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Reduce yield fallout by avoiding over and under at-speed testing

Posted: 14 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:SoCs? phase-locked loops? Static Timing Analysis?

In the nanometer technology used for automotive SoCs, most defects on silicon are brought about by timing issues. As a result, at-speed coverage requirements in automotive designs are stringent. To meet these requirements, engineers expend a lot of effort to get higher at-speed coverage. The principle challenge is to achieve silicon of the desired quality with high yield at the lowest possible cost. In this article we discuss the problems associated with over-testing and under-testing in at-speed testing, which can result in yield issues. We will provide a few suggestions that can help to overcome these problems.

The primary objective of at-speed testing is to detect any timing failure that may occur on silicon at its operating frequency. The most important part to be tested is the logic that generates controllable clock pulses having the same frequency as required for functional operation. The preferred way to supply controlled clock pulses is from the tester (ATE) through the input pads, as this will reduce complexity and minimize the additional test logic that needs to be built over the design.

However, this scheme will have frequency limitations because pads generally cannot support very high frequency clocks. So on-chip phase-locked loops (PLLs) and oscillators are used to provide clock pulses. Free running clocks from these sources cannot be used directly, however, because first we have to shift vectors through scan chains at slow frequency (shift frequency), capture at functional frequency, and then flush out data at shift frequency. We need controllable pulses while capturing at functional frequency, which can be achieved by using the chopper logic. A typical clock architecture with at-speed clocking is shown in figure 1.

Figure 1: A typical clock architecture with at-speed clocking.

For any SoC, STA (Static Timing Analysis) sign-off is integral to validating the timing performance. Timing sign-off ensures that the silicon will operate at the desired functional frequency. The same logic applies to at-speed testing as well. STA signoff must be done for at-speed mode along with the functional modes because the clock path might be different in at-speed mode, and added test control logic needs to be timed as well. The chopper logic is not required in normal functional mode, so we need to meet the timing requirements of the chopper logic as well.

Ideally speaking, closing timing in at-speed mode should not be a problem if the change in clocking is done in the common path, such as at the start of the clock path, so that the change is common for both launching and capturing flops, and hence does not affect setup and hold timing of the design. The test control logic generally works at slow frequency or is static and hence not very difficult to meet timing.

Typical SoC clocking scheme
However, modern Soc designs are not that simple. High performance and low leakage requirements result in the designs having various clock sources within a single SoC, such as PLLs, oscillators, clock dividers, etc. Depending upon the architecture, there can be a number of IO interfaces operating on an external clock running at a few MHz, such as SPI, JTAG, I2C, etc. As a result, different parts of the SoC can operate at different frequencies.

Here's where things get complex. The clocking solutions (chopper logic) discussed earlier for at-speed clocking are not sufficient for complex chips operating at different frequencies. In at-speed testing, these complexities raise problems known as Under-testing and Over-testing, which then lead to the need for optimal testing.

Over-testing happens when logic is tested at a higher frequency in at-speed mode compared to the frequency of operation in functional mode. Over testing happens if a pll_clock is provided to any low frequency modules like watchdog and RTC during at-speed mode. The one key reason for such an approach is simplicity of the test clock path, as this approach will require only minimal change in functional logic. In our example, we just need to bypass all divided clocks/RC osc clocks/external clocks by scan clock which in turn will be controlled by the pll clock.

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