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Cambridge Silicon Radio Case Study

Low-cost tools get Bluetooth silicon up and running

When Bluetooth specialist Cambridge Silicon Radio (CSR) went looking for a design tool to help design the RF and analogue parts of its products, the company did not go to the big guns of electronic design automation. CSR wanted to avoid paying the high licence fees and maintenance costs associated with those tools.

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CSR makes extensive use of analogue design techniques and that expertise has helped the company ship more than 10 million Bluetooth ICs. The world’s leading supplier of Bluetooth silicon has achieved its position by designing products that put a full 2.4GHz RF front-end on the same chip as the digital baseband circuitry. Successive generations of product have seen the company add flash memory, ROM and even a digital signal processor to its single-chip Bluetooth devices to support a variety of vertical market needs. This mixture of functions calls for a flow that allows digital and analogue circuits to be combined easily.

Partitioning between the mixed-signal and digital parts of the design is crucial to the design flow used by CSR. The company’s designers pay careful attention to estimating the relative size and shape of key blocks and interfaces that match closely between them has helped streamline the flow. Many companies believe they need to buy specialised chip-assembly tools in addition to their digital and analogue design tools. But CSR found that there was one custom-IC layout tool optimised for analogue design that could handle the job. That was Tanner EDA’s L-Edit.

CSR’s Paul Egan has used L-Edit since working as a consultant, specialising in mixed-signal design. When he started at CSR, his first choice was L-Edit. And this decision made a lot of sense for CSR as well. As a start-up with limited resources, cost was a big issue.

“We could not see the sense of loading ourselves down with $60,000 to $70,000 of licences. And we had the confidence of being able to do the designs with L-Edit,” said Egan. A further consideration for CSR was the cost of support: “One overhead we don’t have is a large CAD department supporting high-end tools. It really does work straight out of the box.”

L-Edit is used to design the analogue and RF parts of each IC. For the digital part of the layout flow, CSR uses the Synopsys Astro tool. Once the digital blocks have been laid out and routed, they are assembled together with the analogue and RF blocks using L-Edit into a full-chip layout in the form of a GDS II file.

An important aspect of analogue-circuit design tools that is in distinct contrast to the features of digital design tools is the support for custom automation. Mixed-signal designers will often design quite similar circuits for different ICs or different cores for the same IC, such as phase-locked loops that drive different digital bocks. Laying them out polygon by polygon each time is a tedious process.

Scripting and programming interfaces prove highly effective in this environment. L-Edit provides two forms of scripting – the UPI and T-cell programming interfaces – which can automate the jobs such as generating multi-fingered transistors and other complex shapes that would take a long time to draw by hand. T-cells, being object-oriented, make it possible to build complex hierarchies of circuit elements that can be parameterised and generated quickly before being tuned by the experienced hand of the designer.

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CSR has made extensive use of the UPI interface to generate transistor and other circuit-element shapes automatically.

“Using the UPI interface, we developed some nice, neat little tools,” said Egan. “We’re starting to use T-cells for our next generation: we’re trying to use T-cells exclusively.”

Capacity and speed have not proved to be issuesthrough the four generations of CSR Bluetooth silicon that the company has developed. “We like the speed and if we’re worried about running out of capacity, all we have to do is buy a faster PC. When we get to the point when we think L-Edit can’t cope, we find that a faster PC will do it,” said Egan.

He said the set of features provided in L-Edit are a good fit with the way that CSR does its designs. “A lot of what we do is down to the craft rather than the tools. High-end features would probably get in the way,” said Egan.

The CSR experience with cost-effective mixed-signal tools is going beyond the company to others in the Cambridge area, Egan reported: “Cambridge is a small design community. It’s interesting. I get a call every couple of weeks asking me to give someone our experience with L-Edit. Start-up companies are going down that route: they don’t have a lot of money to spend on tools.”

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