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- Tutorial 6 - Active high
- =========================
- This tutorial pulls together several concepts needed to understand active state in the context of a keyboard.
- Skip to the end of this tutorial if you just want to copy an active-high keyboard.
-
- Pull-up resistors
- -----------------
- There are many sources that explain "pull-up resistors", so I won't repeat it here.
- Here is a [good tutorial on Pull-up Resistors](https://learn.sparkfun.com/tutorials/pull-up-resistors/what-is-a-pull-up-resistor).
-
- Active low
- ----------
- All the keyboards up to this point in the tutorial series have used active low with internal pull-up resistors.
-
- "Active low" means that if a switch is pressed (active state), the read pin is low.
- When the switch is released (inactive state), the pull-up resistor pulls the read pin high.
-
- The following table traces the strobe current from left to right (0 is ground, 1 is power).
- If the switch is closed, the strobe current passes through the switch and pulls the read pin low.
- If the switch is open, the pull-up resistor pulls the read pin high.
- <br>
-
- |Strobe pin on | Diode orientation | Switch position | Pull resistor | Read pin state |
- |:------------:|:------------------:|:---------------:|:-------------:|:---------------:|
- | 0 | cathode -:<- anode | close | 1 pull-up | 0 active low |
- | 0 | cathode -:<- anode | open | 1 pull-up | 1 inactive high |
- <br>
- Arduino boards have internal pull-up resistors, which saves on parts and labor compared to manually adding external pull resistors.
-
- To make a keyboard active low:
- * Orient diodes with cathode (banded end) towards the write pins (row)
- * Define strobe on and strobe off in the sketch like this:
- ```
- const bool Scanner_uC::STROBE_ON = LOW;
- const bool Scanner_uC::STROBE_OFF = HIGH;
- ```
-
- Active high
- -----------
- "Active high" means that if a switch is pressed (active), the read pin is high.
- When the switch is released (inactive), the pull-down resistor pulls the read pin low.
-
- The following table traces the strobe current from left to right (0 is ground, 1 is power).
- If the switch is closed, the strobe current passes through the switch and pulls the read pin high.
- If the switch is open, the pull-down resistor pulls the read pin low.
- <br>
-
- |Strobe pin on | Diode orientation | Switch position | Pull resistor | Read pin state |
- |:------------:|:------------------:|:---------------:|:-------------:|:---------------:|
- | 1 | anode ->:- cathode | close | 0 pull-down | 1 active high |
- | 1 | anode ->:- cathode | open | 0 pull-down | 0 inactive low |
- <br>
- Arduino boards do not have internal pull-down resistors.
- If you want to use active low, you will have to add external pull-down resistors to the read pins.
-
- To make a keyboard active high:
- * Add an external 10k pull-down resistor to each read pin
- * Orient diodes with cathode (banded end) towards the read pins
- * Define strobe on and off in the sketch like this:
- ```
- const bool Scanner_uC::STROBE_ON = HIGH;
- const bool Scanner_uC::STROBE_OFF = LOW;
- ```
-
- Making a breadboard keyboard active-high
- ----------------------------------------
- This tutorial converts the basic breadboard keyboard from tutorial 1 to active high.
- By comparing the above tables, one can see what changes need to be made:
- * add external pull-down resistors to the read pins
- * flip the diodes so that the cathode (banded end) are towards the read pins
- * swap the STROBE_ON and STROBE_OFF values
-
- The red bus is grounded.
- The pull-down resistors plug into the red bus and column read pins.
-
- The [keybrd_6_active_highsketch.ino](keybrd_6_active_high/keybrd_6_active_high.ino) is the tutorial 1 sketch with STROBE_ON and STROBE_OFF values swapped.
-
- ![pull_down_resistors.JPG](keybrd_6_active_high/pull_down_resistors.JPG "Active-high diodes and pull-down resistors")
-
- <br>
- <a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png" /></a><br /><span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">keybrd tutorial</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/wolfv6/keybrd" property="cc:attributionName" rel="cc:attributionURL">Wolfram Volpi</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>.<br />Permissions beyond the scope of this license may be available at <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/wolfv6/keybrd/issues/new" rel="cc:morePermissions">https://github.com/wolfv6/keybrd/issues/new</a>.
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