SDMMC 4 Bit wide bus dir voltage converter option

[Rookie38]

Hello Community,

I use the STM32L4R9ZITx and would like to save data on a MicroSD card via SDMMC1. Usually I use the SD 4 bit wide bus mode. After an update of CubeMX and the HAL layers now two more options are offered. One is "Auto Dir Voltage Converter", which occupies another pin (SDMMC1_CDIR) and the other is "Dir Voltage Converter", which selects an other pin (CKIN) in addition to CDIR. Since the identifiers are unknown to me and until now the pure 4 bit mode worked, I wanted to know what these two options are used for? Googling the identifiers was not very informative. I look forward to more information.

[Tesla DeLorean · Accepted solution]

I'd imagine the reference manual has pictures, the H7 where these transceivers were first used did. Look at the H7 docs if the L4+ ones aren't illustrative.

The transceivers were use where the card voltages are dropped to support high data rates, clocks and DDR modes.

The CKIN provides a full-loop feedback to understand how the clock/data has been skewed end-to-end, allowing for correct recovery of data placed on those edges.

The direction pin controlling the buffers and it transitions through phases of the data/command transfers with the cards.

If you're driving your cards at 2.9V - 3.3V, probably should just stick to the D0..D3, CLK, CMD wiring, and SDR data clocking. Should be able to hit 50 MHz with minimal design effort.

NXP SDMMC Transceiver

The original STM32H743I-EVAL used this now obsolete IP4856CX25

https://www.nexperia.com/products/esd-protection-tvs-filtering-and-signal-conditioning/esd-protection/low-capacitance-esd-protection-for-high-speed-interfaces/IP4856CX25.html

https://www.nxp.com/products/peripherals-and-logic/voltage-level-translators/sd-3-0-sdr104-compliant-integrated-auto-direction-control-memory-card-voltage-level-translator-with-emi-filter-and-esd-protection:NVT4857UK

Not entirely on point, but good foundation

https://www.st.com/resource/en/application_note/dm00525510-getting-started-with-stm32h7-series-sdmmc-host-controller-stmicroelectronics.pdf