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Low Power ESP32 Handheld

Pocket sized ESP32 display board with 300µW Always On Display

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This handheld board is powered by an ESP32 and features a transflective Sharp memory LCD.
Similar to my previous Chronio smartwatch the focus of this project is on low power consumption.
Using the ESP32's ULP core, the board can go into deep sleep with an active display.
The software includes a menu interface with a simple RSS reader.


Some of the key features are:
- 400x240px 2.7" SHARP memory display
- 350 mAh LiPo battery with USB charging
- Always On Display with 300 µW power consumption
- 4-way joystick and buttons
- Date and time using built in RTC with NTP sync
- RSS Feed / Website parser

Layout files and Code on GitHub: https://github.com/CoretechR/ESP32-Handheld

  • 1 × SHARP LS027B7DH01A
  • 1 × ESP32 Wroom
  • 1 × 350 mAh 3.7V LiPo

  • Low Power Considerations

    Max.K12/24/2019 at 14:28 6 comments

    While the battery can power the handheld for only a few hours, I wanted to include a standby mode that utilizes the low power display. This way the device would be usable as a clock and calendar. There are a few things necessary to keep the display active and the battery life long. Although the memory display does not need constant refreshing over SPI, a >1Hz clock signal must be supplied to its EXTCOMM pin. Otherwise the screen shows burn-in. This means that part of the ESP32 needs to remain active during standby mode.


    At 70-200mA, the ESP32 can be power-hungry with its dual cores and WiFi modem. But not everyone knows that it has some nice power saving features. In sleep mode the ESP32's main cores are powered down, reducing the current to only 10µA. During this, the internal Real Time Clock controller is still active. The RTC itself can't do much, but it can wake the ESP32's third core, the ULP coprocessor. This Ultra Low Power processor can do basic tasks like checking sensors or reading and writing pin states while only needing ~150µA. Exactly what it need for the 1Hz signal. The ULP can be woken up periodically by the RTC using:

    ulp_set_wakeup_period(0, 1000 * 1000);

    For the 1Hz signal the ULP coprocessor has to be programmed in assembler. Luckily Espressif has an example for a blinking LED that does exactly what I wanted:

    https://github.com/espressif/esp-iot-solution/blob/master/examples/ulp_examples/ulp_rtc_gpio/main/ulp/rtcio.S

    The ESP32 can activated by different wakeup sources. For this project GPIO interrupts are needed as the device should wake up with the press of a button. The two external wakeups are ext0 and ext1. While ext0 can only be assigned to one pin, ext1 can be assigned to a map of different pins. There are a few things to consider when using the ULP core: Only the pins that can be accessed by the RTC can be used (RTC GPIO). Also I found that the internal pullup/pulldown setting are not reliable. I used hardware pulldowns for my design.

    // This wakes the ESP32 with buttons on GPIO32 & GPIO33
    #define BUTTON_PIN_BITMASK 0x300000000
    esp_sleep_enable_ext1_wakeup(BUTTON_PIN_BITMASK,ESP_EXT1_WAKEUP_ANY_HIGH);

    I want to update the time every full minute. This can't be done without waking the ESP32 for a short time:

    esp_sleep_enable_timer_wakeup((60-now.tm_sec) * 1000000);

    With the standby mode configured, the ES32 should only draw a few µA with short spikes when the ULP or the main cores are woken up. But there are other components on the board that need to be powered as well. The display itself draws another 10µA but also needs a 5V supply. As the LiPo battery is only at 3.7V a DC-DC converter is needed. After some searching around I found the MCP1640 from Microchip. It's a boost regulator with a quiescent current of just 19µA. The IC is small and only needs a couple of external parts. The 3.3V power supply for the ESP32 was a bigger problem. It turns out that it is hard to find a regulator with a high output current but small quiescent current, that can also be hand soldered. I ended up with the AP2112K, which can supply 600mA with a quiescent current of 55µA. 

    Once I had the actual PCB, testing the real power consumption was also tricky. The shunt resistance for the µA-range on most multimeters is too high to power up the ESP32. And constantly switching between ranges or swapping wires was to tedious. A user on stackexchange came up with a clever solution (https://electronics.stackexchange.com/questions/340330/measure-wide-range-of-current-800-%c2%b5a-1-5-a/340353#340353): 

    A diode in parallel to the multimeter will limit the voltage drop when the current is high. In sleep mode the diode barely lets current though. This way I was able to measure the power consumption in sleep mode conveniently with my 20$ multimeter. 

