Intel MCS® 151 and MCS® 251 Microcontrollers

Backgrounder
April 1996

The Intel MCS® 51 Architecture: a Classic
The New MCS® 151 Microcontroller: 5x Performance,
The MCS 251 Microcontroller: 15X Performance, Advanced Features
Comparison between the MCS 51, 151 and 251 Microcontrollers
The New MCS Microcontroller Family Meeting Broad Design Needs

The Intel MCS® 51 Architecture: a Classic

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 New MCS® 151 Microcontroller: 5x Performance,
"Drop In" Solution

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: 15X Performance, Advanced Features

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.

Comparison between the MCS 51, 151 and 251 Microcontrollers

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

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

The New MCS Microcontroller Family Meeting Broad Design Needs

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.