How to succeed with programmable analog
Embedded systems are all around us, in our homes, factories, e-bikes, and cars. Just about every device we use daily is controlled by an embedded system. Our smart, wearable devices all rely on embedded microcontrollers. Interacting with the analog world in the digital domain is part of virtually every embedded system. That applies if it is measuring your heart rate, controlling the speed of a motor, or the temperature of your home.
Interfacing between the analog and digital domains requires an analog front end (AFE). The design of an AFE differs depending on the application and may be implemented in several ways. Using discrete analog components and ICs, such as op-amps and data converters, to build the AFE is one approach. Discrete components offer a flexible route to a circuit that meets every specification. However, a discrete design also has limitations. Today's products are also measured commercially, in terms of space, cost, and power. A fully discrete analog approach may now only be found in high-resolution, sensitive test and measurement equipment. A more common approach is to use a mix, with some dependence on microcontrollers (MCUs) with integrated analog peripheral functions.
Programmable analog: the pinnacle of embedded design?
Designing a programmable analog IC is a complex task. By definition, a programmable analog IC should provide every analog signal chain function needed. The requirement will vary massively, from a sensor interface with signal conditioning, to data conversion and scaling. To do that, the IC must incorporate or be able to emulate many analog functions.
By comparison, an MCU with integrated analog will have finite analog peripherals. Integration will limit performance, function, and flexibility. For example, it will likely feature a hardware-based fixed op-amp configuration. Many applications will need integration that balances MCU capabilities with analog functionality and offer a space-saving footprint.
In addition to a comprehensive range of analog functions, a programmable analog device should support multiple input and output channels. Configuration will require different design skills, but good software tools will enable engineers to architect the signal chain they need.
Programmable analog beginnings
Although other semiconductor companies have attempted programmable analog in the past, Cypress Semiconductor was really the first to successfully develop a single device integrating programmable analog and a microcontroller. The company is now part of Infineon. In the early 2000s, Cypress initially focused on memory and application-specific ICs but was eager to profit from the growing MCU market. However, faced with a market saturated by dominant competitors, Cypress management looked for a way to differentiate its MCU from others.
The team realized that although the MCU had become the embedded developer's dream 'go-to' device, it lacked vital analog functionality. Deciding on an MCU and architecting embedded software was relatively straightforward, but a hardware team still needed to design the analog signal chain.
The MCU was just one component of the design's bill-of-materials (BOM); all the analog parts added BOM count and costs, and additional space. With this insight and an understanding of the value a configurable and programmable mixed-signal MCU would represent to the engineering community, Cypress embarked on what would become PSoC™ 1.
PSoC™ 1, a programmable system-on-chip, was launched in early 2002. Hardware engineers now had an alternative way of architecting a flexible, comprehensive analog front end. PSoC™ 1 integrated an 8-bit M8C Harvard MCU core with a programmable array of mixed-signal functions. Unlike an integrated analog MCU, a programmable analog MCU can also reconfigure itself 'on the fly' in much the same way an FPGA can.
Analog programmable functions include op-amps, comparators, analog-to-digital converters, digital-to-analog converters, switched-capacitor/continuous-time circuitry, and switchable passive arrays. The PSoC™ IDE permits routing individual parts to create essential blocks, including an analog-to-digital converter (ADC), a programmable gain amplifier (PGA), or a programmable bandpass filter. Complementing the programmable analog functions are a set of programmable digital blocks, including a host of primitive logic gates, serial communication interfaces, and digital filter blocks.
Switched-capacitor technology: the foundation of programmable analog
The underlying architecture behind Infineon's (Cypress) PSoC™ devices is the switched-capacitor analog block. Analog circuitry involves using passive components, such as capacitors, resistors, and inductors, along with active IC devices. Resistors and passives are commonplace in a discrete design. Fabricating resistors on an IC die is difficult but capacitors are relatively easy to make and easy to switch. Achieving precise capacitance values is much easier to accomplish than for a resistor, and capacitors occupy less space. Switched capacitor-based analog building blocks have become popular for architecting analog (continuous time) semiconductor devices.
