Microcontroller Solutions TechZone Magazine, April 2011 - Digikey

Introducing a Second MCU
to Embedded Designs
by Nicholas Cravotta
There are numerous reasons to add a second
MCU to your design and numerous errors to
avoid once you do. This article considers the
arguments and options while pointing out the
pitfalls along the way.
With the wide variety of processors available today, there are a
great many reasons why you may find it to be a more cost-effective
choice to add new features to an application by introducing a
second microcontroller (MCU) or microprocessor (MPU) to your
design rather than trying to perform every task on a single, higher
performance processor:
• Cost: Migrating to a higher performance system MPU can have a
higher cost differential than adding an 8- or 16-bit MCU to take
on the added load.
• Power: Rather than expend the power to have the main
application processor continuously turn itself on to perform a
relatively simple operation such as monitoring a sensor bank
or checking if a key has been pressed, an 8-bit processor
can be introduced to perform these tasks. In essence, the
8-bit processor serves as a “power gate” by letting the main
processor sleep and waking it only when an event occurs that
requires its attention. This is especially effective with resistive
and surface capacitive touchscreens.
• Determinism: User interface (UI) functions involving compute
intensive tasks such as animation or audio can stress the
real-time deterministic behavior of a system. Rather than risk the
reliability of critical tasks such as motor control, a second MCU
can assume all “real-time” UI responsibilities.
• Diversity: Many systems can benefit from a modular design. For
example, a display module can be implemented with a different
processor to provide additional graphics, or streaming video
capabilities. By taking a modular approach, the main application
code does not need to be changed to add new features to the
system or to support different models.
• Simplicity: A product line may offer a wide variety of options.
For example, a product may have four screens, three keyboards,
and three connectivity options. When a single processor is
used, the design team has to maintain, test, and verify a
separate code version for each possible variant (4x3x3=36
distinct variants). When a secondary processor is used to
implement a new function, maintenance is required only
for each option (4+3+3=10). Note that with the first option,
the entire system needs to be tested and verified with every
variant, a complex and time-consuming process. When using
the second option, the main application code only needs to be
verified once. Each of the options can be verified separately
and with significantly less complexity.
• Conservation of I/O: An MCU can appear to be several devices
over an interface by responding to several addresses. For
example, the main application processor could issue a command
to turn on a fan which is intercepted by an MCU serving as a
virtual hub. This is an effective way to conserve I/O on the main
application processor since only one communication peripheral
is required to read and control multiple devices.
• Certification: Changing the user interface or introducing a new
interface like Wi-Fi to the main system requires the entire design
to be recertified for some markets. Implementing these functions
as an add-on module with a separate processor simplifies
the recertification process, significantly accelerating both
development and time-to-market.
UART, I 2 C, and SPI
There are many options available for interconnecting processors.
The most basic interfaces used for this purpose are Universal
Asynchronous Receiver-Transmitter (UART), Inter-Integrated Circuit
(I 2 C), and Serial Peripheral Interface (SPI). Today, many processors
offer hardware and software support for one or more of these
interfaces. For example, STMicroelectronicss’ STM32 Cortex-M3 32-
bit processors and 8-bit STM8L MCUs support all three. Alternatively,
Cypress’ PSoC family of programmable system-on-chip processors
have analog and digital blocks which can be configured to provide
multiple instances of each of these interfaces, allowing the interface
mix to be altered as required for different target markets. The
question becomes how many interfaces you’re going to need.
A UART operates up to 250 kbps, compared to 100 to 400 kbps for
I 2 C. SPI, which supplies its own clock, can operate at speeds over
1 Mbps. A UART link can typically be implemented in software using
an interrupt-based driver, so long as the driver doesn’t place too much
load or latency on the processor based on the application’s needs. An
I 2 C master can often be implemented in software, but a slave needs to
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