[coreboot] Dual SPI Flash

Oliver Schinagl oliver at schinagl.nl
Wed Mar 7 16:28:59 CET 2012

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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