Max speed of 3.3V Mega128?

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Hi all,

Does anyone have any experience with running a 3.3V Mega128 beyond 8 MHz? I know the datasheet says 8 MHz max for 3.3V operation, and 16 MHz for 5V. Has anyone run faster than 8 MHz at 3.3V? I'm thinking that 12 MHz would be nice...

Thanks,

Frank.

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Also the datasheet does not specify the maximum VCC voltage for the 128L. Anyone know this, I would like to run at 3.6v

John

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John,

Front page of the datasheet...

• Operating Voltages
– 2.7 - 5.5V for ATmega128L
– 4.5 - 5.5V for ATmega128

regards,
Mike

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I have run the M128L at 16MHz, 3.3v no problem...

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re operating voltage

Thanks Mike, I was foolishly looking uder the Electrical Characteristics section ;>)

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internal RC's are supply voltage sensitive (?)

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Hello,

Well there are two answers to your question.

The absolute maximum speed that the chip will work reliably under is 8 MHz. As soon as you exceed 8 MHz, stuff might start to drop out. So if the project will just be sitting on your bench and won't matter too much, you can overclock. If its commercial, or matters don't overclock!

The maximum speed will depend on your chip, 12 MHz would likely work. But you'd have to experiment to find out... also test everything (such as EEPROM) at 12 MHz before trusting it, and even then be careful.

A helpful hint: you might have to drop the frequency to program it, but then raise the frequency again to run the program.

Regards,

-Colin

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Hi,

I'm trying to interface with an FTDI FT245BM to send some data to an ATMEGA128. I was wondering if anyone had experience with these chips and their throughput. Their specs talk about a maximum USB speed of around 11Mb/S, but I need a minumum of 1Mb/S to keep my client happy.

I've seen the appnotes on the FTDI website on how to write data in large blocks to keep the driver running at full speed, but the best I can get is about 100Kb/sec.

Anyone have any hints on the best way to get maximum speed out of these things?

Thanks!

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I've been using the chip a lot, and can get about 300-600 kByte/s, but that's probably the maximum possible.

11 Mbps (or 12 Mbps actually) is the speed on the USB medium. There's TONS of overhead on that and you'll never get the full speed.

1Mbps / 100kByte/s should not be a problem, but you can NEVER get a guaranteed transfer speed in Bulk mode !

/Jesper
www.yampp.com

/Jesper
http://www.yampp.com
The quick black AVR jumped over the lazy PIC.
What boots up, must come down.

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Hi Jesper,

Can you tell me the most efficient way to use the chips? I was using the "Direct" driver and a sample program called USBTEST that I found on the FTDI site. There was a section that sent 128 bytes to the USB chip and I just expanded that to send my data to the AVR.

I was wondering if the virtual COM port driver was perhaps faster? Or, is there something I need to do to insure that I'm sending at maximuim speed.

Another question I had was wether I was harming my data transfer speed by sending a message back to the PC periodically.

Any help you have would be awesome.

Thanks!

Chris

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The DXX drivers are the fastest, although the difference is pretty small.

Block up the data in large blocks (16-20 kB), to get most speed.

Are you sure the mega128 end is not slowing thingd down ?

Try making a simple loop on the mega128 end that simply monitors the RXF line and then toggles the RD line, ignoring the data.
Then you will be able to experiment with the code on the PC side and find the best speed.

/Jesper
www.yampp.com

/Jesper
http://www.yampp.com
The quick black AVR jumped over the lazy PIC.
What boots up, must come down.

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Hi guys
FTDI chips are USB1.1 and seem to work well but are a bit on the slow side. I have asked them about a USB2.0 version and they are working on it but won't have silicon for another year.
USB2.0 is 480Mbits/sec and I am getting around 20MBytes/sec transfer rates on my test system. I am using a QuickUSB module from Bitwise Systems;

www.bitwisesys.com

This is a similar type module as FTDI, with supplied device drivers etc. (although no serial port emulator). It uses the Cypress 68013 EZ USB chip.

My FTDI module has now gone into an old Radio Control transmitter to give my son a USB controller for our model plane simulator - it's more fun than work and keeps him from crashing my real models!

Good luck.
Tony

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Jesper,

Thanks for the help! I tried your experiment and found that just reading characters in a tight loop was still slower than I expected. It turns out that I had the fuse bits programmed accidentally for the internal R/C oscillator and was running my AVR in slow motion.

With the fuses set for crystal oscillator I get about 250K Bytes per second transfer rate. I think this will be sufficient.

Thanks again for your advice!

Chris

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