Trying to learn and understand the A20 SOM schematic

Started by Buchi, December 09, 2014, 11:36:57 AM

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Hello all,

my name is Michael and i am interested in learning how the A20 SOM internally
I am studying the PDF schematic here:
and a little bit confused about the part where the SDRAM is connected to the A20 CPU.
Namely, it is the address line SA15 that i cannot find.
SA0...SA14 are directly 1:1 connected to their counterparts at the two SDRAM IC´s.
SA15 (which in the A20 data sheet should be at ball AA4) is named "SCS1" here, and goes via
RM11G4 (0R resistor) to an unnamed line in the bus that goes to the SDRAM IC´s.
At the SDRAM IC´s, i cannot see any SCS1 line (the ball is labelled SCS1 in the schematic)
But interestingly, there is a line with no name coming out of the bus and going to ball M7 of
both SDRAM IC´s (marked "n.c." both in the schematic and in the Hynix SDRAM data sheet).
Is this the missing "A15" data line ? Is that one required for proper operation ?

Or do we need only 15 address lines because the SDRAM chips are 16 bits wide ? But then,
what is with the unnamed lines to the "n.c." balls ?

Thank you !


To me it looks the same as with their other A20 boards but I don't know if it should be different.

Might be a good time to email them direct :)


Mariah Albert

Mariah Albert


Hmmm strange. The same for all A20 boards ?
I do have the schemtic for the Allwinner BananaPi board, which also uses
two 16-bits wide SDRAMs, and there the baal AA$ is used as address line "SA15".

Another schematic is from the Allwimmer A20 reference manual, but uses four 8-bits wide
SDRAM. But here also this ball is used as SA15.

So, to make an educated guess:
this line is not needed in the Olimex A20 SOM ?