0

I use gpio #32 (wPi #26) as PWM output to control the speed of a fan based on temperature. The job starts at system startup with a dedicated account.

I would like to display the current value that is set with gpio command on a dashboard using a different account (eg when gpio pwm 26 50 is issued then I am interested in the 50), but I cannot not find any way to read the current PWM configuration and values. For example, $ gpio readall does not tell me the duty cycle et cetera, just tells me the below snapshot that is meaningless in my use case.

 +-----+-----+---------+------+---+-Pi ZeroW-+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
                                        | 32 | 1 | ALT0 | GPIO.26 | 26  | 12  |

How can I read the current PWM output values of wPi #26 from the cli?

Improve this question
6
  • You can't. May we know why you don't store the value when you set it?
    – joan
    Dec 12, 2020 at 18:25
  • @joan I did not store it because the common sense dictates not to store something that you can access anytime as writing the sd card regularly shortens its life. But if it cannot be read then I guess I will have no other option but to write it somewhere so that other accounts may access it based on need.
    – ben
    Dec 12, 2020 at 18:53
  • If you are that concerned about SD card writes perhaps store in a temporary RAM file system.
    – joan
    Dec 12, 2020 at 19:00
  • That is done already. I just didn't find that too elegant and rather seemed to be like a workaround to me.
    – ben
    Dec 12, 2020 at 19:05
  • I've added how that can be done but if someone finds another way to read this data would be great.
    – ben
    Dec 13, 2020 at 21:05

1 Answer 1

Reset to default
0

A potential solution is to store the values in a RAM filesystem and write the values into a file on the ramdisk when PWM output changes.

Job to create the filesystem:

[Unit]
Description=Creates a small ramdisk that can be used to share data between accounts
After=syslog.target network.target

[Service]
Type=oneshot

User=root
Group=root

WorkingDirectory=/folder
ExecStart=/folder/ramdisk.sh
Restart=no

[Install]
WantedBy=multi-user.target

/folder/ramdisk.sh

#!/bin/bash

TARGET="/ramdisk"
SIZE="16k"

[ -d $TARGET ] || mkdir $TARGET

mount -t tmpfs -o size=$SIZE myramdisk $TARGET

Then this small one-liner in your other shell scripts, where you want to write to the ramdisk from, will ensure the things are written to the ramdisk and never gets written to the SD card in case the ramdisk creation fails

[ ! -z "$(df /ramdisk/ | grep myramdisk)" ] || exit -1
Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.