How-To use UART7/8 on i.MX6UL

[#1]

       There are 8 UART ports on i.mx6ul and one uniform Linux driver for these UARTs. Form UART1~UART6, there is no special operation or attention to use them. But for UART7/UART8, there is a special rule to enable them.

      According to i.mx6ul RM, we can see UART7/8 RTS pins are muxed with ENET TX_CLK pins. When SION bit of ENET_TX_CLK is set, we need switch to other MUX mode as input signal for UARTx_RTS. Otherwise, UARTx_RTS will be interrupted by loopback ENET clock signal. So we should set IOMUXC_UART7_RTS_B_SELECT_INPUT and IOMUXC_UART8_RTS_B_SELECT_INPUT registers to 0x2/03 to avoid ENET clock's conflict no matter whether we enable UART7/8 RTS/CTS function or not.

Let's summarize the different scenarios to enable UART7/8 as follows:

1. ENET driver is disabled and UART7/8 is enabled. There is no special operation to do, just use UART7/8 like other UARTs

2. ENET and UART7/8 are both enabled. There are two use models, RTS/CTS enabled or disabled.

    2a. If we enable RTS/CTS feature and configure RTS/CTS pins in the device tree, of course, we should avoid the conflict between UART CTS/RTS pins and ENET TX_CLK pins. There is no special operation to do becuase your RTS/CTS device tree would automatically set  IOMUXC_UART7_RTS_B_SELECT_INPUT/ IOMUXC_UART8_RTS_B_SELECT_INPUT register to correct value.

    2b. If we don't enable RTS/CTS feature and no RTS/CTS pin configuration in devcie tree, we should manually add code to set  IOMUXC_UART7_RTS_B_SELECT_INPUT/  IOMUXC_UART8_RTS_B_SELECT_INPUT register because the default value is 0x0(ENETx_TX_CLK_ALT1)

Here is an example to show how to use UART7 on EVK board in scenario 2b.

1. modify imx6ul-14x14-evk.dts to enable UART7

    a. remove all  LCD settings to disable lcdif because we configure UART7 TX/RX pin pad to LCD data line

    b. add UART7 related settings

               &uart7 {

                    pinctrl-names = "default";

                    pinctrl-0 = <&pinctrl_uart7>;

                   status = "okay";

                };

              &iomuxc {

                  pinctrl-names = "default";

                  pinctrl-0 = <&pinctrl_hog_1>;

                  ....

                          pinctrl_uart7: uart7grp {

                          fsl,pins = <

                                      MX6UL_PAD_LCD_DATA16__UART7_DCE_TX 0x1b0b1

                                      MX6UL_PAD_LCD_DATA17__UART7_DCE_RX 0x1b0b1

                          >;

                 };

2. add code to set IOMUXC_UART7_RTS_B_SELECT_INPUT register in arch/arm/mach-imx/mach-imx6ul.c

         static void __init imx6ul_init_machine(void)

         {

              struct device *parent;

              void __iomem *iomux;

              struct device_node *np;

              ...........

              imx6ul_pm_init();

              np = of_find_compatible_node(NULL,NULL,"fsl,imx6ul-iomuxc");

              iomux = of_iomap(np, 0);

              writel_relaxed(0x2,iomux+0x650);

           }

3. build zImage and imx6ul-14x14-evk.dtb

4. Test in linux console

     root@imx6ulevk: ls /dev/ttymxc*                      //you can see ttymxc6 is in the list

    root@imx6ulevk: echo hello > /dev/ttymxc6    

    root@imx6ulevk:

[wangjia]

thanks your infomation.

but we want to use the

pinctrl_uart7: uart7grp {

                        fsl,pins = <

                              MX6UL_PAD_ENET2_RX_EN__UART7_DCE_TX     0x1b0b1

                              MX6UL_PAD_ENET2_TX_DATA0__UART7_DCE_RX  0x1b0b1

                        > ;

                };

and disable the ENET2 , we already done that .

but it seemed no TX out signal , i don't know why , could you help me with that , thank you .

[jedrumh75]

Thank you for posting this information. I have a really simple question that I can't seem to find the answer too. How do I build imx6ul-14x14-evk.dtb? I modify the dts and attempt to "make" but it never rebuilds. Can you please enlighten me. I'm new to all of this.  

[jiaweiren]

Thank you for posting this information.You mean the LCD and uart7/8 conflict?

[ramgopalkota]

Hi Sir,

We are not using LCD but wants to use both Ethernets and UART7 and UART8. I followed your steps.

UART7 is working but UART8 is not working. When I echo on UART8 , board hangs.

Please let me know what could be wrong .... We are stuck .. Please help ...

As per the instructions if we are using UART7/UART8 CTS/RTS in DTS nothing needs to be added in the code.

I have also tried adding the below lines

writel_relaxed(0x3, iomux + 0x650);
writel_relaxed(0x3, iomux + 0x658);

UART7 and UART8 DTS IO MUXING..

&uart7 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart7>;
fsl,uart-has-rtscts;
linux,rs485-enabled-at-boot-time;
status = "okay";
};

&uart8 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart8>;
fsl,uart-has-rtscts;
linux,rs485-enabled-at-boot-time;
status = "okay";
};

pinctrl_uart7: uart7grp {
fsl,pins = <
MX6UL_PAD_LCD_DATA16__UART7_DCE_TX 0x0001B0B1
MX6UL_PAD_LCD_DATA17__UART7_DCE_RX 0x0001B0B1
MX6UL_PAD_LCD_DATA06__UART7_DCE_CTS 0x0001B0B1
MX6UL_PAD_LCD_DATA07__UART7_DCE_RTS 0x0001B0B1
>;
};

pinctrl_uart8: uart8grp {
fsl,pins = <
MX6UL_PAD_LCD_DATA20__UART8_DCE_TX 0x0001B0B1
MX6UL_PAD_LCD_DATA21__UART8_DCE_RX 0x0001B0B1
MX6UL_PAD_LCD_DATA04__UART8_DCE_CTS 0x0001B0B1
MX6UL_PAD_LCD_DATA05__UART8_DCE_RTS 0x0001B0B1
>;
};

[s_mazumder]

Hi,
your answer was helpful but I am stuck on a particular region.
I have a Connect Core 6UL SBC Express board. I want to enable the other UARTs.
UART 4 is enabled on the expansion connector and UART 5 is routed to console.
I want to enable any other UARTs.

Thanks in advance.