[LINK] Microcontroller Units (MCUs)
stephen at melbpc.org.au
stephen at melbpc.org.au
Mon Jul 13 22:08:18 AEST 2009
Microcontroller Units will probably be in *lots* of products soon ..
Embedded flash technologies, a foundation of flash MCU growth
By Young Choi and Xu Chang EE Times (07/13/09, 12:01:00 AM EDT)
A Microcontroller Unit (MCU) is a small computer on a single integrated
circuit that typically contains a central processing unit (CPU) core,
static random access memory (SRAM) modules, embedded flash memory
modules, a system integration module and peripheral modules including a
timer, an analog-to-digital converter (ADC), serial communication and
In contrast to microprocessors, which are used in personal computers and
other high-performance applications, microcontrollers are designed for
You can find them used in automotives, consumer electronics, computers
and peripherals, wired and wireless communications, smart cards and other
simple applications using repeatable programs.
According to market research firm iSuppli, in 2008 the total revenue for
MCUs was close to $16 billion (USD), with the top three manufacturers
(Renesas Technology, Freescale Semiconductor and NEC Electronics) each
having more than 10 percent of the MCU market share.
Microcontrollers with embedded flash memories (flash-MCUs) are widely
used in real-time control application markets.
The programmable code storage provided by on-chip flash memories
contributes to the reduction of production costs and expansion of real-
time adaptive control applications, realizing a value innovation with
remarkable cost/value advantage.
In 2007, flash-MCUs accounted for more than 30 percent of the MCU market
and are predicted to make up more than 50 percent of the MCU market by
Almost all the MCU market segments now use embedded flash solutions.
Automotive is the single largest user of flash-MCU as it accounts for
more than 50 percent of flash-MCU applications in 8-, 16- and 32-bit
products (Kevin Zhang, Embedded Memories for Nano-Scale VLSIs, Springer,
Embedded flash memory technologies deviate significantly from dominant
discrete flash memory technologies due to their specific requirements
such as host-logic CMOS process compatibility, performance, cost and
Minimizing costs is a major technology requirement for embedded flash
memory. This is achieved by fewer process steps and voltage reduction for
less area penalty to the base CMOS logic, rather than mere cell-size
scalability. The extra masking steps should be as small as possible and
the area penalty reduction in small capacity of memory frequently
encountered in embedded applications is focused on smaller periphery
These requirements for embedded applications often do not allow the
direct import of technologies from discrete high-density flash memories,
like NOR and NAND. Embedded uses often require faster access time to
match the on-chip processing speeds, which further deviates from the
discrete flash memory in technology and design.
Currently, the most widely used embedded flash memory are ROM
technologies that use either a floating-gate or a charge-trapping
In recent years, emerging nonvolatile memories such as ferroelectric
random access memory (FRAM), magnetic random access memory (MRAM) and
phase change random access memory (PCRAM) have been widely explored for
embedded applications. Some of these emerging memories offer much
improved performance over ROM technologies in write speed and rewrite
endurance; however, they also have higher costs, due to the new materials
involved in these technologies. The base technology node available for
embedded flash memories lags behind the most advanced CMOS logic
processes because the embedded flash memories have seen time-consuming
optimization of the technologies.
Semiconductor Insights has recently finished analyzing embedded flash
technologies implemented in flash-MCUs from seven leading MCU
manufacturers: Renesas, Infineon, Fujitsu, ST Micro, Toshiba, Microchip
and Atmel. To find out more about this analysis, see
This analysis (a chart, on the quoted website) compares embedded flash
memory technologies based on the topographical and cross-sectional
analysis of seven medium-density flash-MCUs selected from the top ten MCU
vendors' 32-bit products. The mature process technology currently used to
fabricate flash-MCUs is 150/180-nm CMOS process using aluminum
metallization. Adoption of 130-nm CMOS process using copper metallization
has begun in some high-end products. The selected flash-MCUs operate at
the CPU clocks, which vary from 40 MHz to 160 MHz with embedded flash
memory capacities ranging from from 128 Kbytes to 1 Mbytes.
The analysis revealed that each flash-MCU has different approaches when
it comes to selection of embedded flash technology.
The one-transistor stacked gate cell is extensively used in embedded
applications as well as in discrete memory products. It is a mature
technology that provides a scalable path to high-throughput embedded
The split-gate (1.5T) cell technology is used to overcome the well-
known "overerase" problem in one-transistor flash cells. 2T floating-gate
type cell uses two different techniques: either a selecting transistor
between the floating-gate memory transistor and the source connection, or
a selecting transistor to provide an access to the drain diffusion from
the bitline contact side for F-N tunneling operation. SONOS and MONOS-
type flash memory technology using charge trapping mechanism has been
explored by several manufacturers for embedded and stand-alone
Estimated process overhead for each embedded flash process technology
shows the overhead in terms of additional masking steps varies from 4 to
7. The extra masking steps devoted to the flash memory module should be
kept as small as possible, as the additional process is levied on the
rest of the chip that does not benefit from it.
The amount of embedded flash memory in MCUs is expected to grow,
especially for high-end MCUs.
The size of embedded flash memory is forecast to increase up to 8 MB for
code storage and 128 KB for data storage. High performance and reliable
technology for embedded flash, with low cost overhead is essential to
meet growing demands.
Young Choi is senior manager, Engineering
Xu Chang is senior process analyst at Semiconductor Insights, a division
of TechInsights Technology & IP Services.
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