Guidelines for Chip DFT Based on Boundary Scan Reference to an article by Ben Bannetts By Regev Susid
Specify 1149. 1 Compliance It is important for all the devices to be 100% compliant with the 1149. 1 standard. The optional functions should also be included: • IDCODE – Reading the ID of the device. • CLAMP – Scanning the cell data into the output. • HIGHZ – Outputs a High-Z state. And also to add the TRST.
BSDL (Boundary-Scan Design Language) The device’s design should be done according to the BSDL. It is also extremely important to check the BSDL thoroughly In both syntactically and semantically (some commercial Checkers are available like ASSET etc’) The last part of checking the BSDL is to compute 1149. 1 conformance tests and to apply them to a device.
Assigning IO and OZ Control Cells Using Bidirectional (IO) and three-state (OZ) pins is recommended in order to manage their status while in test mode. By doing so we can prevent unnatural test configurations that may might place some devices on the board in a dangerous state.
TRST option is important in order to maintain testing consistency and in order to be sure of the correctness of the results. 2 Choices are available: • Adding TRST. • Adding POR (Power On Reset).
Shorts The first step of checking the board is for shorts. This part is very important (though it may take some time) because we need to be sure our test results are true.
Increasing Shorts coverage It is important that all the devices should be BS When a BS device is connected to a non-BS device it is harder to discover shorts. This problem can be solved by using BC_7 pins.
BC_7 cell is designed for bidirectional signal pins, and when in drive mode, it is able to sense the value of the driven signal, thus monitoring the “transmitted” data”.
Ground Bounce is actually a state in which the DC power supply drops the voltage for a small amount of time, this situation might cause a disruption in the TAP Controller’s operation. Its important to ensure our Device is protected from such Thing, and it could be tested by checking an “Extreme” situation of input by using EXTEST command.
Accessing the internals DFT By allowing access to other internal chip DFT features like: • Memory BIST (Built-In Self-Test) • Logic BIST We can re-use these features once the chip is assembled on the board which In turn supports improved board-level diagnostics. While using the RUNBIST (Built-In Self-Test) and INTEST (Built-In Self-Test) the device switches into test mode which gives the control to the boundary scan cells on the output.
Safe States: Power-Up & EXTEST Boundary Scan Devices should be powered up in safe states meaning IO in input mode and OZ in output mode, thus avoiding the risk of powering up into a Contention state (Bus fight).
TAP Pin Layout It is suggested to position the TDI, TDO, TMS, TRST and TCK pins away from the Power and Ground Pins in order to avoid the situation that shorts will behave very deterministically as strong stuck-at-1 or stuck-at-0.
Reduced Pin-Count Test (RPCT) Problem: How to test a 600 -pin chip on a 500 Driver/Sensor channel tester? Solution: 1. Buy a new machine. 2. Use 100 bits of the BS register to indirectly access the low – frequency pins.
Design for RPCT High data-rate pins like: Scan-In, Scan-out, Clock are Contacted directly while the Low data-rate functional pins are accessed by part of the BS register. The BS register is designed to allow segments to be addressed through the TAP using Instructions like INTEST-RPCT.
Why Using RCPT • Reduces cost of test: re-use older testers, no need to buy new D/S channels. • Reduces number of probe points during wafer probe: - lower cost - lower contact problems • Enables multi-site testing at both wafer probe and packaged die test. • Only a few high-speed, high resolution pins needed.
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