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Topic: Why are there no 8-bit CPUs with GHz clock-rates? (Read 3 times) previous topic - next topic

Tom Carpenter

There is no technical reason why you can't make an 8 bit CPU at GHz clock speeds. You can make a 32 or 64 bit one, so you just reduce the bus width.

The question is, what would you use one for?

The faster you vibrate electrons in a material, the more energy the dissipate as heat. This means that a 1+GHz processor could not be used in a low power application due to the energy wasted (part of that is smaller transistor size though), nor could you use one without a large heatsink to dissipate the generated heat.

A hobbyist couldn't in all likelyhood use one because with a 1GHz clock, even a 1ns propogation delay would be a missed clock cycle. This means you would need a precisely designed circuit board with all traces in a databus exactly the same length.
Breadboards would be totally out of the question, so would protoboard. Even a slight amount of stray capacitance would essentially mask a 1GHz signal.

Such high speeds require expensive manufacturing methods, meaning a £2 avr at those speeds would be out of the question - think £50 and up, just for the CPU and that still leaves you requiring fast enough program memory, ram, peripherals, etc, which wouldn't be on chip.

For a computer, performing calculations using 8 bits would result in an incredible performance reduction (unsigned long long math takes about 100 instructions on 8 bit, whereas it would be about 8 maybe less on a 32bit processor), meaning they would simply not be suitable.

If you are going to go to the effort of designing a high speed microcontroller, why stick to an 8 bit bus, when you can easily use 32 or 64 bit (the technology is already there). It would be like building a ferrari and sticking a moped engine in it.

20MHz isn't the limit though. Look at the atxmega series. 8 bit microcontrollers, with similar peripherals to the atmegas, but running at 64MHz.


The great part about the avr is its very general purpose, easy to put in many situations
Faster cpu(like the rasberry pi) needs buffering and many other supporting hardware just do do one purpose,
if all you had was a processor you couldn't run leds straight off the pins(with resistors) or directly interface with alot of things
you can run a 328 off a 3.7lipo, with capacitors, reset switch, and a few leds all attached to the back of the physical ic
id like to see anything faster than 20Mhz do that
also when's the last time you saw a wearable 2Ghz board? The avrs can be put in pretty decently rugged situations and still work reliably
lmao speaking of seeing things, have you ever seen a breadboarded 20+Mhz processor?
the lower speed definetly allowed a more rugged ic, less picky, and more versatile
To raise the speed would lower the market even to applications that need it and further applications that are in a stable enviroment


Microcontrollers [typically 8-bit] are intended for low-power, relatively simple
embedded products at are sold in very high volume. That's how Atmel and others
make billions of $$$ each year, not by selling one-sies to hobbyists. They're not
trying to compete with Intel for PC-level applications, they want to sell chips to
go into millions of products, like automobiles.

If you look at the Atmel AVR and the Microchip PIC product lines, you see dozens
and dozens of different controller chips differing in only a small way between each
other, and with prices that differ by only pennies. You might wonder why. The idea
is that people can choose the most cost-effective chip for their particular embedded
app. They may only save a few pennies from using a bigger faster chip, but the
pennies add up when they sell 100s 0f 1000s of devices.


Simply ask yourself this:

- Who would want to spend the time and money designing a 1GHz 8-bit CPU core when there is already 1GHz+ 32/64-bit CPU cores on the market at a decent price?

It would be like taking a Yugo 3-cylinder car and putting a nitro kit in it.  Yes, it would go faster, but what would be the point?  You'd just look like a tit.  Yes, a faster tit, but still a tit.

If you need the greater speed of a fast core, you don't want to then cripple it with narrow data paths and tiny registers.
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After a little more digging, I think Chagrin may be closest to the mark with his comment about fabrication processes.

Atmel's fastest processors do in fact have on-board ADCs, but what they don't have is PicoPower and high-voltage tolerance.  Both of these features would require thick oxide layers, which in turn requires larger and slower transistors.  Layer-thicknesses cannot easily be varied across a single chip, so if one part requires thick insulators (the IO ports, for example) the rest of the chip is condemned to use the same technology.

That would also explain why ATmegas need 5V to reach their maximum clock rate, whilst other designs can manage with 3.6V or 1.8V.  As Tom mentioned there are faster 8-bit processors, and significantly these chips also require lower supply voltages.


1. My guess,  IC and CPU market is dictated by military lobby. It meas, that if you can manufacture 8-bit 50 GGz microCPU, you can't do it, at least you can't do it for profit. Despite regular market driven by profit/interest there is different rules. Simply, because politicians don't want "smart" weapon spread around the globe and threat world stability at first place, and secondly, there are much higher profit if you selling out "smart" bomb or rocket, than you selling 2$ chip. Who want to fight Taliban if they would have access to most sophisticated weaponry for dirty cheap price from cnina-kong?

