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Intel MCS® 151 and MCS® 251 Microcontrollers
Backgrounder
April 1996
Since its introduction in 1980, Intel's popular MCS® 51 microcontroller architecture is
still going strong. Today it enjoys the support of hundreds of licensees and acceptance by
millions of customers. According to Dataquest (June 1995), in 1994 suppliers shipped more
than 183 million 8051 devices.
In 1994, Intel announced a major upgrade to its MCS 51 architecture with the introduction of
the MCS 251 microcontroller. The MCS 251 microcontroller incorporated a new core design that
featured pipelined rather than sequential instruction execution, up to 24-bit rather than 16-bit
addressing, a register-based CPU versus an accumulator-based processor, and a 16- rather than
an 8-bit internal code bus. The MCS 251 architecture brought high performance (up to 15 times
that of the MCS 51 microcontroller), increased memory mix and addressing, low power, low noise,
efficient high-level language support, an enhanced instruction set, and a high level of integration
to the 8-bit embedded control market.
Today, Intel is introducing the MCS 151 microcontroller, a product designed to bridge the
price/performance gap between the MCS 51 and the MCS 251 microcontrollers. The MCS 151
microcontroller (8XC151SA/SB) is designed to satisfy customer demand for easy MCS 51
microcontroller application "drop in" upgrades at a lower cost, and for quick
time-to-market with little to no development costs.
Most significantly, both MCS 151 and MCS 251 microcontrollers maintain binary code compatibility
with the original MCS 51 microcontroller. With this new addition, Intel provides embedded designers
with a complete 8-bit product portfolio from low- to high-end, allowing customers to grow their
embedded product lines along a gradual and seamless migration path.
The MCS 151 microcontroller is Intel's latest addition to the classic MCS 51 embedded controller
family. It is instruction-set compatible with the MCS 51 microcontroller and pin-compatible with
both the MCS 51 and the MCS 251 microcontrollers, providing backward and forward compatibility
within the MCS line. The MCS 151 microcontroller has 256 bytes of on-chip RAM and is available
with 8/16 Kbytes of on-chip ROM/OTPROM or ROMless options. Key new features include a pipelined
CPU, 16-bit internal code bus, page mode external code fetch configuration and external wait
state capability. Also included is a Programmable Counter Array for applications that require
real-time compare/capture, high-speed I/O and pulse width modulation capabilities, an enhanced
serial port for multi-processor communication, a hardware watchdog timer and power management
modes.
The MCS 151 microcontroller's fast pipelining provides up to five times the performance of MCS 51
microcontrollers at the same clock speed. Because it is instruction set- and pin-compatible with
the MCS 51 microcontroller, the MCS 151 microcontroller can be used as an easy "drop in"
replacement for MCS 51 controllers, providing a quick performance upgrade with zero development.
And because it is pin-compatible with the MCS 251 controller, the MCS 151 also provides embedded
designers with a direct upgrade path to even more performance and advanced features in the future.
Key customer benefits include:
- Faster time to market
- Low development costs
- Software and hardware investment protection
- Reduced RFI due to high performance at low clock speed
- Interface flexibility for slow peripherals and memory through new wait-state capability
- Future migration to MCS 251 microcontroller solution
Target applications for the MCS 151 microcontroller include:
- PC-related devices such as scanners, POS terminals, CD ROM drives, modems, etc.
- Communications applications, such as analog/digital line cards, phone terminals.
- General MCS 51 microcontroller application upgrades requiring more performance in a short time.
The MCS 251 microcontroller remains at the high end of the MCS 51 microcontroller family,
with 15 times the performance of the original MCS 51 microcontroller product, a suite of
advanced features and an enhanced instruction set.
The MCS 251 microcontroller is equipped with a register-based CPU architecture with 40-byte
register file accessible as bytes, words and double words allowing for programming flexibility
as well as an increase in overall performance. Its enhanced MCS 51 microcontroller instruction
set includes 16- and 32-bit arithmetic and logic instructions, compare and conditional jump
instructions and an expanded set of move instructions that simplify code programming, reduce
code size and increase performance. The MCS 251 microcontroller has 18 bit linear code/data
addressing with 256KB memory space and 64KB extended stack space. The MCS 251 microcontroller
has extensive memory options: 512-byte or 1-Kbyte of on-chip data RAM options or 8 Kbytes or
16 Kbytes of on-chip ROM/OTPROM or ROMless options.
Key customer benefits of the MCS 251 microcontroller include:
- High-end performance for embedded applications
- Increased programming flexibility and efficient C language support
- Supports bigger code and data memory requirements
- Reduced code size
- Reduced power consumption and RFI at low clock speeds
- Simple upgrade from MCS 51 and MCS 151 microcontroller applications
Target applications for the MCS 251 microcontroller include the same applications served
by the MCS 151 microcontroller, but for those particular designs requiring more advanced
functionality and performance:
- PC-related devices such as copiers, scanners, printers, POS terminals, CD ROM drives, modems, etc.
- Communications applications, such as analog/digital line cards, phone terminals.
MCS 51 | MCS 151 | MCS 251 |
Sequential instruction execution unit |
Pipelined instruction execution unit |
Pipelined instruction execution unit |
Minimal 12 clocks per instruction |
Minimal 2 clocks per instruction |
Minimal 2 clocks per instruction |
Accumulator-based CPU |
Accumulator-based CPU |
Register-based CPU with 40 bytes general-purpose register file |
16-bit addressing |
16-bit addressing |
18-bit addressing |
64 KB address space for code |
64 KB address space for code |
Maximum 16 MB linear address space for both code and data |
64 KB address space for data |
64 KB address space for data |
Maximum 16 MB linear address space for both code and data |
256 bytes maximum stack space |
256 bytes maximum stack space |
64 KB maximum stack space |
MCS 51 instruction set |
MCS 51 instruction set |
Enhanced MCS 51 instruction set |
8-bit instructions only |
8-bit instructions only |
8-bit instructions plus new 16 and 32-bit data transfer,
logical and arithmetic instructions. With extended
addressing modes that support indirect, relative displacement
and bit addressing. |
8-bit internal code bus |
16-bit internal code bus |
16-bit internal code bus |
Does not support page mode external code fetch |
Supports 8-bit, 2-clock external code fetch in page mode |
Supports 8-bit, 2-clock external code fetch in page mode |
No wait-state capability |
External wait-state capability |
External wait-state capability |
Relative performance = 1X |
Relative performance up to 5X |
Relative performance up to 15X |
With the addition of the MCS 151 microcontroller, Intel now has a broader spread of products
to meet the wide range of price, performance and time-to-market requirements in the embedded
market. The MCS 151 microcontroller closes the price/performance gap between the MCS 51 and
the MCS 251 microcontrollers by presenting a new, mid-range solution. Customers can drop the
MCS 151 microcontroller into an existing MCS 51 microcontroller design and get an immediate
5-times performance boost, without any development costs or code rewrites. Their software and
hardware investments are protected, and instruction set and pin compatibility with the higher-end
MCS 251 microcontroller provide a seamless migration path to future performance and functionality
upgrades.
The MCS 251 microcontroller will continue to be Intel's flagship microcontroller product,
commanding the most sophisticated design wins. However, for MCS 51 microcontroller designs
that need an incremental increase in performance with the shortest development time and effort,
the MCS 151 microcontroller will provide the perfect next step.
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