XMega crap, sodding EBI, pftt, grrr.

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Seeing that the new XMegas devices had a EBI interface with SDRAM support, I quikly hacked a breakout board for a MT48LC16M8 chip (a 16MB * 8-bit SDRAM) which the datasheet claimed the EBI would support.

WOES!

Yes, it would have supported it, IF the chip had had a port more.

8-bit SDRAM support is only supported in 4-port mode, which NONE of the existing Xmega devices have!
Only 4-bit devices are supported in 3-port mode.

The thing is that the XmegaA datasheet that covers all Xmega's, and the EBI application note happily specifies this 4-port mode.
The actual Xmega128A1 datasheet only briefly mentions EBI in a short paragraph!
But deeper investigation show that the Xmega128A1 are missing the PORTL that would be needed for 4-port mode.

So, that means you can only work with 4-bit devices (or just use only 4-bits of an 8-bit chip like I did).
What's the problem then?

The problem is that with 3-port mode you can ONLY use SDRAM, there's no support for SRAM.
Doesn't sound too bad, you say, as 16MB is plenty of memory.
Well, yes, but it also mean you can't connect anything else that uses a regular address/data bus, such as ethernet devices, LCD displays, CF cards or any other memory mapped device.
Also, the configurable chip selects doesn't work, as there's no port pins for it (despite having 5 unused 8-bit I/O ports!!), so you have to bit-bang the Chip Select.

GRRRRRR!

The EBI implementation in the Xmega is crippled.
Crippled so bad it needs a wheelchair on crutches.

The ideas I had for this chip is simply not possible, I have to use the SAM7SE as planned before the Xmega showed up.

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

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Jesper,

I'd have said this post was very much ON topic for AVR Forum so can move this to there if you like?

Cliff

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That a major setback!!!

@Cliff
You're right more folks will receive a heads up there.

Cheers,
John42

Resistance is futile…… You will be compiled!

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Quote:

8-bit SDRAM support is only supported in 4-port mode, which NONE of the existing Xmega devices have!
Only 4-bit devices are supported in 3-port mode.

The thing is that the XmegaA datasheet that covers all Xmega's, and the EBI application note happily specifies this 4-port mode.
The actual Xmega128A1 datasheet only briefly mentions EBI in a short paragraph!
But deeper investigation show that the Xmega128A1 are missing the PORTL that would be needed for 4-port mode.


I'm not an external-memory person but try to follow Xmega conversations.

After looking at the docs, I can understand your confusion: the A1 series that is most fully documented are the biggest As on the block, right? If >>they<< don't have 4-port mode, which models will???

In the 64A1 datasheet, the Figure 3-1 block diagram would seem to only indicate 3-port.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I looked into SDRAM for XMEGAs a few days ago, but I must have overlooked that missing port :?

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I looked for that when, few months ago, I tried to develop an small prototyping board for Xmegas, with external memory support.

Then I found that the 4 bit DRAM's are no so common, and that for a profitable and well fitted external memory, I would need an Xmega with four ports for EBI. So finally I had discarded this board because I couldn't see any use. Buy a nMBytes memory in a x8, x16 or even x32 configurations is not too difficult, but since there are not many x4, then one has to use a x8 memory and 'throw away' half the capacity.

IMHO, this EBI is more or less 'obsolete', since it seems targeted to 'obsoleted x4 DRAM memories'.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Well,
YOu could use an 8bit with an external PLD/CPLD to hold the data and restructure it, but I do concede that the 4bit design sucks.

Jim

P.S.,
Guillem. When you comming to NY so I can set you up at my place and then corrupt you? :wink:

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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clawson wrote:
Jesper,

I'd have said this post was very much ON topic for AVR Forum so can move this to there if you like?

Cliff

Absolutely. That was the plan. Not sure why it actually ended up here, the forum was acting up.

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

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Quote:

IMHO, this EBI is more or less 'obsolete', since it seems targeted to 'obsoleted x4 DRAM memories'.
And even when you do, you loose all other memory mapped capabilities. :-(

I you really wanted to use the full capacity, you could probably "bank" the memory by having a gate directing DQM to DQML and DQMH on a 8bit chip.

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

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Jesper, you are right with the memory mapped peripherals. All of them are designed for static memory, not for dinamic, but probably with the CPLD/FPGA approach that suggests Jim (jgmdesign) will allow to decode dinamic adresses and map peripherals, and probably refit the 4 bits into 8 bit chunks, as well as use all the memory capacity. Of course, with some effort and probably some limitations.

Anyway, It had been a long time since the last memory mapped peripheral I used.

Another solution would be to use an SPI port and serial to paralel/paralel to serial registers to use this kind of peripherals (like LCD's) if you run ot of pins.

@Jim:

If some day I go to NY, I will contact you and try to arrange to have for a beer ;)

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Guillem Planisi wrote:
Jesper, you are right with the memory mapped peripherals. All of them are designed for static memory, not for dinamic, but probably with the CPLD/FPGA approach that suggests Jim (jgmdesign) will allow to decode dinamic adresses and map peripherals, and probably refit the 4 bits into 8 bit chunks, as well as use all the memory capacity. Of course, with some effort and probably some limitations.

Anyway, It had been a long time since the last memory mapped peripheral I used.

Another solution would be to use an SPI port and serial to paralel/paralel to serial registers to use this kind of peripherals (like LCD's) if you run ot of pins.


Any effort beyond a standard connection is not worth it. A SAM7 will do it cheaper, better faster e.t.c.

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

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Guillem Planisi wrote:
Then I found that the 4 bit DRAM's are no so common...

Wow! I know 4-bit SRAMS are almost impossible to find outside of surplus suppliers, but I thought 4-bit DRAM chips were still pretty common. Looks like I'm a little behind the times on DRAM, which is a bit embarrassing since my last job was at a PC memory company.

:oops:

Mark

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Well, I limited myself to check Arrow and EBV, asking directly the salesmen, and then checking Farnell, Cypress and some other DRAM manufacturers. That was about May this year, and then I get convinced that 4bit DRAM's, quite common years ago with PC memorys, are now obsolete.

PC market moves too fast, so 6 months is an eternity for them.

And I must fully agree again with Jesper. Any effort beyond standard connection means that there are better/faster/cheaper choices (ARM7 in fact), even from Atmel itself.

Guillem.
"Common sense is the least common of the senses" Anonymous.