Detailed analysis report on SiLabs Precision32 MCU

This article introduces Silicon Labs' new Precision32 microcontroller product. This is the company's first 32-bit MCU. In addition to an ARM-compatible CPU, the chip also integrates USB and multiple analog components, as well as common Flash memory, SRAM, timers and serial interfaces. This article is sponsored by Silicon Labs, but all opinions and analysis come from the author himself.

Silicon Labs is deeply involved in the microcontroller industry

Most people know Silicon Labs for its analog and mixed-signal technology. The company has CMOS design expertise integrating high-performance analog peripherals with digital circuits. This expertise is reflected in a variety of IP blocks, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), digital isolators, digital phase-locked loops, USB, RF, sensors, etc. The company integrates these blocks to create application-specific products, including SLIC and DAA devices (for VoIP equipment), AM/FM radio receivers and transmitters, silicon TV tuners, capacitive touch sensors, USB bridges, and programmable clock products.

Less well known is that Silicon Labs has been in the microcontroller (MCU) market for many years. This experience dates back to 1999, when a company called Cygnal Integrated Products began to introduce a series of mixed-signal MCU products. When Silicon Labs acquired Cygnal in 2003, the company had released more than 50 8-bit MCU products. The company developed pipelined 805 1-compatible CPUs that ran at up to 100MHz. In addition to fast CPUs, these microcontrollers also included ADCs, DACs, oscillators, voltage regulators, and other analog functions.

Since the acquisition, Silicon Labs has continued to invest in microcontrollers, expand its product line, and integrate a variety of analog product technologies into MCUs. Many MCU versions include RF, USB and other communication interfaces. The company also uses its expertise in analog technology to reduce microcontroller power consumption, including working mode power consumption and sleep mode power consumption.

Now, Silicon Labs offers a variety of mixed-signal 8-bit MCUs with the smallest package being 2rrirr12. The 0 series chips are widely used in industrial, embedded, consumer and communications fields, especially in applications that require small packages, analog features and low power consumption.

ARM-based Precision32 MCU

To expand its microcontroller product line, Silicon Labs recently launched a new 32-bit MCU series. The new Precision32 product contains an ARM CPU, which fully utilizes the software and tool advantages of the ARM architecture. ARM is the most popular instruction system in the industry, with a large number of suppliers and considerable annual shipments. Although most of this success comes from the mobile phone market, ARM is now rapidly becoming a leader in 32-bit MCU shipments.

Silicon Labs Precision32 series products use the ARM Cortex-M3 CPU, which can run at up to 80MHz. Although this clock speed is not faster than Silicon Labs 8-bit MCUs, the ARM 32-bit architecture is more suitable for modern compilers and other codes that use 16-bit or 32-bit data. The Cortex-M3 supports ARM Thumb2 mode, which reduces code size by compressing instructions to 16 bits as much as possible. Therefore, the ARM CPU code size is comparable or even smaller than the code of 8051 or other 8-bit instruction systems.

The Precision32 product combines the Cortex-M3 CPU with a variety of analog components, many of which are also used in the company's 8-bit microcontroller products. These components include: two 12-bit ADCs, two 10-bit DACs, an oscillator, a voltage regulator, and a 16-channel touch sensor. The product also includes Flash memory, timers, counters, and serial interfaces, as shown in Figure 1.

Figure 1. Precision32, microcontroller block diagram. The new Silicon Labs MCU combines a Cortex-M3 CPU, Flash memory, and multiple analog interfaces.

The first Precision32 products released are available in two basic types: with USB and without USB. The former supports USB 2.0 full speed (12Mbps) but not high speed. The Flash memory capacity of each type ranges from 32-256KB. The company offers pinned and non-pinned package types, supporting QFN-40 package (minimum size 6mm), QFN-64 package, QFP-64 package, TQFP-80 package and LGA-92 package (maximum I/O pin configuration). It is worth noting that even the smallest 6mm version of the device supports a maximum flash memory capacity of 256KB. In short, the 32-bit series products launched by the company this time are unique. All initial releases are now available for sampling and are expected to be mass-produced in the second quarter of 2012. [page]

Reduce system costs

The high analog integration of the Precision32 MCU reduces system cost by reducing several peripheral components. For example, most MCUs require an external crystal oscillator to run at faster speeds (such as 8MHz). Such a crystal costs about 10 cents. The Precision32 MCU only needs to use its internal oscillator and PLL to make the USB interface work without an external crystal. The internal oscillator uses clock recovery technology to provide 1.5% accuracy over the PVT (process, voltage, temperature) range, which is within the tolerance allowed by the USB specification. The second internal oscillator generates an 80MHz CPU clock (also with 1.59~ PVT), which is generated using the USB clock (no external crystal required) or an inexpensive 32kHz crystal.

