From the CTOs desk. Why use the ARM CPU?

There are several answers to this question but the big one is power consumption. In a small wearable device it is important that the design is optimised as much as possible for low power use thus increasing the run time for a given power storage device. Given the small size of a wearable device, having a small energy storage device is also important.

Whilst there are some choices in the silicon market, many silicon makers opt to use ARM CPUs in their silicon. The phone you use almost certainly has an ARM processor in it.

BBC Micros at a school

The ARM processor has it’s roots in a small company that started in 1978 in Cambridge, England by the name of Acorn Microcomputer. Acorn became better known as the winner and builder behind the BBC Computer. These became ubiquitous in schools throughout the UK as the appetite to learn all things computing grew. The machines were based on the then popular MOS Technology 6502 also used in the Commodore PET (which incidentally is the 2nd computer I did any significant programming on around 1980 – for a cup of tea I’ll tell you about the first) and VIC-20.

The the development of a follow on product, the Proton, a young computer science graduate from Selwyn College, University of Cambridge, was working away on a new processor for the next generation of computers. In April of 1985 Sophie Wilson‘s design of the first ARM based processor came to life.

Sohpie Wison

Designing a processor from the ground up is no easy task but Sophie had some particular goals in mind. She wanted to use a Reduced Instruction Set Computer (RISC) architecture to keep the silicon size down which helps minimise power consumption as well as optimise the instruction set for stack-orientated block mode structured programming language such as C. She also wanted to optimize for speed and low latency. Another key feature she introduced to the ARM was conditional execution. (For another cup of tea I’ll talk to you about that).

In 1985 the first ARM based computer was launched by Acorn Computers, the Archimedes based on the ARM2. This CPU core used about 30,000 transistors compared to the likes of CPUs from Motorola and Intel that used 40,000 (68K) and 134,000 (80286) to deliver similar performance. Transistor count is important to power consumption (another cup of tea and I’ll tell you how and why).

Acorn Computers created Acorn RISC Machines in 1990 and by 1992, Apple used the ARM6 based ARM610 in their Newton PDA. The ARM6 was a considerable advancement compared to the ARM2 but still only used 35,000 transistors.

Symbian OS phones

By the time I was working in the mobile industry in 2000, ARM was used by almost every mobile device. All the users of the Symbian OS, where I was working were using them. As part of my role at Symbian, I was part of the liaison team to ARM representing our customers interests.

At an ARM TechCon conference in 2001 I had the great pleasure of meeting Sophie over, yes you guessed it, a cup of tea.

The ARM core is licensed to be built into silicon. The Commlet Bracelet uses a couple of ARM based SOCs (System-On-Chip) selected for their use of the ARM core and low power Radio Frequency functionality such as GPS and Cell Phone.

It’s sobering to think that today’s low power wearables owe their processing prowess to the computers of the early 1980s.

Listen to Sophie talk about ARM etc
Part 1
Part 2
Part 3

To watch a docu movie about the times of the BBC Micro, watch Micro Men from the BBC.
Micro Men Movie

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