77 lines
3.9 KiB
Markdown
77 lines
3.9 KiB
Markdown
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Tutorial 6 - Active high
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=========================
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This tutorial pulls together several concepts needed to understand active state in the context of a keyboard.
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Skip to the end of this tutorial if you just want to copy an active-high keyboard.
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## Pull-up resistors
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There are many sources that explain "pull-up resistors", so I won't repeat it here.
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Here is a [good tutorial on Pull-up Resistors](https://learn.sparkfun.com/tutorials/pull-up-resistors/what-is-a-pull-up-resistor).
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## Active low
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All the preceding breadboard keyboards in this tutorial series have used active low with internal pull-up resistors.
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"Active low" means that if a switch is pressed (active state), the read pin is low.
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When the switch is released (inactive state), the pull-up resistor pulls the read pin high.
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Active low requires pull-up resistors.
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The following table traces the strobe current from left to right (0 is ground, 1 is power).
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If the switch is closed, the strobe current passes through the switch and pulls the read pin low.
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If the switch is open, the pull-up resistor pulls the read pin high.
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|Strobe pin on | Diode orientation | Switch position | Pull resistor | Read pin state |
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|:------------:|:------------------:|:---------------:|:-------------:|:---------------:|
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| 0 | cathode -:<- anode | close | 1 pull-up | 0 active low |
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| 0 | cathode -:<- anode | open | 1 pull-up | 1 inactive high |
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Arduino boards have internal pull-up resistors, which saves on parts and labor compared to manually adding external pull resistors.
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If you are designing a keyboard, go with active low.
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To make a keyboard active low:
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* Use internal pull-up resistors if the IC has them
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* Orient diodes with cathode (banded end) towards the write pins (row)
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* Define strobe on and off in the sketch like this:
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```
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const bool Scanner_uC::STROBE_ON = LOW;
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const bool Scanner_uC::STROBE_OFF = HIGH;
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```
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## Active high
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"Active high" means that if a switch is pressed (active), the read pin is high.
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When the switch is released (inactive), the pull-down resistor pulls the read pin low.
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Active high requires pull-down resistors.
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The following table traces the strobe current from left to right (0 is ground, 1 is power).
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If the switch is closed, the strobe current passes through the switch and pulls the read pin high.
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If the switch is open, the pull-up resistor pulls the read pin low.
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|Strobe pin on | Diode orientation | Switch position | Pull resistor | Read pin state |
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|:------------:|:------------------:|:---------------:|:-------------:|:---------------:|
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| 1 | anode ->:- cathode | close | 0 pull-down | 1 active high |
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| 1 | anode ->:- cathode | open | 0 pull-down | 0 inactive low |
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Arduino boards do not have internal pull-down resistors.
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If you want to use active low, you will have to add external pull-down resistors to the read pins.
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To make a keyboard active high:
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* Add an external 10k pull-down resistor to each read pin
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* Orient diodes with cathode (banded end) towards the read pins
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* Define strobe on and off in the sketch like this:
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```
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const bool Scanner_uC::STROBE_ON = HIGH;
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const bool Scanner_uC::STROBE_OFF = LOW;
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```
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## Making an active-high keyboard
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This tutorial converts the basic breadboard keyboard from tutorial 1 to active high.
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By comparing the above tables, one can see what changes need to be made:
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* add external pull-down resistors to the read pins
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* flip the diodes so that the cathode (banded end) are towards the read pins
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* swap the STROBE_ON and STROBE_OFF values
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The red bus is grounded.
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The pull-down resistors plug into the red bus and column read pins.
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The [keybrd_6_active_highsketch.ino](keybrd_6_active_high/keybrd_6_active_high.ino) is the tutorial 2 sketch with STROBE_ON and STROBE_OFF values swapped.
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