Semi-custom analog and mixed signal arrays — the new alternative to full-custom ASICs

Introductory remarks
Almost every electronic appliance is driven by market requirements to become smaller, consume less power and cost less. Add to these requirements the constant pressures Product Design Engineers are under to reduce development cost and design cycle times and the task becomes painfully challenging. A full-custom mixed-signal application specific integrated circuit (ASIC) is worth consideration as a means to meet reduced size, power and unit cost requirements. However, ASICs do not usually match up with the need to reduce development cost and cycle time. ASIC Design has historically been expensive, time consuming and risky—accessible only to organizations with large budgets, high volume requirements and the willingness to tolerate long and unpredictable development cycles. A new alternative to the full-custom mixed-signal ASIC, that minimizes several of the drawbacks with the full-custom ASIC, is a semi-custom mixed-signal device called the via-configurable array (VCA.)

Mixed-signal VCAs are similar to field programmable gate arrays (FPGAs) but are via-programmable as opposed to field programmable. In other words, VCAs are programmed (or configured) during fabrication by creating a unique via layer to connect the required analog and digital cells to implement a mixed-signal circuit. Unlike FPGAs, VCAs contain analog cells allowing for integration of analog circuits in addition to the digital. Another benefit of VCAs over FPGAs is they are not burdened with the overhead of field programmability. Field programmability generally requires more power and more silicon area (higher cost) when compared to a VCA.

VCAs are developed by first combining analog cells and digital cells into tiles. These tiles are then placed in an integrated circuit to form an array. A two layer routing fabric is added to the array which allows analog and digital cells in the tiles to be inter-connected. These VCAs are fabricated on silicon wafers, but held just prior to processing the via-layer. The via-layer is then created to implement a unique mixed-signal circuit and added to these wafers. The wafers are then finished and packaged like a typical integrated circuit. Figure 1 illustrates the VCA development process.

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Figure 1: VCA development process.

Via-configurable technology

VCAs are similar to structured ASICs and gate arrays. A VCA can be thought of as a mixed signal gate array where only a via-layer is necessary to configure it for a specific application. Figure 2 is a simplified illustration showing the process of configuring the VCA fabric. Figure 2(a) shows a fabric square composed of two metal layers with perpendicular routing tracks. The fabric is made up of several fabric squares with each square rotated 90° relative to the adjacent one as shown in Figure 2(b). Orienting the fabric squares this way allows for better routing utilization. Figure 2(c) shows the fabric with vias placed and the resulting routing. Unused tracks in the fabric can be used for shielding or alternative routing. Ultimately the only layer used to configure the entire VCA is a single “configurable via layer” (CVL) between the two metal layers of the fabric.

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Figure 2: (a) VCA fabric square (b) VCA fabric with no vias, (c) VCA fabric with vias inserted.

Designing the configurable via-layer

Designing the configurable via-layer (CVL) for a VCA is similar to mixed signal printed circuit board design. The only difference is the analog and digital libraries used to create the schematic and synthesize the HDL code (Verilog or VHDL) are based on the VCA platform being used for the design. This also applies to simulation. The via-layer is created by and place and route tool called ViaPathTM . Inputs to ViaPath are a netlist describing the mixed-signal circuit and placement constraints. The netlist is derived from the schematic and HDL code. This is normally an automated process and included with the schematic capture tool. The placement constraints are manually created and allow sensitive portions of the circuit to be routed with minimal interaction from other circuits (i.e. low noise applications.) Figure 3 shows the VCA configurable via-layer design flow.

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Figure 3: VCA design flow.

Applications

In general, VCAs are an excellent way to replace several discrete analog and digital devices. A system consisting of an FPGA and several analog chips can be integrated into a single VCA resulting in lower cost, lower power and smaller size. VCAs can support applications requiring an on-board microcontroller, power management (high voltage and current) and precision analog (e.g. 16 bit analog-to-digital converter.) In other words, just about any electronic circuit is a candidate for a VCA.

