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SparkFun Soft Power Switch - JST 2mm
SparkFun Soft Power Switch - JST 2mm (PRT-26993)
The Soft Power Switch - JST 2mm is a passive, hard on/off switch with software feedback and control. In other words, it's like the on/off switch on a laptop. A simple press will turn the system on. Another press can (with MCU intervention) turn off the system. And if things go really wrong, pressing and holding the button for ~7 seconds will force a power-down. If you're building something with an enclosed battery and need a good power button, this is the board you need.
The Mk1 Soft Power Switch is great, but we do see occasional issues caused by component tolerances or the operating temperature. Mk2 is based on an application note by Texas Instruments, and is a completely digital design. It is based around a 74LVC1G175 D-type flip-flop - remember those?! - and is insensitive to temperature and component variation. It also includes improvements like: an over-current self-resetting 2A fuse; input and output LEDs. This version has JST 2mm battery connectors.
The PUSH pin can act as an output to a microcontroller indicating the state of the power button (high = not pressed, low = user is pressing power button). This can be used as an input to your firmware to begin to shut down before power is lost. PUSH is an open-drain output and requires a pull-up. Alternatively, the OFF pin can be driven high by the system forcing fast power off via software. (For Mk1, the Fast Off pin was driven low.)
The most common use case is something like this:
- Microcontroller begins running code and checks to see if the power button is still being pressed for X milliseconds.
- If not, it's just a tap, go back to sleep.
- If yes, power up and run normal program.
Later...
- User presses power button in normal program
- If power button is pressed for <X milliseconds, it's just a tap, ignore.
- If power button is pressed for >X milliseconds, it's time to power down.
The wake-up and power-down times can be tweaked for your own user experience but we found 500ms for power up and 2s for power down work really well.
If needed, pads are provided for a larger, external button.
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