community update, Thursday, January 10, 2008

Michael Shiloh michael at
Fri Jan 11 00:43:34 CET 2008

Hi Ian,

ian douglas wrote:
>> Although it would be nice to take advantage of this faster processor, 3 
>  > problems make this impractical
> Michael,
> If I'm reading your message correctly, the GTA02 will indeed have a 
> 500MHz processor, but will not be running at 500MHz because of the three 
> problems you outlined, is that correct?

That is correct. We are installing a CPU that is rated for a maximum 
speed of 500MHz, but clocking it at 400MHz.

This is fairly common in the hardware world as availability of certain 
chips changes.

For example, a manufacturer might find they can make the faster chips 
for the same price as the slower, and it costs them more to stock the 
two speeds, so they produce only the faster.

Another reason might be that the volume of sales of the 400MHz part was 
so low that it wasn't worth manufacturing.

> Does this mean you will be slowing the CPU to 400MHz, or some other 
> speed, or will you only know the answer once more testing has been done?

I'm not sure the expression "slowing the cpu" is accurate. The CPU is 
rated for a maximum clock frequency of 500MHz, but it can be used at 
other speeds as well.

If you are asking whether we intend to increase the clock speed at a 
later time, the answer is it's possible, but it's not planned. Since the 
rest of the circuit is designed for 400MHz it would take some serious 
analysis and testing to convince ourselves that it works reliably at 500MHz.

A big part of hardware design consists of making sure that all signals, 
taking into account worst-case propagation delays and rise and fall 
times, will arrive at their destinations early enough to meet the setup 
times of the destination. This analysis is done at the intended CPU 
clocking frequency, which in our case was 400MHz. There is no 
expectation that these conditions will be met when the CPU is clocked at 
500MHz - rather, every single signal in the circuit must be analyzed at 
this higher frequency.

This is a tremendous amount of work, and is properly prioritized below 
getting GTA02 into manufacturing.

> Your description of problem 2 makes it sound like memory access will be 
> slower, at 83MHz, if running the CPU at full speed because of the memory 
> bus speed, instead of the anticipated 100MHz, is that correct? 

I can't answer for sure because I didn't work this out myself - rather I 
asked someone else. I can only presume that dividing by 5 was not an 
option. I'll find out.

This frequency pre-scaler is not part of our circuitry; rather, it's 
built-in to the system-on-a-chip (SoC). Typically, those system provide 
pre-scalers, and typically those pre-scalers provide a limited range of 
fixed numbers for dividing the incoming frequency. So just because "4" 
and "6" were available divisors does not indicate that "5" is available 
as well.

> Does 
> anyone there anticipate that this 17% decrease in memory access speed 
> will be noticeable to the end user? What's the memory access speed on 
> the GTA01?

There is no decrease in memory speed - the system was designed to run 
with a CPU clock of 400MHz, and the memory at 100MHz, and that's what it 
will do.

> Thanks,
> Ian


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