The use of a capacitor to emulate a resistor was first discovered by James Maxwell in 1873 and is the basis of programmable analog devices available from Infineon (PSoC™) and Renesas (GreenPAK). Maxwell observed the change of current flow due to reversing the capacitor during experiments. By controlling the flow of charge in and out of a capacitor using semiconductor switches, the capacitor behaves as a resistor.
Surrounding a simple op-amp with a bank of switched capacitors enables the creation, configuration, and operation of the op-amp in various modes. For example, it can be a comparator, programmable gain amplifier, differentiator, integrator, or an analog-to-digital converter.
The basic concept uses a bank of switched capacitors with an op-amp as an analog-to-digital converter.By controlling the reference input signal (ARefMux in the diagram), it is possible to establish if the input is less than or greater than the reference. The feedback path from the output signal enables this control. The detailed theory of switched-capacitor analog blocks is beyond the scope of this short piece, but you may find this article helpful.
Renesas' GreenPAK programmable mixed-signal devices also use the switched-capacitor technique. Silego Technology initially developed GreenPAK in 2010, and acquisition by Dialog Semiconductor followed in 2017. Renesas acquired Dialog in 2021 and now promotes the GreenPAK product portfolio.
Choosing a programmable analog IC
Infineon's PSoC™ family of programmable mixed-signal microcontrollers continues to advance, with the PSoC™ 4 and PSoC™ 6 series representing the most comprehensive portfolio.
The PSoC™ 4 series suits industrial, commercial, and automotive applications and features Arm Cortex-M0 / Arm Cortex-M0+ cores. Integrated connectivity includes USB, CAN, and a Bluetooth Low Energy wireless transceiver. An automotive, AEC-Q100-compliant variant extends wired network support with SENT, LIN, and CAN FD protocols.
A single-chip motor control circuit for a home appliance using Infineon’s PSoC™ 4 is shown below. Featured programmable functions include an analog multiplexer, an ADC, op-amp filters, a programmable gain amplifier, and a comparator.
The PSoC™ 6 series, with its ultra-low-power Arm Cortex-M4 and Cortex-M0+ cores, is designed for battery-operated IoT/IIoT designs, from sports performance watches to wall-mounted heating and ventilation control panels.
A comprehensive set of software and hardware development tools for PSoC™ includes ModusToolbox™, PSoC™ evaluation boards, PSoC™ reference designs, and YouTube tutorials.
Renesas offers a slightly different approach with its GreenPAK family of non-volatile memory programmable mixed-signal devices. GreenPAK provides a blank canvas of analog and digital elements from which you can create the exact analog signal chain the design requires. Like PSoC™, GreenPAK aims to shrink BOM cost, and component count, and minimize board space. GreenPAK does not have an MCU core but provides vital analog and digital support functions. With GreenPAK you can integrate a host of separate mixed-signal functions into a single compact IC.
The GreenPAK line-up includes programmable and reconfigurable devices with high-voltage H-Bridge functions, custom state machines, low-dropout (LDO) regulators, and automotive-qualified ICs.
The diagram shows how three discrete power supply and sequencing tasks are incorporated into a single GreenPAK IC. GreenPAK design tools include reference designs, evaluation boards, the Go Configure IDE toolchain, and the GreenPAK Cookbook.
Of the two programmable analog devices highlighted above, only the Infineon PSoC™ can reconfigure itself in situ. For an application that requires the embedded system to switch between two distinct use cases or that requires a highly integrated MCU, PSoC™ is an ideal candidate.
Programmable analog - saving BOM cost and shrinking PCB footprint
In this article, we've highlighted the three ways you can incorporate analog circuitry into a design; discrete, integrated analog MCU, and programmable analog. Programmable analog offers the greatest variety of analog peripherals with integrated analog MCUs delivering a good balance of features and ease of development. That said, the number and flexibility of analog functions integrated alongside an MCU are growing and coming close to the programmable feature set.