[LINK] Microcontroller Units (MCUs)

[stephen at melbpc.org.au]

Microcontroller Units will probably be in *lots* of products soon ..

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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)
http://www.embedded.com/underthehood/218400747


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 
networking. 

In contrast to microprocessors, which are used in personal computers and 
other high-performance applications, microcontrollers are designed for 
small applications. 

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 
2010. 

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, 
2009).

www.springer.com/engineering/circuits+&+systems/book/978-0-387-88496-7

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 
reliability. 

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 
circuitry. 

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 
technology. 

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 
www.semiconductor.com/resources/reports_database/view_report.asp?pid=5392.


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 
flash memories. 

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 
applications.

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.

--
Cheers,
Stephen