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
CMD during initialization but changes to push-pull afterwards. The card
internal pull-ups to prevent bus floating during setup and will disable
after switching to 4bit mode (or 8bit for eMMC when available).
But even after switching to push-pull drivers there are states the bus
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
to HIGH by the card respectively host output driver, while Z-bit is
(respectively kept) HIGH by the pull-up resistors Rcmd respectively
Enabling the pull ups on the CPU would be the right choice considering
most boards will not have external pull-ups. Even if they would have
adding the internal in parallel would work in almost all cases and the
increase in power consumption would be negligible.