Likewise, the use of an on-chip voltage regulator eliminates the need for an off-chip regulator, which costs about 15 cents. The on-chip voltage regulator allows the Silicon Labs chip to operate directly from a 5V power supply, and if necessary, it can even be powered directly from USB. The on-chip voltage regulator can also drive the output power to the external drive, and the voltage is programmable, which allows the MCU to provide power to external ICs without the need for an external voltage regulator. In addition, the output power can also be used as a constant current source to drive the backlight of the LED display. A stand-alone LED backlight controller costs about 30 cents.

The MCU also provides six high-drive output pins, each capable of outputting 300mA or inputting 150mA. Enough to directly drive power MOSFETs, high-power LEDs, buzzers and similar components. Since a high-drive off-chip transistor adds about 4 cents to the system cost, these outputs can save up to 24 cents in total. In addition, the chip can directly connect up to 16 capacitive touch sensors. This type of sensor is increasingly used in consumer electronics and other devices to replace mechanical buttons, which are usually more expensive and may cause failure. If an external touch controller is used, it may cost up to 50 cents more.

Figure 2: Integrated analog components can save costs. This picture shows that in a typical MCU-based design, the highly integrated Precision32 chip can replace several peripheral components.

For USB systems, the chip integrates a complete USB PHY and analog front end that can be connected directly to the USB connector. Most other MCUs require an external USB pull-up resistor and termination circuit, adding small components that cost about 5 cents in total. The total component cost savings from using a Precision32 MCU can be as high as $1.34. Of course, many systems do not require LED backlighting or touch controllers, which reduces the cost savings, but most designs will be able to save at least 30 cents. As shown in Figure 2, the reduction in external circuitry also reduces board area.

Reduce power consumption

Silicon Labs focuses on low power design of Precision32 MCU in working and sleep modes, especially for Cortex-M3 CPU. In working mode, the whole chip consumes only 22mA (80MHz running speed), or 0.28mA/MHz. Although some of Freescale's latest Kinetis MCUs can also run at 0.25mA/MHz (maximum 72MHz running speed), Silicon Labs' low power consumption level still exceeds most other MCU products of the same type. The sleep mode current of Precision32 MCU is even more amazing: when starting RTC, the current is only 0.35uA, which is the lowest power consumption in the industry.

The company is also focusing on reducing system power consumption, as many MCU applications are powered by batteries or through the restricted levels of the USB specification. For developers, integrating the analog devices mentioned above, such as oscillators and USB terminations, can shorten signal path lengths and integrate peripheral components, thereby reducing overall system power consumption.

In addition, the on-chip oscillator allows the CPU to run at any desired frequency; if an 8MHz external crystal is used, the MCU must run at a multiple of 8MHz. For the same reason, Silicon Labs allows the CPU and USB frequencies to be set independently, rather than in fixed ratios. This flexibility enables system designers to fine-tune the CPU frequency to provide the desired performance while maintaining minimum speed and power consumption. [page]

Reduce design time

With its experience in designing 8-bit MCUs, Silicon Labs built a flexible I/O system into the Precision32 design. As shown in Figure 3, the chip includes two crossbars that can connect any I/O function to different pins. In the TQFP-80 package example, the crossbarl connects pins 9-40, which does not include pins 28 and 29, which are power and ground pins. The crossbarl supports 14 different internal functions, including various serial interfaces, timers, and comparators; these functions can be connected to any external pins supported by the crossbar. Some of these pins can also be mapped to ADC inputs or 5V compatible outputs.

This great flexibility provides multiple benefits to system designers. Designers can configure chip pins to simplify board design, such as configuring output pins close to the chip they are connected to. In some cases, this approach can even allow the use of fewer routing layers on the PCB, saving costs. In addition, pin reconfiguration can easily cope with the challenges of final board changes.

This approach provides greater flexibility when choosing which functions to connect to pins and which to leave floating. Flexibility is especially important when using small packages with a small number of I/O pins. Most other MCUs offer some configurations, but may only support one or two functions on each pin, limiting the number of options and leading to potential pin conflicts.

So many configurations can be confusing, so Silicon Labs provides designers with a software tool to simplify configuration. The Application Builder tool provides a graphical interface that allows designers to configure pins using drag-and-drop functions. Once the configuration is complete, the tool can generate boot code that loads the configuration into the MCU. The tool can work with commercial IDEs, including Keil and IAR, as well as the popular Eclipse, which Silicon Labs has adapted to support Precision32 products.

Figure 3. Precision32 I/O crossbar. Silicon Labs MCUs include two internal crossbars that enable any I/O function to be routed to different pins.

Industry-leading low power consumption

With multiple vendors offering ARM-based 32-bit microcontrollers, Silicon Labs needed something special to stand out. As shown in Table 1, competitors include Freescale Semiconductor's new Kinetis, STMicroelectronics' STM32 series, and Texas Instruments' Stellaris. Other vendors, such as Atmel, EnergyMicro, and NXP, also compete in the market, but we'll focus on the market leaders. To facilitate comparisons with other vendors' broad product lines, we'll try to select products with similar CPU speeds and memory capacities, as well as USB ports.

