[Oberon] RISC.img format

[enso]

Thanks Magnus!  RTFM, as they say.  I've wasted a lot of time building 
speed switching circuits in my life, for no reason!

On 02/13/2016 05:35 PM, Magnus Karlsson wrote:
> >> Ignoring the multiplicity of standards overlaid onto SD / MMC, you 
> can talk to all cards with a simple SPI port; the only complication is 
> the requirement to go slow until a faster rate is negotiated.
>
> This is a common misunderstanding.  In SPI mode there is no need to go 
> slow initially, you can go at full speed right away.  The need for 
> slow speed is in SD or MMC mode where the pins are initially 
> open-drain in order to support multiple cards on the same bus.  In SPI 
> mode the signaling is always push-pull and you can clock the card at 
> typically up to 25 MHz from the start.
>
> See Appendix A in this document 
> http://www.circlemud.org/jelson/sdcard/SDCardStandardv1.9.pdf
>
> /Timing specifications //
> //Design engineers must meet the rise, fall, setup, hold, and other SD 
> Card and MultiMediaCard bus timing specifications.  If they want to 
> support MultiMediaCards in their design, the clock speed should be 
> controllable by the host.  This is due to the MultiMediaCard's 
> open-drain mode; the MultiMediaCard powers up in the open-drain mode 
> and cannot handle a clock faster than 400 Khz.  Once the 
> MultiMediaCard completes the initialization process, the card switches 
> to the push-pull mode.  In the push-pull mode the MultiMediaCard can 
> run at the maximum clock speed
> /
> Magnus

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