Using PWM on the beaglebone black

[James Fitzsimons]

Hi all,

Problem solved!

After coming back to this fresh the next day I realised I had gone down a
blind alley with the device tree thinking. The PWM driver does all hardware
configuration required - no fdt required.

I started double checking everything in the driver code and eventually
found the bug. The register offsets for the PWM output on P9_14 and P9_16
were wrong. I've created a patch for this and submitted it to the developer
list.

Cheers,
James Fitzsimons

On Sat, 4 Jul 2020 at 22:36, James Fitzsimons <james.fitzsimons at gmail.com>
wrote:

> After thinking about this some more I realised the pinmux setup is part of
> the PWM driver code, so I assume the fdt overlay isn't required to set the
> pin mux. I tried reducing the overlay to just the following:
>
> /dts-v1/;
> /plugin/;
>
> / {
> compatible = "ti,am335x-bone-black-wireless", "ti,am335x-bone-black",
> "ti,am335x-bone", "ti,am33xx";
>
>     fragment at 0 {
>         target = <&epwmss1>;
>         __overlay__ {
>             status = "okay";
>         };
>     };
>
>     fragment at 1 {
>         target = <&ehrpwm1>;
>         __overlay__ {
>             status = "okay";
>         };
>     };
> };
>
> uboot now loads the fdt correctly and the RTEMS application runs, but
> there is still no PWM output on pin P9_14.
>
> I'm at a bit of a loss. If anyone can shine a light on what I'm doing
> wrong please let me know!
>
> Cheers,
> James Fitzsimons
>
> On Sat, 4 Jul 2020 at 20:46, James Fitzsimons <james.fitzsimons at gmail.com>
> wrote:
>
>> Hi all,
>>
>> I'm planning to start development on adding the QEP driver to the
>> Beaglebone BSP soon, but want to make sure I can use the PWM driver
>> properly first as this will almost certainly require some changes to add
>> the QEP functionality.
>>
>> I've been able to get the PWM driver to output a signal on P8_13 (which
>> is EHRPWM2B by default). However, trying to get the same program to output
>> the PWM signal on P9_14 is not working.
>>
>> Questions:
>> 1. Am I correct in assuming that I will need to add a fdt overlay to
>> enable P9_14 for PWM output before that will work?
>> 2. If yes to 1. above, is there anything special that needs to be added
>> to enable fdt support in RTEMS for pinmux changes?
>>
>> I have tried creating an overlay (source at the end of this email) and
>> adding it to the u-boot start up but when RTEMS starts I get the following
>> error displayed in the console output.
>>
>> ...
>> Loaded environment from /uEnv.txt
>> Importing environment from mmc ...
>> Checking if uenvcmd is set ...
>> gpio: pin 56 (gpio 56) value is 1
>> Running uenvcmd ...
>> 81815 bytes read in 8 ms (9.8 MiB/s)
>> 53130 bytes read in 6 ms (8.4 MiB/s)
>> 373 bytes read in 2 ms (181.6 KiB/s)
>> 1065 bytes read in 2 ms (519.5 KiB/s)
>> failed on fdt_overlay_apply(): FDT_ERR_NOTFOUND
>> ## Booting kernel from Legacy Image at 80800000 ...
>>    Image Name:   RTEMS
>>    Created:      2020-07-04   5:52:20 UTC
>>    Image Type:   ARM Linux Kernel Image (gzip compressed)
>>    Data Size:    81751 Bytes = 79.8 KiB
>>    Load Address: 80000000
>>    Entry Point:  80000000
>>    Verifying Checksum ... OK
>> ERROR: Did not find a cmdline Flattened Device Tree
>> Could not find a valid device tree
>> ....
>>
>> Obviously this  error "failed on fdt_overlay_apply(): FDT_ERR_NOTFOUND"
>> is signifying something wrong with my overlay, but I'm struggling to figure
>> out what. I have a much more complex overlay working on Linux.
>>
>> Any advice greatly appreciated!
>>
>> Cheers,
>> James Fitzsimons
>>
>> overlay source:
>>
>> /*
>>  * Copyright (C) 2018 James Fitzsimons
>>  *
>>  * This program is free software; you can redistribute it and/or modify
>>  * it under the terms of the GNU General Public License version 2 as
>>  * published by the Free Software Foundation.
>>  *
>>  *  It sets up the following devices:
>>  * - P9.14 PWM ehrpwm1A on epwmss1 for motor control
>>  * - P9.16 PWM ehrpwm1B on epwmss1 for motor control
>>  *
>>  */
>> /dts-v1/;
>> /plugin/;
>>
>> //#include "am33xx.dtsi"
>> //#include "am335x-bone-common.dtsi"
>> //#include "am335x-boneblack-common.dtsi"
>>
>> / {
>> compatible = "ti,am335x-bone-black-wireless", "ti,am335x-bone-black",
>> "ti,am335x-bone", "ti,am33xx";
>>
>> // -- PWM section --
>>     fragment at 0 {
>>         target = <&am33xx_pinmux>;
>>         __overlay__ {
>>         pwm1_pins: pinmux_pwm1_pins {
>>                 pinctrl-single,pins = <
>>                     0x048 0x06  //AM33XX_IOPAD(0x848, PIN_OUTPUT_PULLDOWN
>> | MUX_MODE6) /* (U14) gpmc_a2.ehrpwm1A  P9_14 */
>>                     0x04c 0x06  //AM33XX_IOPAD(0x84c, PIN_OUTPUT_PULLDOWN
>> | MUX_MODE6) /* (T14) gpmc_a3.ehrpwm1B  P9_16 */
>> >;
>>             };
>>         };
>>     };
>>
>> fragment at 1 {
>>    target = <&ocp>;
>>        __overlay__ {
>>        pwm_pin_helper {
>>         compatible = "rtems";
>>            pinctrl-names = "default";
>>            pinctrl-0 = <&pwm1_pins>;
>>            status = "okay";
>>        };
>>    };
>> };
>>
>>     fragment at 2 {
>>         target = <&epwmss1>;
>>         __overlay__ {
>>             status = "okay";
>>         };
>>     };
>>
>>     fragment at 3 {
>>         target = <&ehrpwm1>;
>>         __overlay__ {
>>             status = "okay";
>>         };
>>     };
>> };
>>
>>
>>
>>
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