Electronic Design

16-Bit MCU And DSP Confluence

Along with 16-bit microcontrollers (MCUs), there are some very attractive DSPs that perform like MCUs, yet have real DSP capabilities. The typical DSP works on 16-bit data and is managed by a host processor—a microprocessor (MPU) or MCU that sets up the DSP; performs scheduling, data processing, and control functions; and manages the entire system. A few vendors now offer these blended processors by combining the attributes of both.

Such blended MCU-DSPs have the Harvard-style bus structure that lets a DSP access two or three memory spaces at once, as well as the fast single-cycle MAC operations. Bit manipulation, logical operations, conditional branching, software stacks, and intricate memory indexing add to the normal DSP functions of single-cycle multiply-accumulates (MACs), modulo addressing and circular buffers, no-overhead looping, overflow handling, and flexible interrupt structure. With numerous peripheral circuits on board to help the blended processor, these parts can be very powerful.

Blended MCU-DSPs typically perform comparably to 16-bit MCUs, challenging traditional DSPs at the same clock rate. Applications such as higher-end motor control, mid-range telephony, datacom, consumer audio, uninterruptible power supplies, industrial, and certain automotive applications benefit from fewer chips. The second chip consumes space and power, adds cost, and tacks on that many more pins to solder down and test. Eliminate it and that's one less chip to be ordered and inventoried, and maybe one less vendor to qualify on the bill of materials.

For many years, Motorola has sold a blended MCU-DSP. The 56800 DSP family includes 16-bit 56000 DSPs with registers, addressing, and controller instructions modeled after the 68HC11 MCU and a bit of the 68000 MPU. A few years ago, Motorola revamped the design with a faster, enhanced version. It now offers a dozen or so parts, many flash-based, running up to 120 MHz.

Texas Instruments brought out a DSP with MCU instructions about five years ago. But the 'C2700 architecture faded away once it failed to gain a foothold in the disk-drive industry for which it was targeted.

Two years ago, Microchip announced that it would enter the 16-bit MCU market with a new blended MCU-DSP architecture. The "dsPIC" is still being worked on, but 20 configurations are shown. Digital signal processing requires more sophisticated programming, so hopefully Microchip can remove some of the fear that engineers may have in stepping into DSP, as they did with MCUs.

Working out the interdependencies between control code and signal processing code may be more difficult with far more resources being shared in a blended chip. But in separate chips, this can be more cumbersome simply because the same development tool might not support both the MCU and DSP, which could come from different vendors. At least with the blended chip, the tools and the compiler will obviously support both the control code and the DSP code as it is one and the same.

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