IoT Design Challenges

Low-cost IoT designs that interface with the real world incorporate multiple design domains that individually are challenging for today’s engineers, so it’s no surprise that putting them all together creates extreme pressure on IoT design teams.

The typical IoT device contains a sensor and an actuator that interface to the Internet. The sensor creates a signal based on some real-world activity it is monitoring, often via a MEMS sensor. It sends the signal to an analog signal processing device that is either 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.

The big challenge in IoT design is that analog, digital, RF, and MEMS design domains are all combined in one end device. All four design domains must work together, especially if they are going on the same die. Even if the components are on separate dies that will be bonded together, designers from these different disciplines will still need to work together during the integration and verification process. Typically, several components support 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. That means 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.

Figure 1: A top-down design flow for IoT design, unifying the four design domains.

Tanner provides a single, top-down design flow for IoT design, unifying the four design domains (Figure 1). Whether you are designing a single die or multiple die IoT device, design teams can use this flow for creating and simulating an IoT device in the following way:

• 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. T-Spice simulates SPICE and Verilog-A representations of the design while ModelSim simulates the digital, Verilog-D/RTL portions of the design.
• Simulating a mixed-signal design. S-Edit creates a 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. 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 implementation language, the designer drives the simulation from S-Edit and the design is automatically partitioned across the simulators. Then, the designer 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 the designer 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.
• Verifying the MEMS device. The SoftMEMS 3D Modeler is integrated within L-Edit to automatically generate the 3D model from the 2D masks using a set of specified fabrication steps. The designer can perform 3D analysis of the MEMS model against physical requirements and then make changes to the 2D masks if there are any issues.

Conclusion
The prime challenge of IoT design is creating a device that incorporates four design domains: analog, digital, RF, and MEMS. It is best to use a design flow that 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, click here.