Beaglebone Black pinmuxing

[O'Connor, Daniel]

Hi,
I have a BBB with a GPS engine attached to UART4 (P9_11 - UART4 Rx, P9_13 - UART4 Tx) but I am having trouble communicating with it.

I have tried both an overlay and editing the main DTS file directly but while the UART is probed and attached I can't see any data coming in, nor see any go out (by putting a 'scope probe on P9_13 while sending data out cuau2).

By my reading of the TRM and some googling P9_13 is gpmc_wait0 who's pinmux offset is 0x870, P9_11 is gpmc_wpn, offset 0x874. The pinmuxes get 0x800 subtracted from this (based on what is already in the DTS file)

This is the overlay source..
root at generic:/ # cat /boot/dtb/overlays/am335x-beaglebone-uart4a.dtso
/dts-v1/;
/plugin/;

/ {
        compatible = "ti,am335x-bone-black", "ti,am335x-bone", "ti,am33xx";

        fragment at 1 {
                target = <&uart4>;
                __overlay__ {
                        status = "okay";
                        pinctrl-names = "default";
                        pinctrl-0 = <
                                        0x074 0x06      // gpmc_wpn.uart4_txd_mux2 -> mode 6
                                        0x070 0x2e      // gpmc_wait0.uart4_rxd_mux2 -> mode 6
                        >;
                };
        };
};

Compiled with..
dtc -@ -I dts -O dtb -o am335x-beaglebone-uart4a.dtbo am335x-beaglebone-uart4a.dtso
And I put this in loader.conf
fdt_overlays="am335x-beaglebone-uart4a"

After that didn't work I tried modifying the /boot/msdos/dtb/am335x-boneblack.dts file directly with these diffs..
--- am335x-boneblack-orig.dts   2018-08-11 12:27:38.000000000 +0000
+++ am335x-boneblack.dts        2018-08-11 13:25:42.000000000 +0000
@@ -785,6 +785,13 @@
                                                pinctrl-single,pins = <0x170 0x30 0x174 0x0>;
                                                phandle = <0x4f>;
                                        };
+                                       pinmux_uart4_pins {
+                                               pinctrl-single,pins = <
+                                        0x070 0x2e      // P9_11 gpmc_wait0.uart4_rxd_mux2 -> mode 6
+                                        0x074 0x06      // P9_13 gpmc_wpn.uart4_txd_mux2 -> mode 6
+                                               >;
+                                               phandle = <0x7a>;
+                                       };
                                        pinmux_clkout2_pin {

                                                pinctrl-single,pins = <0x1b4 0x3>;
@@ -1153,7 +1160,7 @@
                        clock-frequency = <0x2dc6c00>;
                        reg = <0x481a8000 0x2000>;
                        interrupts = <0x2d>;
-                       status = "disabled";
+                       status = "okay";
                        phandle = <0x7a>;
                };
                serial at 481aa000 {
@@ -2189,6 +2196,7 @@
                hdmi_0 = "/ocp/i2c at 44e0b000/tda19988/ports/port at 0/endpoint at 0";
                gpio1 = "/ocp/gpio at 4804c000";
                uart0_pins = "/ocp/l4_wkup at 44c00000/scm at 210000/pinmux at 800/pinmux_uart0_pins";
+               uart4_pins = "/ocp/l4_wkup at 44c00000/scm at 210000/pinmux at 800/pinmux_uart4_pins";
                i2c2_pins = "/ocp/l4_wkup at 44c00000/scm at 210000/pinmux at 800/pinmux_i2c2_pins";
                pm_sram_data = "/ocp/ocmcram at 40300000/pm-sram-data at 1000";
                sha0_fck = "/ocp/l4_wkup at 44c00000/scm at 210000/scm_conf at 0/clocks/sha0_fck";

And while in both cases the UART attaches..
uart2: <TI UART (16550 compatible)> mem 0x481a8000-0x481a9fff irq 15 on simplebus0
.. I can't see any data from the GPS engine with..
( stty clocal -crtscts 9600 ) < /dev/cuau2

Any help much appreciated, thanks!

--
Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
 -- Andrew Tanenbaum