Freescale's Kinetis products are extremely competitive. Its Cortex-M4 CPU is similar to the M3, but includes a DSP and optional FPU. In the K20 version, the CPU can reach 100MHz, although the emerging low-power version is limited to 72MHz. In this low-power mode, the new Kinetis K20 has an operating current comparable to the Precision32 MCU, while running only slightly slower than the Silicon Labs product. However, in sleep mode, the Kinetis consumes 4.0uA - more than 10 times the current consumption of the Precision32 MCU.

Kinetis products have comparable analog performance to Precision32 MCUs in most categories, and even offer higher-resolution ADCs and DACs. However, Precision32 has some advantages over Kinetis, including a smaller package for the low-end market. More concerning is that Kinetis is lagging behind in product development, with low-power versions not yet sampling (sampling is planned for the end of the first quarter of 2012). In addition, Freescale's additional features, such as DSP and FPU, will increase the chip size and price.

Precision32 mixed-signal MCUs compared to major competitors *Also offers a 100MHz operating rate with higher power consumption; *Also offers 128KB flash memory, 20KB SRAM, and a 6mm QFP-36 package without DAC; **Powered only by battery, the current is 4.0uA when powered by Vdd. (Source: various manufacturers)

ST offers a variety of ARM-based MCUs, and the STM32F10x is the best match, with a 72MHz, Cortex-M3 CPU and full-speed USB interface. ST is the only competitor in the Precision32 series with a small 6mm package. However, ST's 6mm package limits the size of Flash and SRAM to only about half of Silicon Labs' products of the same size; the smallest package also prohibits the use of DAC. The full-featured version requires a 10mm package.

At the same clock rate, the STM32FlOx consumes about 50% more power than the Precision32 MCU, and in sleep mode, the difference is more than 5 times. ST offers an "ultra-low power" version of the processor with a sleep current of 0.23mA/MHz and 0.27uA; these characteristics are slightly higher than those of Silicon Labs, but ST's ultra-low power devices limit the CPU to 32MHz. 128KB Flash memory, 16KB SRAM, in order to maintain its ultra-low power characteristics.

For TI's Stellaris family, we chose the 3000 series for comparison. Among these products, TI limits the Cortex-M3 CPU to 50MHz, which is slower than the Precision32 CPU. To reach an 80MHz CPU, it must choose to use the more expensive 5000 series, which is targeted at the automotive market. The Stellaris 3000 uses a larger package than the Precision32 MCU, consumes more power, has limited ADC resolution, and does not have a DAC.

The Precision32 family offers some unique features that differentiate it from all of these competitors. The design flexibility provided by the chip's I/O crossbar is unmatched by other MCUs. The high drive output and LED backlight control capabilities are unique and can provide significant cost savings. While other leading MCUs currently do not offer capacitive touch interfaces, perhaps Kinetis will have this capability when it provides samples.

stand out

Although Silicon Labs does not yet offer a broad portfolio of 32-bit MCUs, the initial release of the Precision32 chip is well suited to a variety of end applications. For industrial uses, such as motor control and monitoring, the chip's 5 V-compatible inputs and high-drive outputs are particularly useful. Battery-powered devices such as blood glucose monitors, GPS positioning systems, sensor controllers, and thermostats can benefit from the MCU's low power consumption; barcode scanners, card readers, and other peripherals require highly integrated USB interfaces; and the 6mm package will help meet the form factor requirements of lOGbps optical transceivers.

The unique and flexible pin configuration of the chip is an important advantage for almost all designs. It can shorten the design time and allow final design changes, thus accelerating the time to market. This approach also allows designers to use MCU products with smaller packages and fewer pins, because these pins can be allocated to support almost all peripheral combinations.

Silicon Labs applies its unique analog design experience to new 32-bit MCU products, making them feature-rich, such as precision oscillators, voltage regulators, high-drive outputs, capacitive touch interfaces, etc. Although some competitors also have similar features, no one manufacturer can cover all features. Even in the functions that competitors can also provide, such as analog-to-digital converters, Silicon Labs products are often better in accuracy and voltage range.

Compared with competing products, the new MCU designed by Silicon Labs has the lowest sleep power consumption: the current is only 0.35 uA, which is less than 1/3 of the closest competing product. Its working power consumption (mA/MHz) is also the lowest among similar MCU products in the industry. The processor provides multiple power modes and clock options to help designers minimize power consumption.

With its latest 32-bit microcontroller product line, Silicon Labs is expanding from 8-bit products to the highly competitive ARM-compatible MCU field. To stand out, the new Precision32 products offer low power consumption, flexible I/O and analog integration. These features make them ideal for a wide range of industrial, medical, communications and consumer electronics applications. Developers who often use MCU products from large suppliers should pay attention to this new member of the 32-bit MCU market.