs to be held low prior to and during power-up of the target

• Recommend these pins route to the TAP header in place of a ground pin. When the JTAG controller plugs into the TAP, the pin will automatically be pulled low

Compliance Enable Pins Must Be Satisfied

• Compliance pin states are listed in the BSDL file

• Correct compliance pin states must be maintained prior to and during JTAG testing to maintain test compliance

• For example, the BSDL file for the Motorola MPC106 device describes the following compliance enable pin:
  
  attribute COMPLIANCE_PATTERNS of mpc106: entity is "(LSSD_MODE_L) (1)";
  
  indicates that pin LSSD_MODE_L must be a logic high prior to and during boundary-scan testing for correct JTAG operation
   

Considerations for Flash In-System Programming

• In-system programming of flash devices through JTAG is done by emulating read & write cycles to the flash device using surrounding JTAG logic including address pins, data pins, chip enable, output enable, write enable, and optionally reset, write protect and ready/busy

• The driving boundary-scan devices must have separate control cells for the address, data, and control pins on the memory device

• To reduce programming time:

  • Utilize boundary-scan devices with faster TCK rates

  • Ensure that your scan-chain is short

  • Remove unnecessary constraints

  • Use external write strobe

• External Write Strobe (Flash In-System Programming)

  • The WRITE_STROBE* signal is active low and should be pulled high with a 1K resistor on the target board. It should be logically OR-ed with the WRITE_ENABLE* signal so that assertion of either the WRITE_ENABLE* signal or the WRITE_STROBE* signal will assert the flash WE* pin

Flash In-System Programming Theoretical Speed Formula

(#bits in chain) * (#scans/write) * (#writes/location) * (#locations)
                                      TCK frequency

Where:
#bits in chain - effective length of the boundary-scan chain (assuming unused components are placed in BYPASS)
#scans/write - number of DR scans which are required in order to write a data value to the flash
#writes/location - number of data values that must be written to program each location
#locations - number of data locations to be programmed
TCK frequency - frequency of the JTAG test clock (TCK) signal

PCB Layout Considerations

• Component Placement

• Trace Routing

• Test Points

• Multiple Scan-Chains

General

• TCK needs to be as free as possible of glitches and spikes, since all
  operations are triggered by rising and falling TCK edges.

• Connection of TDO of last device in scan chain to board TDO should be as
  short as possible.

• TCK and TMS fan out to every device is most critical.

• When using fan out buffers to distribute TCK, TMS and TRST_N, put
  termination resistors on the primary side of the buffer (signals coming
  from the JTAG controller) .

• A series resistor on the TDO of the last device in the chain should be close
  to that device’s TDO pin.

Component Placement

• Provide adequate room around the TAP connector to plug the cable in

  • Consider cable access when the product is enclosed or fully assembled

• Easier debug access when termination resistors are placed consistently close to the TAP connector along with a clearly labeled ground point

• Do not place the TAP connector near noisy analog components such as voltage regulators

• Consider access to probe the scan-chain if things don’t work

• Place JTAG devices such that a star topology can be implemented on the JTAG signals

Trace Routing

• Recommend general documented layout guidelines such as

  • A Practical Guide to High-Speed PCB Layout, Analog Devices

  • Guidelines for Designing High-Speed FPGA PCBs, Altera

  • High-Speed Board Layout Guidelines, Altera

  • Basic Principals of Signal Integrity, Altera

• Recommend JTAG traces be a minimum 10 mil width with minimum 10 mil spacing with length s short as possible JTAG signals should be routed on the outer layers away from noisy analog voltage regulators 36

• TMS and TCK are broadcast lines. They should be routed in accordance to high-frequency bus rules. Use fan out buffers to avoid overload of TMS and TCK signals coming from the JTAG controller.

• Route TCK and TMS in a star topology

• Signal quality is a key factor for successful JTAG testing

  • JTAG test signals should be assigned as critical for first pass route

  • Most common problems are reflections and ringing on TCK

  • Faster TCK results in faster in-system programming times

Test Points

• Compliance enable pins should be accessible. It is a good idea to place pads for both pull-up and pull-down resistors on the pin when in doubt

• Even though TDI and TDO are point to point, consider an accessible test point at each link for debug if required

• Always ensure there are convenient test point locations for ground

  • Debug is much easier with the ability to clip one lead to ground

Multiple Scan-Chains

• A working scan-chain should be one of the highest priorities

• Breaking up a scan-chain may provide easier debug capability

• Different groups use the JTAG chain differently

  • Designers need JTAG for in-circuit emulation and in-system programming

  • Test engineers need JTAG for interconnect testing

For more tips on boundary-scan chains, see boundary-scan chain tips.

 

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