In the 13.0 version of the Cyclone V SOC dev kit (Rev C) the BootSel default is set to 4 which (according to various tables etc) should be a 1.8V uSD card.This seem odd as the uSD socket power I connected to 3.3v. Looking at late documentation(and the Arrow SOC dev kit), the BootSel should be set to 5 (3.3V SD) where option 4 is no supported. My questions are: 1) for the Rev C board, should Bootsel 0 be Low (as per defaults and which seems to work) or high as per other later documentation? 2) for our own PCB using the HPS, should we do something special to support 1.8V or in fact, should we be using option 5 (Bootsel2 high)?
From what I understand is that all SDCards uses the 3.3V interface at startup. Then a configuration(ACMD41/CMD11) is needed to turn the interface to 1.8V. This is only the voltage used on the data&control lines, the power supply should always be 3.3V (2.7V - 3.6V)You can read a summary on wikipedia : http://en.wikipedia.org/wiki/sdcard there is a topic "electrical interface". or the complete story in https://www.sdcard.org/downloads/pls/simplified_specs/archive/part1_301.pdf page 15 The SD/MMC controller of the SOC of the Cyclone V can work in 4 modes. DS/HS and requires an interface of 3.3V and SDR12/SDR25 that requires 1.8V. The speed is however the same, maximum 25MBps. This will thus probably depends if you are using/supporting the "old" SD cards or only the SDHC/SDXC. I did not find what will happen when you connect an old SD to a cyclone configured to 1.8V