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Original question:

Linux/AM4372: Trying to port Linux to new AM437x processor based design.

Linux/AM4372: Question about VTTREG enable code for AM437x GP in u-boot

Timothy Barr
Genius 3370 points
Part Number: AM4372
Other Parts Discussed in Thread: TPS51200, TPS65218

Tool/software: Linux

The DDR memory on my AM437x design is layed out similar to the GP EVM, which uses a Vtt regulator. But I had to change the GPIO pin to GPIO5_13, as GPIO5_7 was being used elsewhere. I am looking at the code in am437x-gp-evm.dts file in the arch/arm/dts directory of uboot in the SDK. There is code for vtt_fixed that has GPIO5-7 defined for controlling the regulator.. But when I look at the am43xx.c file in board/ti/am43xx it looks like the vtt_reg enable pin is being hard coded in the enable_vtt_regulator code. I also did a grep on vtt_fixed and it only seem to be in dts and dtsi file in the u-boot code. Is this something that has not been fully implemented yet, so I have to change the code in enable_vtt_regulator in order to get this pin to work?

  • 0
    TI__Genius 13567 points

    Hi Timothy,

    You do not need to control VTT with a GPIO. The TPS51200 enable pin can be connected to V3_3D. The VTT power sequence will follow VDDS_DDR / REFIN.

    The code in U-boot's am43xx.c file does not actually enable / disable the VTT regulator. By default, the AM437x SoC has most pins set to High-Z with internal pull-up resistors so before any code runs on the chip VTT is already enabled because of the default state of the AM437x pin. The code in am43xx.c only enables the GPIO5_7 output but does not change the state of VTT.

    The Linux / u-boot device tree code also does not change the state of VTT. It just defines a regulator that could be controlled by GPIO5_7. No application can turn it off however because it's marked as "regulator-always-on". 

    Controlling VTT and the TPS65218 Load-Switch for DDR only becomes relevant if you were designing a system that supports a low power mode called suspend to RAM.