[Arm-netbook] A minimal A10 SBC

[luke.leighton]

On Thu, Mar 28, 2013 at 4:22 AM, Ajith Kumar <ajith at iuac.res.in> wrote:
> P1
>>
>> ---
>>
>> B1, B2, B3 - call +1.5V "DRAM_VCC" and +0.75V "DRAM-REF", then put 1
>> of the capacitors connected directly to the DRAM-REF, and mark it
>> "keep as close to DDR3 as possible", to remind yourself.  joe will
>> know what value those need to be, but that ref voltage *must* be kept
>
>
> All the decoupling capacitors kept on one sheet for ease of handling,

 ... but then you don't know how many you actually need, or where they
are supposed to go!  this is a mistake

>  will
> note it down in the schematic about the critical positions. Joe is going to
> attempt a layout. Could you give some suggestions regarding layers, the ones
> to be dedicated for ground etc. My understanding is that there should be s
> ground layer next to the DDR3 tracks to get the characteristic impedance
> correct and also to give good return path. Something like:
> L1 : tracks , L2 : ground, L3 : power, L4 : tracks, L5 : GND, L6 : tracks

 that looks about right.

> Since you already have done it,

 i have not.  what i have done is asked SoC manufacturers to supply
full EVB schematics.  i have then NOT ONCE MADE ONE SINGLE ALTERATION
TO THE DDR3 LAYOUT, precisely because i know that it is beyond my
experience and time to learn.

> please make some concrete suggestions on
> that.

 i can't.

>
>>
>> A6, A7 - 24mhz XTAL is connected to RTC :)  that'll need to be to N23
>> and N22 :)  then, if you are going to have an RTC, you *need* that
>> 32mhz crystal - you cannot leave it off.  remember the 10M/1% resistor
>> which needs to be put in parallel with the 32mhz XTAL.
>
>
> That was a mistake, intention is to use only the 24MHz oscillator, not the
> 32k one. Corrected.

 .. as in added in the 32k xtal as well?  if you do not do this, you
*will* not have time on the device - period.

>>
>> E4, E5 - if the SY8008C is the same as the SY8008B, then you want
>> resistors 10x that size, and you want them @ 1%.  so: R10 = 150K/1%,
>> R11 = 100K/1%, and R13 = 442K/1% and R16 = 100K/1%.
>
>
> SY8008B or C would do 1A and 1.2A,

 right - i see, the datasheet rev #0.9 page 3 says SY8008C does 1.5A,
SY8008B does 1.2A

> the resistors calculated as per the data
> book. Olinuxino seems to be using 1.1K and 4.99K, I thought it is too much
> of current wasted and used higher values. 100k & 1500k

 150k not 1500k!!!!

> will make the
> feedback path current 4 micro amps, will it make it more prone to noise.
> Should I change it ?

 i have no idea.  the circuits from wits-tech use 100k and 150k and it's fine.

>>
>>
>> also you're missing the capacitors which *must* be close to the SY8008
>> - you want a 10uF on the IN as well as the OUT, but also on the OUT
>> you want a 2nd much *smaller* capacitor (joe - 0.1uF?) as this catches
>> the high frequencies.  make sure you put these as connected directly
>> to the SY8008 because that tells you, when you come to do the layout,
>
>
> The instructions for layout will be marked in the schematic, so that is it
> is not overlooked.

  you're missing the point: you should *move* the capacitors onto the
same schematic.  how the hell will you know which ones, out of a huge
long line of 30 to 50 capacitors on page 2, are supposed to be close
to which components??

 it doesn't make any odds - you have space on P.1 for two more capacitors.

>
>> to each and every single pin.  my recommendation here: follow the
>> example schematic on p5 of the AXP209 datasheet.
>
>
> I did not find an English version, learned somewhat from a lower version
> AXP202.

 find the english translation.  look harder, and use the correct
example.  AXP202 could be *different* from AXP209 and you would blow
up the board as a result.

>>
>> C7 - what's the 6-pin thing connected to DM0/DP0?  it's not marked.
>
>
> Another mistake, it is only a USB header with two shield pins. Corrected.

 :)

>
>>
>>
>> other comments:
>>
>> * no JTAG!  no JTAG+UART equals very very baaad :)
>
>
> Should I add a 20 pin JTAG connector or just bring  TDI0, TDO0, TMS0, TCK0
> and RESET to a header. Please suggest.

 i'd bring them to a header.

l.