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Maximizing Design Potential with a Mixed-Signal MCU
By Ross Bannatyne, Director of Marketing for Silicon Laboratories
Historically, the minimum requirement of a mixed-signal microcontroller (MCU) was
support for analog-to-digital converters (ADCs) and/or digital-to-analog converters
(DACs) on a single integrated circuit (IC). However, designers should expect more from
a device called “mixed-signal.” A truly mixed-signal MCU is a system-on-a-chip that is
able to handle both analog and digital inputs and outputs without sacrificing performance
when compared to a discretely implemented solution. While few offerings deliver fullfeatured mixed-signal MCUs, those that do offer a host of advantages over traditional
solutions.
Reducing the Bill-of-Materials
Mixed-signal MCUs simplify partitioning problems because they are capable of handling
different types of signals on the same chip. The integration inherent in a mixed-signal
MCU simplifies the design while reducing the board real estate required.
In the design process, once the system specification has been defined and the
implementation partitioning options have been determined, a decision must be made on
how to optimize the system based on design constraints such as maximizing performance,
power consumption, physical space constraints, overall system cost, etc. There is an
infinite number of different products for the designer to choose from – sensors, actuators,
discrete analog components such as transistors/diodes/capacitors, analog chips such as
power supplies and amplifiers, digital chips such as programmable logic chips, fixed
function controllers, memory chips, microprocessors and MCUs.
A typical constraint is enabling multiple types of signals required in the system while
minimizing board space and system complexity. For example, a sensor may provide a
very small analog signal in the mV range. Programmable processors use digital signals
that are usually in the 1.8 – 5.5 V range. System-controlled actuators will likely need a
power stage with higher voltages and currents that are analog in nature. Because there is a
wide variety in the types of signals that are used in most electronic systems, the
partitioning problem is complex.
Using a typical embedded system as an example (see Figure 1), sensors provide analog
signal inputs that need to be conditioned by a combination of ADCs, comparators and
programmable gain amplifiers (PGAs) before they can be processed by an MCU. The
MCU outputs are typically digital and need to be reconditioned back to analog by DACs
or physical layer chips (PHYs) before they can be used to drive analog components. The
remaining functions in an embedded system are housekeeping functions that are usually
analog in nature – temperature sensor, voltage references, oscillator, charge pumps
(required for supplying the higher voltage needed for programming flash EEPROM) and
voltage regulators.
Many of the functions described, including all of the typical analog functions usually
implemented in discrete analog chips, can be integrated onto a single MCU; this is the
essence of a mixed-signal MCU.
Figure 1 – Typical Embedded System Block Diagram
Input
Signal
Conditioning
Inputs
Sensors
Serial
Parallel
MCU
ADC
Comparator
PGA
Output
Signal
Conditioning
DAC
PHYs
Outputs
Actuators
Indicators
Networks
Other
Temp. sensor
V reference
Oscillator
Charge pumps
V regulator
Figure 2 – Component Count Reduction Using a Mixed-Signal MCU
uCONTROLLER
C
PGA
C
R
R
R
R
R
C
RE F
C
C
ADC 12
R
DA C 12
CMP
XTA L
19.4 x 16.5 mm = 320 mm 2
Figure 2 compares a typical MCU with a number of discrete analog components –
programmable gain amp (PGA), ADCs, DACs, comparators (CMP), crystal oscillator
(XTAL), reference voltage (REF), resistors and capacitors - and a mixed-signal MCU
that provides all of these functions in a single system-on-a-chip. The size of the circuit is
reduced, which is very important in many applications. In fact, it is often the number one
consideration in applications that are required to be ‘wearable’ such as portable MP3
players.
Reliability is another benefit of a more integrated solution. The main source of reliability
problems in embedded systems is interconnections – solder joints and mechanical
connections. The more interconnections that exist, the more failures can be expected.
Mixed-signal MCUs reduce components and interconnections and therefore markedly
increase system reliability.
Lower Power Consumption
Reduced power consumption is another advantage that a single chip mixed-signal MCU
provides. The advantage of the lowest power MCU on the market is lost if it needs to be
used in a system with a power-hogging standalone ADC or other power-hungry analog
chips. Because mixed-signal MCUs have the power supply overhead requirements of
only one chip, they have a lower power budget than a system that uses several
components.
Noise Reduction
The noise performance of the system for both noise emissions and susceptibility will be
better for a mixed-signal MCU-based system than a system that has separate analog and
digital chips. There are no high-speed digital signals connected to standalone ADCs and
DACs so emissions are lower. Overall system susceptibility to noise is also improved
because there is a reduction in the number of system components that can be affected.
Lower System Cost
A highly integrated system lowers overall system cost. The reduction in components and
the economies of single package, die and test costs are an important advantage of the
integrated mixed-signal MCU solution. There are other cost savings as well – the
software and overall design are less complex. This translates to reduced development
time, lower development cost and faster time-to-market.
The Challenge: Design Complexity
Digital and analog chips have traditionally used different types of manufacturing process
technology. The “workhorse” process for digital integrated circuits such as MCUs and
memory chips is complimentary metal oxide semiconductor (CMOS). CMOS is
commonly used for digital-only chips while analog ICs usually use different process
technologies such as BiCMOS and Bipolar. Because CMOS is the lowest-cost process in
the industry, the objective of mixed-signal ICs is to couple both analog and digital
circuits on a monolithic chip using industry-standard CMOS process technology. This IC
design challenge requires specialized knowledge and skill. There are significantly fewer
engineers in the industry proficient in analog design than digital design. There are also no
standardized design tools for mixed-signal ICs.
In addition, the performance of analog functions in many MCUs is degraded in the
presence of the high-speed digital bus. To achieve the goal of replacing multiple chips in
the system, a truly mixed-signal MCU should have an on-chip 12-bit ADC, on-chip DAC
and oscillator at minimum.
It is a significant challenge to develop and manufacture mixed-signal MCUs, and
technical problems that are encountered are significant. For example, it takes a skillful
designer to ensure that digital noise does not degrade the performance of analog
peripherals. It is difficult to develop and produce mixed-signal MCUs, but the
advantages they offer are significant. By integrating high-precision, high-speed analog
peripherals with digital control circuits on the same chip, a mixed-signal MCU allows
designers to bypass system level analog-digital integration issues.
Figure 3 – Silicon Laboratories’ C8051F020
Looking closely at a mixed-signal MCU as defined here, the analog circuits on the chip
are visibly less “regular” than the digital circuits. The RAM and Flash arrays are regular
structures (except for the charge pump circuits on the right side of the 64K Flash block).
The two 12-bit DAC blocks, the 8-bit ADC block and the 12-bit ADC block are clearly
more handcrafted architectures than the sea-of-gates structure of the digital functions
(CPU, Ports, Timers, UARTs, etc). Other analog functions included on this chip are an
on-chip oscillator that replaces an external crystal or resonator, on-chip temperature
sensor and a pair of comparators with programmable hysteresis that can be used for
wake-ups or interrupts.
The availability of a mixed-signal MCU that meets the specifications on the data sheet
gives design engineers a host of new options in the architecture of their system design.
Ideal for applications such as industrial and process feedback control systems, cellular
base stations, portable and stationary test equipment, smart transmitters and weigh scales,
mixed-signal MCUs offer designers an affordable alternative for smaller, faster MCUs
with better analog.
More details on mixed-signal MCUs are available at www.silabs.com/MCU