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BEE WATCH MK1

Introduction

I am very interested in making small gadgets so this is my first attempt to working on a simple OLED watch, I was inspired from N|Watch project, My version is arduino compatible.The main component of this watch is it’s RTC, The RTC IC used here ISL1208 and my friend Vishnu M Aiea is developed the arduino library for ISL1208, If you are interested in that please take a look to his documentation about ISL1208 .


Specification

  • 1.3” OLED Display
  • ATMEGA328P Microcontroller
  • Alarm feature
  • Timer
  • BPM Sensor
  • Temperature Sensor
  • Game

HARDWARE

BEE WATCH MK I SCHEMATIC
On the hardware side the watch contains an Atmel ATmega328P microcontroller, 2.5V regulator, ISL1208 RTC IC from Intersil RTC, 1.3″ 128×64 monochrome OLED, Heart sensor, a buzzer sounder, 3 way switch for navigation, powered by a 150mAh LiPo battery which can be charged via Mini USB

The ATmega328P uses its internal 1MHz oscillator and runs on 2.5V from a linear regulator. Its current draw is around 1.5mA when active and 100nA in sleep mode.Here I used MiniCore instead of Optiboot

The Intersil ISL1208 is a low power RTC chip with I2C interface. It uses an external 32.768KHz crystal to keep track of the time and has month-date-hour-min-sec alarm registers. It only consumes around 400nA in battery (VBAT) operation and a maximum of 1.2uA on external supply voltage (VDD). The operating voltage is from 1.8V to 5.5V. What makes this a good candidate are the low power consumption and the month-date alarm feature. Normal RTCs such as DS1307 do not have a month setting in alarm register.

The battery charging circuit uses a Microchip MCP73832 along with some additional components for load sharing, where the battery can charge without the rest of the watch interfering with it.

As the microcontroller is running on 2.5V the battery voltage needs to be brought down a bit to obtain an ADC reading. This is done by a simple voltage divider. However, with the voltage divider connected across the battery there would be a current of around 350uA constantly flowing through it, this is a huge waste of power. A P-MOSFET (and some voltage level conversion for it, which I forgot about in the first version so it was always stuck on) was added so the divider can be turned on only when needed.

The 2.5V regulator being used is a Torex XC6206, primarily chosen for its tiny quiescent current of just 1uA.

Why a linear regulator and not a switching regulator? The switching regulators I looked at had an efficiency of at least 80% with a 2mA load, but that efficiency quickly dropped off to less than 50% with loads of 100uA. Since the devices connected to the regulator draw 2-3uA in sleep mode, a switching regulator would have performed incredibly poor compared to a linear regulator. The 2.5V linear regulator efficiency is 60% with 4.2V input going up to 83% with 3V input.

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