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The eMMC signalling voltage is determined by VCCQ which is provided to the card by the host. Signalling is not required to begin at 3.3v and, if the host and card both support a particular VCC/VCCQ combination, it can be used immediately. In contrast, SD Cards must begin with 3.3v signalling and may switch to a lower voltage signalling if instructed to do so in CMD11. A message is required to coordinate this operation because the card only receives a 3.3v VDD and must know when to use the 1.8v produced by its internal regulator. It makes sense for the core to begin with 3.3v signalling but when that can't be set, 1.8v and 1.2v signalling also should be attempted. This is especially important when an external regulator with a limited range is used to supply VCCQ to an eMMC part. Signed-off-by: Tim Kryger <tim.kryger@linaro.org> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org> Signed-off-by: Chris Ball <chris@printf.net> |
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