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Going from 8bit to 32bit MCUs takes tools

Posted: 03 May 2004 ?? ?Print Version ?Bookmark and Share

Keywords:microcontroller? mcu? asic? cpu? microprocessor?

Since the main concern in migrating from an 8bit MCU to a 32bit device is how to reuse existing software, designers need 32bit choices that offer peripheral compatibility with 8bit MCUs. In addition to simplifying migration from one product generation to another, this compatibility enables the creation of multiple versions of a system that offer different feature sets at different price points, but share the same code base.

This is one of several software issues that are defining the shape of today's and tomorrow's MCU choices. Rather than force designers to adapt to available hardware resources, MCUs must be made to accommodate their software requirements. In addition to compatible 8bit and 32bit versions, MCUs can offer a wide variety of software IP--minimizing the need for ASIC hardware--and generous amounts of on-board flash memory for code storage.

Several examples illustrate the usefulness of peripheral compatibility across 8bit and 32bit MCUs. Utility power meters provide one good example because they have shifted from the traditional mechanical meter to electronic versions based on 8bit devices, followed by versions based on 32bit MCUs. The meters provided more detailed information about power usage, such as the amount of power consumed at peak times. An 8bit MCU supports a simple infrared output function that allows meter readers to gather data electronically.

Upgrading to a 32bit MCU lets the meter exchange detailed information with the utility provider via a phone line or the power line itself. The meter can include a card reader that allows customers to buy power using a debit card, a solution well-suited for developing countries whose power companies may not have a strong billing infrastructure. The 32bit MCU can run software that makes modem hardware unnecessary and that measures power consumption with integrated circuitry, implementing a sophisticated but inexpensive one-chip system.

Power meters are used worldwide under a variety of conditions, and the companies that make them need a design that can adapt as needed. The basic functions of the 8bit and 32bit meters are the same: measure the power consumed in specific time periods and communicate that information to the utility. With 8bit and 32bit MCUs that have compatible peripherals, the companies can use the same basic code for making measurements and calculating tariffs across the product line.

For this approach to work, the MCUs' functional registers, timers, UARTs and other resources need the same names and configurations. A header file can handle any minor differences in memory map addresses. Upgrading to a 32bit MCU is necessary to implement networking and security features and makes it practical to execute functions in software that would otherwise require external hardware.

Many motor control applications face similar reasons for upgrading to 32bit. In the past, motor control algorithms were usually simple and could be handled by 8bit devices. Now many applications require more sophisticated and high-accuracy motor control algorithms that, due to the need for 32bit math, can only be implemented by a 32bit MCU or a dedicated motor control DSP. These algorithms can reduce power consumption and cost with no additional sensors.

Typical motor control applications that use these 32bit MCUs include standard household electronics like washers, dryers, refrigerators, freezers, dishwashers and airconditioners, as well as hybrid cars and factory machines.

Another upgrade exists in applications that contain multiple 8bit MCUs or a combination of an 8bit MCU and a dedicated hardware engine with a DSP architecture for tasks like voice or image processing, or both. Integrating the functionality of the various processors into a single 32bit device can reduce costs, save space and simplify system control.

In all of the upgrades, the 32bit MCU lets engineers leverage software-based functions to reduce or eliminate dependence on external hardware. Software IP lets engineers take advantage of assembly-code-optimized functions that make maximum use of the MCU. These functions span a broad range and may include image processing, audio processing and user interfaces. These types of functions have typically been regarded as hardware blocks in an SoC. By exploiting spare cycles from a 32bit MCU, engineers can reduce the complexity of an ASIC or eliminate the ASIC altogether.

In other cases the designs can migrate in the other direction, and it becomes necessary to eliminate separate MCU and ASICs and roll all of the hardware into a single SoC. Thus, it may be useful to design embedded applications around a 32bit MCU whose functionality can be transferred as IP directly to an SoC. Again, this strategy allows the reuse of existing software.

Future upgrades

The desire for a single-chip solution has led to MCUs that include as much as a megabyte of flash memory. Flash capacities will likely continue to expand to accommodate increasingly complex software functions, with the benefit of flexibility to upgrade or change the functions remotely in the field and make masked-ROM versions of MCUs unnecessary.

In many MCUs, the flash occupies more silicon area than the 32bit processor itself, even though innovations such as split-gate flash are reducing the size of memory cells. The use of RISC processors is also helping to reduce the relative size of the processor. Compared with CISC architectures, a RISC processor executes instructions much faster for a given clock speed and consumes much less power, so RISC-based MCUs are growing in popularity.

Another emerging trend is to use multiple 32bit cores in one MCU rather than trying to push a single CPU's performance capabilities. This multiple-CPU approach may seem like the mirror image of consolidating multiple 8bit MCUs into one 32bit device, but the idea is to pack the 32bit CPUs into a single device controlled by a sophisticated multiprocessor RTOS. This strategy reduces power consumption by optimizing the software task allocation among multiple processors. Using compatible 32bit cores also supports the migration of existing application software-the key to cost-effective MCU design.

Figure 4: Common peripherals eliminate need to recompile software when applications migrate from 8bit to 32bit MCUs.

- Steve Ikei

NEC Electronics America Inc.

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