    With the ESP32 in sleep mode the current is at around 97µA. 59µA are caused by the 3.3V power...

    Read more »

  • Introduction

    Max.K12/24/2019 at 11:15 0 comments

    Earlier this year Panic, the software studio behind the Untitled Goose Game, introduced their Playdate console. It's a retro handheld device with a monochrome display and a crank as an input device: https://play.date/. I really liked the simplicity of it and decided to make something similar. It was also a chance to improve on my 2016 Chronio Smartwatch concept which features a similar display but only an ATmega328.

    The display board is using the same 2.7" Sharp memory display as the Playdate. These displays only consume a few µAs and because of their in-pixel-memory they don't need constant refreshing. They are also readably under direct sunlight with no need for backlighting.
    As its brains the board is using an ESP32. The dual core chip has enough processing power to drive the display while also featuring WiFi and Bluetooth. And it has a ULP co-processor for low power applications. I decided to include a small 5-way joystick and two buttons to enable simple games. As I wanted the device to work as a bare PCB without 3D-printed parts, a mechanical crank was out of the question.

    The size of the needed PCB is defined by the display and buttons. With the display on the front, the ESP32 and all other components fit nicely on the back of the device. A milled slot enables the display flex cable to go through the PCB and into its connector.

    I was able to reuse some of the parts and circuits from my ESP32 robot but it was challenging to keep the power consumption to a minimum. More on that in a separate project log.

View all 2 project logs

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Discussions

Jose Baars wrote 01/08/2020 at 22:46 point

For a 3.3V LDO take a look at the XC6220. A few (20-60 mV depending on load) dropout and down to 8 uA when no power is required.

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Jan wrote 01/09/2020 at 12:01 point

Nice part, it does automatically switch into low Iq mode under 1mA output load! With 0.2€/pc buying 10 it's not expensive too https://lcsc.com/product-detail/Low-Dropout-Regulators-LDO_Torex-Semicon-XC6220B331MR-G_C86534.html

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ostropest wrote 01/10/2020 at 19:05 point

A co myślisz o panelu słonecznym? chodzi o to by cały czas jednak doładowywać. Trzeba by jakas elektronikę dorobic?

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kwapiszon wrote 01/08/2020 at 17:04 point

nice

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Anthony DiGirolamo wrote 01/07/2020 at 19:54 point

This is amazing, thank you for sharing! I wasn't aware the esp32 had an ULP core. Any interest in making a v2 with a keyboard? I have a prototype board that uses the http://www.ti.com/lit/ds/symlink/tca8418.pdf See the project logs here: https://hackaday.io/project/162281-teensy-thumb-keyboard/ 

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Max.K wrote 01/07/2020 at 20:38 point

Thank you! I was considering using something like the blackberry keyboard at first. But it would have ended in a much larger device. Maybe I'll be able to include one in  a second version.

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ostropest wrote 01/04/2020 at 23:21 point

You are planing create kickstarter/ indeigogo campainin in future?

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Max.K wrote 01/07/2020 at 20:29 point

I'm not planning to do that.

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Elliot Williams wrote 01/04/2020 at 19:31 point

That is sexy!  The ULP is one of the coolest parts of the ESP32, IMO.  You make great use of it here.  

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ostropest wrote 12/24/2019 at 13:50 point

Is possible to add solar panel, for power for screen and meybe for other system.

meybe in bottom (other side than main screen) , small hole for carry on rope.

Please think about more memory for mruby.org it need 400KiB memory and of course more buttons /key ;) 

This is very simmilar idea xircom Rexx [56]000. This is very good idea! http://www.reviewsonline.com/articles/976862308.htm on network are still exist some developers on this machine similar Newton machine.

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Max.K wrote 01/03/2020 at 12:29 point

Hi, sorry for not answering earlier. 

A solar panel on the backside would be cool, not sure if this is practical.

Including a hole for a strap is a really good idea for a second version.

I don't know what the requirements for mruby are. 400KB of what - flash or SRAM?

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ostropest wrote 01/04/2020 at 23:20 point

Meybe this will be more clarify

https://github.com/kyab/mruby-arduino

https://github.com/mruby-esp32/mruby-esp32

https://www.youtube.com/watch?v=EXvDKb2LTYM

Solar panel on bottom have sense. Make a test this are good start https://blog.voltaicsystems.com/powering-a-raspberry-pi-from-solar-power/ today solar panel are very good in small light

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