[coreboot] Dual SPI Flash

Oliver Schinagl oliver at schinagl.nl
Tue Mar 20 22:42:26 CET 2012

So after a weekend and some playing with gEDA and pcb (learning curve 
that was) I drew up a schema [1] from what we talked about earlier.

After spending some hours routing I decided to make a second version 
(there is plenty of room on a 5x5 pcb) with the header in the middle, 
instead of the edge and thus a 2 design PCB was born [2]. Before I send 
this off to the PCB fab and have some boards made, I was hoping what you 
guys thought and whether it is sound. I've attached a more or less 
'complete' render [3] and one without the silkscreen [4] (probably most 

(M)any pointers appreciated, this being my first board ever designed :)

[1] dspif_sch.png
[2] dspif_photo.png
[3] dspif.png
[4] dspif_nosilk.png

On 03/07/12 16:28, Oliver Schinagl wrote:
> On 06-03-12 19:33, Peter Stuge wrote:
>> Oliver Schinagl wrote:
>>> Pin 1, 'chip select enable' is an inverted? pin. enables and
>>> disables device operation. When chip select is high, the device is
>>> de-selected and the serial data pins are at 'high impedance'.
>> Correct.
>>> So if I understand all this correctly, the chip can be
>>> connected in parallel with the exception of the Chip Select Enable.
>>> A simple switch to either connect it directly to the
>>> board/socket/other end and toggle it to connect to ground (via
>>> 'some' resistor').
>> Right. This is what you can see demonstrated in the photos linked to
>> at the bottom of http://stuge.se/m57sli/ i.e.:
>> http://stuge.se/m57sli/overview.jpg
>> http://stuge.se/m57sli/U5.jpg
>> http://stuge.se/m57sli/U9.jpg
>> These photos are not from a PC mainboard but the principle hopefully
>> shows. The connection you describe is indeed how GIGABYTE boards
>> implement Dual BIOS. What is not shown in my photos are the
>> resistors, which are mounted onto the GIGABYTE board on pads for that
>> very purpose.
> After this mail-conversation, those images make perfect sense!
>>> I tried to make a simple schematic in ascii, but failed horribly so 
>>> i've
>>> attached it to this message as monochrome BMP (only format that I could
>>> quickly think of to be smallest in size).
>> Hint: png
> I thought I tried and came out to 54kb, I redid them in this new 
> version and it is only 998 bytes! Nice!
>>> I don't know what value those resistors need to be (and if the
>>> schematic can be even more simplified, with a single resistor), but
>>> I belive this is the schematic used for the dual-SPI flash 'module'
>> Not quite, the resistors need to be pull-up and not pull-down. See
>> e.g. http://stuge.se/flash_switch.png which shows the principle with
>> resistors, but connects the switch common to GND, instead of to the
>> mainboard as must be done.
> Hmm, I made a new 'design' and I put the common of the switch to the 
> GND, but you say it should connect to the motherboard? Why is this?
>>> This seems sensible to me, but my knowledge in
>>> this field is very limited.
>> You're already learning more. Your schematic is correct, but
>> resistors need to pull up to 3.3V and not down to GND. The values
>> are, as I wrote earlier, not really critical, just don't go too
>> much under 1k or you will potentially waste some current.
>> Also make sure that your switch is the break-before-make type.
> Learn I did, I'll now try to learn some gEDA and design a basic PCB 
> for this purpose!
>> //Peter

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