Digital Implementation in the Laker Custom Environment
Chances are, if you are the one doing the digital blocks in a “big A , little D” (primarily Analog) environment, you are either handcrafting the digital design from a schematic, or – for larger blocks – sending it out to a megabucks digital place and route tool, and then modifying it afterwards. Either way, you know the pitfalls of these approaches first hand – especially at advanced nodes.
But what if you could get both environments in one system? (Hint: you can.)
In 2010 SpringSoft launched two new controllable automation engines – the Laker Custom Row Placer and the Laker Custom Digital Router – to enable a faster, more efficient custom digital design flow. Both tools run completely within the Laker™ Custom Layout Automation System. If you can imagine a digital place and route tool with all of the ability of a high-end layout editor built in, then you get the idea.
How does that work? Very well, actually! The Laker Custom Digital Place and Route tools have been very well received and are in daily production use for designs down to 28nm at a number of major semiconductor companies. The flexibility to automatically or incrementally build up a high-performance custom digital block using standard cells and to do as much or as little of the job as you want to by hand is the perfect combination for many of you.
Let’s begin at the beginning: Regardless of whether you are working with a SPICE (transistor-level) or digital (gate-level) netlist, the Laker system can auto-generate a corresponding schematic for you in the Laker schematic-driven layout (SDL) environment. The schematic and hierarchy views appear in the Laker layout window as in analog design.
Prior to placing any cells, you can use the Custom Row Placer’s floorplanning capabilities to query the design data and estimate the area needed to create a right-sized block, or you can simply draw any orthogonal-shaped area (or areas) in which to locate the digital cells. The resulting block may be queried for the utilization rate to assure successful placement and routing before you start. In today’s mixed-signal designs where dozens of small digital components may be mixed into an analog block, the convenience of in situ digital block generation is quite attractive. However, when the exact shape of the digital block is not as critical, you can also specify any number of variables from utilization rate to height or width dimensions and let the placer automatically define the actual block size. It doesn’t get any easier than that!
Next, you can use automated Power and Ground (PG) routing to create the PG rails around the periphery of the block and again, you have the option to tweak them by hand.
Because Laker knows the desired connectivity from the schematic, it can automatically generate the pins associated with the digital block in the row placement area. Of course, in this custom environment you have the freedom to just move them wherever you want them if you so choose.
Placing: With the placement area defined, you could just let the placer place the entire design. But that’s probably not as “custom” as you want, so you have the option of using Laker’s SDL capabilities to drag critical cells from the schematic and hand-place them into the digital block. When a cell is hand-placed in this manner, you will have flight lines to the pins and previously placed cells to help guide your placement. In addition, when placing the cell, it will automatically snap to a legal placement in the relevant row. Even better, as a cell is dragged up or down across rows, it automatically flips back and forth to match that row's power and ground signals.
After you are done placing the critical cells, you can select the remaining cells in the schematic and instruct the Laker Custom Row Placer to automatically place them, which it will do following all design rules and constraints while minimizing wire length. The Custom Row Placer works with the Custom Digital Router to minimize congestion and assure a routable and DRC-clean placement. It will perform incremental selection and placement, as well as internal iterations to pack the placement area.
The Laker Custom Digital Row Placer
But this is only the start: You can use all of the Laker layout system’s normal editing functions to move cells, or hand-route critical nets, for example. Or, if you subsequently import a modified netlist for a digital block, the Laker system’s ECO capabilities will automatically identify any changes and assist in placing only those portions of the design. Similarly, if you place a new cell or cells into a previously placed area, the system can automatically legalize these cells and shift surrounding cells around to make room.
While performing its tasks, the Custom Row Placer is constantly aware of routing issues. If you request a congestion map, the Custom Digital Router will determine which areas will be difficult to route and highlight those areas. This gives you the opportunity to use the Laker layout capabilities to move things around, create blockages and/or routing channels, and to re-check for potential congestion. This is the kind of interactive, controllable automation needed for high-productivity custom digital layout.
Congestion maps check routability
When the placement is done: The Custom Digital Router performs routing directly in the Laker database – or in OpenAccess – and is fully supported with LEF/DEF import and export capabilities. It is a hybrid router that can employ both grid- and shape-based routing technologies. The grid-based router is used for initial routing to provide better performance (speed), and then the shape-based router takes over to address off-track pin connections and any remaining design rule violations. The router employs a full suite of DRC avoidance utilities – such as support for advanced end-of-line spacing, min-edge/min-area, and enclosure edge rules – and offers post-route yield optimization to remove small notches and jogs and add redundant vias where possible. Routing is DRC clean down to 28nm.
The Laker Custom Digital Router
What’s custom about this router? With placement complete, you can tell the router to route the entire design automatically, but a custom digital block typically requires some amount of customization or it wouldn’t be custom, right? Because the Custom Digital Router is incremental and interactive, you can use the various built-in routers in the Laker Layout system to hand-route critical nets and then complete all or some of the remaining routing automatically using the Custom Digital Router. At any point in this process, you can run a global route and review the congestion map to ensure a successful completion.
The Custom Digital Router also takes design changes in its stride. Before now, changes to the digital design made after the design was routed often required a complete rip-up and re-route, causing all of the painstakingly hand-drawn routes to be lost. Our router will recognize both floating and added connections and make the necessary corrections without disturbing existing connected routes. Throughout the process, the seamless integration of built-in Laker Layout features and Custom Digital Router features work together to create a far more efficient flow.
Summary: The Laker Custom Row Placer and Laker Custom Digital Router work in conjunction with each other to minimize wire lengths and assure routable placements. These new custom place and routing technologies allow you to work in a single comprehensive design environment for maximum productivity and achieve high quality of results (QoR) in a fraction of the time it would take to place and route the digital blocks by hand. For the first time, there truly is a "mixed” design flow for mixed-signal design that can improve quality, increase productivity, and accelerate time to market and time to profit.
