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Topic: Missing RAM Pins (Read 4 times) previous topic - next topic

Tom Carpenter

I think the bigger problem is A5. With A6, its a pain, but you could always solder a thin wire to the chip and run that wire out to a pin header glued to the side of the communication header. It's not an ideal solution, but I have done this before only with the Mega1280 to get access to one of the Timer inputs.
With A5, there is no way around it. It is shorted out to the NRD pin which means, unless there is a way to disable the NRD pin, you can't actually use the SMC controller as two of its output signals would short out.
~Tom~

Graynomad

#6
Oct 27, 2012, 05:35 pm Last Edit: Oct 27, 2012, 06:07 pm by Graynomad Reason: 1
A6 is nc on PC27, so yes you could run a wire.

NRD is available on PA29 which they seem to be using for a CS for an SD card.

A5 is on PC26, which is a PWM or another SPI slave select, but as you say it's shorted to PA29(NRD). What possible reason could there be for that?

But these two pins aren't used for any other SMC function so as long as you disable both PC26 and PA29 from being used by other things it should be OK wouldn't it?

Either way it's obvious that external memory was not a priority in the design.

_____
Rob

Rob Gray aka the GRAYnomad www.robgray.com

Tom Carpenter

I thought the SMC uses both at once to access an SRAM chip. The NRD pin is connected to the SRAMs read enable pin, and A5 is an address pin. What if you need to read from address 16 for example. Read enable would be low (n meaning it is reversed logic), and A5 would be high. This would mean the SMC controller has essentially just shorted the chip out.
It isn't a simple case of always having A5 low either. What happens when you try and write to the SRAM. Read enable goes high, and again you will have just shorted the chip out.

Unless of course it is possible to set PC26 as an input when you are using the SMC. If that is the case then it won't be a problem. I haven't studied the datasheet in detail yet - I am waiting until I have a Due (which the arduino store hasn't shipped yet! :().
~Tom~

Graynomad

You're right, when using the SMC you need both PC26 and PA29 as they both have functions with the SMC.

As they are shorted together you are screwed.

Quote
set PC26 as an input when you are using the SMC.

Doesn't help I think, you'll lose A5 if you do that.

No I think the Due cannot use external memory, if so that's a major shortcoming IMO.

The connection of those two pins make no sense as far as I can see, it's just possible that the PCB doesn't actually do that and the schematic is wrong. I think Massimo said the other day that the MACB connector is not on the PCB, not just DNP but not even allowed for, and yet it's on the schematic. So one has to wonder about the accuracy of the schematic.

It's 2AM here but tomorrow I may grab the Eagle files and have a look at PC26 and PA29.

_____
Rob



Rob Gray aka the GRAYnomad www.robgray.com

Tom Carpenter

Unfortunately they are connected in the eagle file.

It would be possible to disconnect them as the trace joining the two is accessible on the bottom of the board, however you would still be left having to run a wire to bring the NRD pin out.

So really, unless you can set the A5 pin to be an input, and then use the external memory with structs which contain a maximum of 32bytes, and store those structs in the memory aligned at 128byte boundaries, then sadly the external memory capability is not possible.
That right there severely cripples the Due's capability and the number of possible uses for it.
~Tom~

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