MMC2 Linux Device Tree Configuration For SD CARD on ARM PART i

Update: 

• This blog is useful for ARM microprocessors running Linux Kernel versions 4.1.2-ti-r4 to 4.4.0.
• It could still be relevant for earlier kernel releases but earlier releases have not been tested.
• If your ARM microprocessor is running kernel's 4.4.16-ti-rt and newer go to part II:
• MMC2 Linux Device Tree Configuration For ARM PART ii

Interfacing a second SD card reader to the Beaglebone Black 

• I could not find any tutorials or guides in forums on how to interface another SD card to the beaglebone black, so I thought I'd share and show you how I got mine up and running.  I won't explain the device tree bindings in detail but you can use my solution as a reference.

• Firstly, my beaglebone black, at the time, was running the 4.1.2-ti-r4 Linux Kernel.  And I was interfacing Digilent's PmodSD: Full-sized SD Card Slot to the mmc2 lines of the microprocessor.
• The PmodSD reference manual can be found here.
• Schematics are always fun to look at: AM335x schematics

MMC2 PINMUX CONFIGURATION:

Added in file: am335x-bone-common.dtsi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

mmc3_pins: pinmux_mmc3_pins { pinctrl-single,pins = < 0x44 (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_a1.mmc2_dat0, INPUT_PULLUP | MODE3 */ 0x48 (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_a2.mmc2_dat1, INPUT_PULLUP | MODE3 */ 0x4C (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_a3.mmc2_dat2, INPUT_PULLUP | MODE3 */ 0x78 (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_ben1.mmc2_dat3, INPUT_PULLUP | MODE3 */ 0x88 (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_csn3.mmc2_cmd, INPUT_PULLUP | MODE3 */ 0x8C (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_clk.mmc2_clk, INPUT_PULLUP | MODE3 */ 0x40 (PIN_OUTPUT_PULLDOWN | MUX_MODE7) /* gpmc_a0.gpio1_16 */ 0x74 (PIN_INPUT_PULLDOWN | MUX_MODE7) /* mmc2_sdcd, p9_13, Note: Dont know why but we set card detect pinout to be GPIO */ 0x15c (PIN_INPUT_PULLDOWN | MUX_MODE1) /* mmc2_sdwp, p9_17, Note: Write protect is not configured in the device tree settings*/ >; };

 

• Note that the mmc0, mmc1, and mmc2 lines on the beaglebone refer to mmc1, mmc2, and mmc3 in the device tree.

• Also notice that the mmc2_cmd pin is the T13 ZCZ Ball: http://www.ti.com/product/AM3358/datasheet/terminal_configuration_and_functions#bc_15x15_T13 and this T13 line is tied to the R13 line which happens to be gpio1_16.  In the pinmux configuration I am referring to:

1 2 3 4

0x88 (PIN_INPUT_PULLUP | MUX_MODE3) /* gpmc_csn3.mmc2_cmd, INPUT_PULLUP | MODE3 */ ... ... 0x40 (PIN_OUTPUT_PULLDOWN | MUX_MODE7) /* gpmc_a0.gpio1_16 */

• You can see that the two lines are connected:

MMC2 DEVICE TREE BINDINGS:

• Pinctrl-0 is set to mmc2's pinmux (mmc3_pins)
• cd-gpios (Card detect) is set to gpio0_31 and is active high, i.e. gpio0_31 is high when SD card is inserted in the SD card reader. Therefore, using the "cd-inverted" property means, that the CD line is active high.
• Max-frequency is set to 25MB/s because of SD bus speed specifications.
• dmas parameters: <&edma 12 &edma 13>
• use dma channels 12 and 13
• 12 and 13 are mapped to crossbar events 1 and 2

• ti,edma-xbar-event-map = /bits/ 16 <1 12 
                                       2 13>;

• Search EDMA Events in AM335x Sitara Processors Technical Reference Manual (Rev. N) 
• Here's the mmc device tree bindings documentation:  
• http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/mmc/mmc.txt  

Added in file: am335x-bone-common.dtsi

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
&mmc3 {
      vmmc-supply = <&vmmcsd_fixed>;
      ti,dual-volt;
      ti,needs-special-reset;
      ti,needs-special-hs-handling;
      pinctrl-names = "default";
      pinctrl-0 = <&mmc3_pins>;
      cd-gpios = <&gpio0 31 GPIO_ACTIVE_HIGH>;
      cd-inverted;
      bus-width = <4>;
      max-frequency = <25000000>;
      dmas = <&edma 12
              &edma 13>;
      dma-names = "tx", "rx";
      status = "okay";
};

&edma {
 ti,edma-xbar-event-map = /bits/ 16 <1 12 
                                     2 13>;

};