2. Radioactive susceptibility  grows up with lowering size of transistors. GGz chip would have a lot of troubles to operate on board telecom/GPS satellite, and even regular airplane flight at 10 km is exposed to space radiation.



Did you mean GHz?
Radioactive susceptibility  grows up with lowering size of transistors. GGz chip would have a lot of troubles to operate on board telecom/GPS satellite, and even regular airplane flight at 10 km is exposed to space radiation.

I am guessing that they are shielded anyway. Remember the gold foil on Apollo capsules?


Yes, GHz, typo. 
Remember the gold foil on Apollo capsules?
Gold is good for IR or heat shielding. Radiation 'd require thick plate of lead, which is heavy and cost a pile of money to launch in space, where each gram is accounted.


Remember the gold foil on Apollo capsules?

Do you mean the gold-coloured kaptan over aluminium foil?
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The foils on spacecraft are for thermal insulation.  Since there's no air they only have to combat radiative-heating or cooling, and the best way to do this is with multiple layers of something light and reflective.  Metalised mylar or polyimidie is commonly used.  The same stuff is used to insulate ultra-low temperature cryostats.

The foils do nothing to shield against the high-energy radiation which can disturb electronics (and astronauts).


But the electronics is still shielded somehow, since it works out there.


Aug 13, 2012, 08:13 pm Last Edit: Aug 13, 2012, 09:24 pm by Far-seeker Reason: 1

The foils do nothing to shield against the high-energy radiation which can disturb electronics (and astronauts).

Right, during Apollo the astronauts knew they would have minimal radiation protection while on the Moon.  However, because they were only going to be on the moon for at most a few days and the missions were planned to avoid peroids of time with the highest solar radiation and incidence of flares (which have the potential to be fatal very quickly and the LEM and/or Apollo suits was no protection against); they decided traveling to the Moon was worth the significant increase to the risk of developing cancer or other health problems later in life.  Also, most of them that wanted families had fathered children by then...

As for the electronics, radiation was a concern as well, but the Apollo vintage gear would generally be more innately rad tolerant than most modern equipment.  This is because there are three things that tend to make electronics more intrinsically vulnerable to radiation; smaller feature size, lower signal voltages, and higher operational frequencies.  While we tend to think of areas of "high radiation", radiation only interacts with substances on the atomic or sub atomic levels.  Therefore the smaller the size of transistor gates the greater potential difference a stray ion passing through or embedded in it will make, and the higher the feature density the more potential damage a single ion can do.  Similarly, the lower the logic level voltage the easier it is to erroneously change it.  Finally, the higher the operational frequencies the more transient errors can affect the system in a given period of time.

But the electronics is still shielded somehow, since it works out there.

See above.  Also remember that "out there" is subjective.  The vast majority of space hardware is in orbit around Earth low enough to be within the Earth's magnetic field.  While it's subjected to more radition than it would be within the atmosphere, it's still significantly less than what's in or beyond the Van Allen belt.  Deep space probes usually do have significantly more sheilding than satellites because of this fact.

Edit: And to be clear "sheilding" in the context of space probes is mostly placing as much of the less radiation sensitive parts of the spacecraft (e.g. structure, radiators, batteries, fuel, etc...) between what you are trying to shield an the outside of the craft as possible.  Mass is always at a premium, the small the amount of material added just for radiation sheilding.


Aug 13, 2012, 08:34 pm Last Edit: Aug 13, 2012, 08:39 pm by P18F4550 Reason: 1
One more thing i don't know if it's already been mentioned but timing is an issue, did you ever see those squiggly lines on motherboards and graphics cards, they seem to make no sense but in reality they make sure that all the data lines are the same length and that data arrives on the bus at the same time, imagine if you had to cut all you jumper wires to the same length, if you didn't you'd have to wait until all the data lines are energised, waiting wastes time and so no point in having a fast processor,

I dont know much about atmega's but PIC's have a clock prescaler and there are advantages to having a slow clock in pic case's as low as 32khz because you dont need huge delay proceadures

just found this http://downloadsquad.switched.com/2009/07/20/how-powerful-was-the-apollo-11-computer/


For sure clock-skew could become problematic for high-speed I/O, even for serial data transfer.  OTOH the I/O doesn't necessarily have to run at the same rate as the CPU, and in fact usually doesn't.  Even on the ATmegas the interfaces are usually run far below 20MHz.  The benefit of a faster CPU is lower latency, which is especially useful when multitasking or doing something slow like floating-point maths.

In the case of the ATmegas it's clear that the clock is limited by the electrical properties of CPU and not by any external considerations.  If high clock-rates are problematic in any particular application the user is free to slow things down to his or her taste.  And at the same time any user who needs to use the highest speeds can do so - on condition that he/she deals with the consequences that brings.


I found this interesting. The fastest CPU isn't always the best one for a particular job.
An iPhone 4S has four times the CPU power of Curiosity

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