View detail for Designing for Efficient Production with In-System Re

View detail for Designing for Efficient Production with In-System Re
Designing for Efficient Production
with In-System Re-programmable Flash µCs
By: OJ Svendlsi
For products where time-to-market and efficient production is important, selecting the right microcontroller
architecture plays a big role. To get the shortest possible development time, the following requirements
should be filled:
• Good and easy to use development tools
• Enough resources on-chip to meet requirements
• Efficient for high level languages
• Flash program memory for fast and
reliable programming
The megaAVR family from Atmel has all
these features and more, making them a
perfect choice for advanced products
requiring short time-to-market.
This white paper discusses the advantages of
this family related to getting the shortest time-to-market and the most efficient production once the product
is ready.
During development of a new product, the microcontroller is normally the most important module, and
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always the component where the majority of the engineering hours are spent. Thus, making sure the microcontroller has what it takes to ease the development
should be given some priority.
Selecting the Right Development Tools
When selecting a microcontroller, you are also selecting
the development tools you are blessed with. These
development tools range from the simplest evaluation
kits to high-end In-circuit emulators and production
programmers. Depending of the complexity of the end
product, different debugging solutions will be optimal. A
1-2K microcontroller with a minimum of on-chip
resources can often be debugged using a free architectural simulator such as AVR Studio. For complex
applications on big parts with many peripherals, using
an advanced In-circuit emulator with advanced features
such as Trace capabilities, advanced breakpoints and
Code coverage functionality might reduce the design
time significantly.
Parts with on-chip debug capability offers a very low
cost alternative for the debugging. The JTAGICE mk-II
from Atmel is available for only $299, but still provides
debugging capabilities like breakpoints, data watch,
single stepping through code and full overview of
processor internal resources. An additional important
advantage of on-chip debug is that the debugging is
done on actual production silicon. Because of this,
many of the problems designers run into when switching from an emulator platform to the real thing is eliminated.
Starter kits are useful when evaluating a microcontroller
that might fit in the application. Many starter kits can
also be used as target hardware during initial code
development before the target hardware is ready.
Programming in a High Level Language
By programming the microcontroller in a high-level language (HLL), like for instance ‘C’, it is possible to reduce
the development time significantly compared to writing
in assembly. Generally one can say that an experienced
designer can write the same amount of lines of code
per day in C and assembly. However, the code-lines
written in C will do much more than the same number
of lines written in assembly.
Typically, a program written in a high level language will
also be much more structured than a similar program
written in assembly. Because of this, it is generally easier to debug a program written in a high-level language.
Most 8-bit microcontroller architectures come with a Ccompiler. However, there is a big difference in how efficient the architectures are for high-level languages, and
how the C-code should be structured to be efficient
with one particular microcontroller architecture.
Generally one can say that accumulator-based architectures like the 8051 architecture from Intel works
best with global variables, while register-based architectures like the AVR from Atmel works best with local
The benefits of using local variables are that the code
becomes more structured and the portability and main-
tenance of the code is simplified when compared to
code written with very many global variables. It is also
much easier to reuse code when it is written with
extensive use of local variables. When all the parameters going in and out of a subroutine is defined in the
function call, it is very easy to port that subroutine into
a new project.
The biggest drawbacks of writing code in a HLL are that
the code normally becomes bigger and slower than a
similar program written in assembly. However, as the
number of code-lines increases, the gap in size
between the HLL code and the assembly code starts to
shrink. For a typical AVR user, the crossover point where
the HLL and assembly code is the same size is around
4K. However, the HLL code will almost never be faster
than the assembly code. If execution speed of a certain
part of the program is critical, the solution is often to
write the code for the critical parts in assembly, and
write the skeleton and less critical subroutines in HLL.
The level of integration in a microcontroller can also
affect the development time, in addition to power consumption and board space. Integrated functions such
as Brown-out protection, Watchdog timer and Power-on
reset circuitry gives the most reliable operation of the
microcontroller, while functions like integrated EEPROM, internal RC oscillator and strong push pull port
drivers eliminates external components and reduces
cost and complexity of the design. With fewer external
devices on the PCB, the possibility of running into noise
related problems is also reduced. Especially the internal
RC simplifies the design from a noise tolerance point of
In-System Programmable
Flash Program memory
Having In-System reprogrammable flash memory simplifies the development of a microcontroller application
significantly compared to ROM/OTP solutions. The
devices can be soldered into the application, and then
be reprogrammed when a problem with the code is
found. Using the AVR microcontrollers, the reprogramming can be done either through the SPI interface, or if
using a JTAG on-chip debug or programming tool,
through the integrated JTAG interface. If using a boot
loader with the self-programming memory, other communication channels can also be used to reprogram the
devices. The non-volatile memories of the AVR devices
can be used to store history when debugging a program. The contents of the memories can then be read
out after the program is finished.
The advantages of using modern flash microcontrollers
do not stop when the development is completed. Also
when in full volume production, the benefits are many
as outlined here.
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When using Flash microcontrollers, it is much easier to
handle the procurement of the products. It is often easier to handle surprise orders since the microcontroller
used is a standard line item. The chance that other
companies are using the same microcontroller is usually big. This makes it possible to stock the microcontroller both at the distribution level and at the device
manufacturer. If the same microcontroller is used for a
number of applications, inventory can be moved from
one product over to another to maximize the revenue.
Handling multiple versions
With a Flash microcontroller the same device can be
used to handle multiple software versions. In many
products, the only difference between a high-end and
a low-end version of a product is the firmware. In these
cases, a flash microcontroller is ideal. The PCB can be
assembled and completely tested in advance. Once the
order comes in for a specific product, the code for this
product can be programmed in, and the product
shipped to the customer.
Many larger manufacturers are selling the same product to different OEM customers. In many cases, the differences between the products shipped to these OEMs
lies in the software. Again, an In-System reprogrammable flash microcontrollers can be programmed with
the correct code at the production line. Alternatively, the
OEM account can program the parts of the code that is
individually changeable by using the boot-capability of
the megaAVR devices.
Many analog sensors have a relatively large offset error
that needs to be calibrated out to achieve good measurements. Devices with integrated non-volatile data
memory and high performance Analog to Digital converters are ideal for this. With the AVR microcontrollers,
special calibration software can be used to run the calibration and store the calibration values in the internal
EEPROM. After this, the main code can be programmed
in, and use the values stored in the EEPROM.
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