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Identifying the Prime Challenge of IoT Design

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By Jeff Miller, Product Marketing Manager, Mentor Graphics Corporation

Introduction

In his blog post for Semiconductor Manufacturing & Design, Pete Singer shared how the acquisition of Tanner EDA by Mentor Graphics provides a solution to meeting the design challenge of Internet of Things (IoT). Low-cost IoT designs, which interface the edge of the real world to the Internet, mesh together several design domains. Individually, these design domains are challenging for today’s engineers. Bringing them all together to create an IoT product can place extreme pressure on design teams. For example, let’s look at the elements of a typical IoT device (Figure 1).

Figure 1: A typical IoT device.

This IoT device contains a sensor and an actuator that interface to the Internet. The sensor signal is sent to an analog signal processing device in the form of an amplifier or a low-pass filter. The output connects to an A/D converter to digitize the signal. That signal is sent to a digital logic block that contains a microcontroller or a microprocessor. Conversely, the actuator is controlled by an analog driver through a D/A converter. The sensor telemetry is sent and control signals are received by a radio module that uses a standard protocol such as WiFi, Bluetooth, or ZigBee, or a custom protocol. The radio transmits data to the Cloud or through a smartphone or PC.

This device points out the prime challenge to IoT design: analog, digital, RF, and MEMS design domains all live together in one device. IoT design requires that all four design domains are designed and work together, especially if they are going on the same die. Even if the components are targeting separate dies that will be bonded together, designers still need to work together during the integration and verification process. In this design, there are several components in multiple domains, such as the A/D converter, digital logic, a RF radio, a MEMS sensor, and an analog driver that connects to an external mechanical actuator. The design team needs to capture a mixed analog and digital, RF, and MEMS design, perform both component and top-level simulation, layout the chip, and verify the components within the complete system.

The Tanner Solution

The Tanner solution delivers a top-down design flow for IoT design, unifying the four design domains (Figure 2).

Figure 2: The Tanner IoT design flow.

Whether you are designing a single die or multiple die IoT device, you can use this design flow for creating and simulating this device:

  • Capturing and simulating the design. S-Edit captures the design at multiple levels of abstraction for any given cell. Each cell can have multiple views such as a schematic, RTL, or SPICE and then you choose which view to use for simulation. T-Spice simulates SPICE and Verilog-A representations of the design while ModelSim simulates the digital, Verilog-D/RTL portions of your design.
  • Simulating the mixed-signal design. S-Edit creates the complete Verilog-AMS netlist and passes it to T-Spice. T-Spice automatically adds Analog/Digital connection modules and then partitions the design for simulation. T-Spice simulates the analog (SPICE and Verilog-A) and sends the RTL to ModelSim for digital simulation. Both simulators are invoked automatically and during simulation the signal values are passed back and forth between the simulators whenever there is a signal change at the analog/digital boundary. This means, that regardless of the design implementation language, you drive the simulation from S-Edit and the design is automatically partitioned across the simulators. Then, you can interact with the results using the ModelSim and T-Spice waveform viewers. Behavioral models of MEMS devices can be created in Verilog-A or as equivalent lumped SPICE elements that are simulated along with the digital models for system-level verification.
  • Laying out the design. The physical design is completed using L-Edit which allows you to create the layout of the analog and MEMS components for the IoT design. The parameterized layout library of common MEMS elements and true curve support simplify the MEMS layout.
  • Completing the flow. Of course, there are other steps in the flow, such as digital synthesis, digital place and route, chip assembly, physical verification, static timing analysis, and full system verification. However, these steps are beyond the scope of this discussion.

Implementing the MEMS Device

One of the most challenging aspects of IoT design is implementing the MEMS device. So, in this article we focus on the physical design flow for this device. Let’s say that the MEMS device in our design is a magnetic actuator. A magnetic actuator is comprised of a coil and a moving paddle. The paddle is suspended by a spring. When current is sent through the coil, a magnetic field is created which moves the paddle in and out of the coil field (Figure 3).

Figure 3: MEMS magnetic actuator.

You could create a 3D model of the magnetic actuator using a 3D analysis tool and then analyze its dynamic response to different currents. To fabricate the actuator you need a 2D layout mask and deriving a 2D mask from a 3D model is error-prone and difficult to validate. A better approach is to follow the mask-forward flow that Figure 4 shows, that results in more confidence that the actuator will not only work correctly but that it can be successfully fabricated.

Figure 4: The mask-forward MEMS design flow.

The mask-forward MEMS design flow starts by creating the 2D mask layout in L-Edit. Then, use the SoftMEMS 3D Solid Modeler (integrated within L-Edit) to automatically generate the 3D model from those masks and a set of specified fabrication steps. Perform 3D analysis using your favorite finite element tool and then iterate if you find any issues. Make the appropriate changes to the 2D mask layout and then repeat the flow. Using this mask-forward design flow, you can converge on a MEMS device that you are confident can be fabricated correctly because you creating the 3D model directly from the masks that will eventually be used for fabrication, rather than trying to work backwards from the 3D model.

Conclusion

The prime challenge of IoT design is working in four design domains: analog, digital, RF, and MEMS. The Tanner design flow is architected to seamlessly work across all of these design domains by employing an integrated design flow for design, simulation, layout, and verification.
For more information about the IoT design flow, see: www.mentor.com/tannereda/mems-design?cmpid=10167

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