Smart sensors play a vital role in the industrial environment. Temperature, pressure, torque, displacement and humidity are a few of the measurements typical for industrial applications. Frequently these analog signals need to be acquired and quantized for processing in the digital domain.

Figure 4 shows a VCA with an embedded ARM Cortex M0 processor (MOCHA). The Cortex M0 is a 32 bit fixed point processor optimized for low power and minimum area. In the MOCHA the Cortex M0 sub-system includes a hardware multiplier, 32k bytes of Flash program memory and 24k bytes of data memory, timers, a peripheral bus interface and a debug interface. The most significant difference between MOCHA and standard VCAs is the Cortex M0 sub-system is implemented as a hard macro (not via-configurable.) The rest of the chip is via-configurable allowing for a wide variety of peripheral analog and digital functions to be implemented.

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Figure 4: Cortex M0 based mixed-signal VCA

Figure 5 shows the MOCHA configured for a smart sensor application. In this case, the data acquisition function is implemented in the via-configurable portion of the chip with the Cortex M0 providing a programmable microcontroller function. In the analog section, programmable gain amplifiers and switched capacitor filters are used to condition the signal before being input to the sigma delta analog-to-digital converters (ADCs.) Digital-to-analog converters (DACs) are used to generate a stimulus signal. The digital section further processes the quantized analog signal prior to presenting it to the Cortex M0 where it is packetized and sent back out on the serial peripheral interface (SPI.) This is just one of many possible applications of the MOCHA.

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Figure 5: MOCHA VCA configured for a smart sensor application.

Other specialized VCAs are available for high-voltage/high-power applications such as power management or motor control. Generally these systems require multiple voltage domains, power sequencing, sleep mode operation, and intelligent control. Many portable appliances require supply voltages of 1.8V and 3.3V for digital ICs, 4.2V for Lithium-ion batteries, 5V for legacy interfaces, 36V for LED backlighting, isolated voltages for sensitive analog circuits, and higher currents for motor control. All of these can be accommodated in a single high-voltage power management VCA.

Cost effective development of a mixed-signal ASICs

If a mixed-signal custom ASIC would provide benefits to an electronic appliance, the VCA should be given consideration. With the VCA approach a system designer can develop a prototype with an off-the-shelf array. This initial VCA should be selected such that it will easily accommodate the product requirements. In other words, it should have more than enough analog and digital resources. Once the design has been debugged and market tested, if market requirements warrant, an optimum VCA can be developed by simply removing the tiles from the initial array used for prototyping. This optimum VCA will meet the cost targets for moderate volumes (typically less than one million pieces annually) and is still via-configurable allowing easy design changes or new product development. For the product whose run rates are higher (e.g. consumer) it is practical to reduce the silicon area further (lowering cost) by creating a full-custom device using only the required cells from the analog and digital tiles in the optimal VCA. This device will be the lowest possible cost, but does give up via configurability.

Conclusion and the future

VCAs offer distinct advantages to the electronic system designer. These new devices, with their unique design approach, solve many of the historic problems associated with full-custom ASIC design, yet can realize all the benefits. Even for high volume requirements, where a full-custom ASIC would certainly be required to achieve cost targets, using the VCA approach where the full-custom device is ultimately derived directly from a proven VCA, will result in faster time to market with much lower risk and cost.

References

  • J. Kemerling, “VIA-Configurable Analog ASICs – Technology and Applications,DesignCon, 2010.
  • J. Kemerling, “Semi-Custom, VIA-Configurable Analog and Mixed-Signal ASICs,” GSA Forum, March 2009.
  • R. Wender, “How to Design a 16 bit Sigma Delta Analog to Digital Converter,” Application Note TSA002, Triad Semiconductor, Inc., January 2007.
  • J. Kemerling, M. Turner, and R. Wender, “Structured Analog IC Design Example using Mentor Graphics tool flow”, U2U, 2006.