Re: [PATCH] ARM: dts: sunxi: Explicitly enable pull-ups for MMC pins

From: klaus . goger
Date: Thu Nov 17 2016 - 16:10:11 EST

On 2016-11-17 10:34, Chen-Yu Tsai wrote:
Given that MMC starts in open-drain mode and the pull-ups are required,
it's best to enable it for all the pin settings.

It's even more complicated than that with MMC. It starts in open-drain mode for
CMD during initialization but changes to push-pull afterwards. The card has
internal pull-ups to prevent bus floating during setup and will disable them
after switching to 4bit mode (or 8bit for eMMC when available).
But even after switching to push-pull drivers there are states the bus would
float and pull ups have to ensure a high level on the bus.

See JESD84-A441 chapter 7.15 ff as reference.

The difference between the P-bit and Z-bit is that a P-bit is actively driven
to HIGH by the card respectively host output driver, while Z-bit is driven to
(respectively kept) HIGH by the pull-up resistors Rcmd respectively Rdat.

Enabling the pull ups on the CPU would be the right choice considering that
most boards will not have external pull-ups. Even if they would have one,
adding the internal in parallel would work in almost all cases and the
increase in power consumption would be negligible.