The motivation of this project was to develop a low power calculator focused on the needs of firmware engineers. Inspired by my love of the TI-36X calculator and simultaneous frustration for its lack of support for computer science operations, the 'HX-36X’ was born.

Firmware and Hardware design available at Octal Github

Programmers Calculator

Hardware Overview

• MCU / Memory

  • STM32L476VGT6 low power microcontroller

  • W25Q128 QSPI Flash

• I/O

  • Sharp LS027B7DH01 Memory LCD

  • USB Micro port for li-ion charge / firmware updates

  • 50 tactile switches

  • ST SPBTLE-RF Bluetooth module to allow clipboard push/pull from computer

User Interface

Optimizations implemented to extend battery life.

The device is low active duty, so standby processor/component efficiency is a top priority. This consideration was taken into mind when selecting potential end product components. This led to the selection of the STM32L476 processor, which is designed for low power applications. Among other features, this processor allows for power savings by doing things like running the core at a lower voltage than peripherals to reduce power consumption and allows for deep sleep and standby modes. To achieve power savings via firmware the processor is put into low-power sleep mode [AN4621] until an external interrupt triggers a wake. After the interrupt is received inputs are polled, processed and output to the user. After this the processor is put back into its sleep state again. Keeping processor and component wake time low is responsible for the majority of the devices power savings.

Power consumption calculations:

[Processor wake time x processor wake consumption] + [Processor sleep time x processor sleep consumption] + [Bluetooth tx/rx consumption x Bluetooth transfer time] + [Bluetooth sleep consumption x Bluetooth sleep time] + [assorted component quiescent currents] = Total power draw of system

 Schematic of STM32L4 microprocessor

 Schematic of USB and lithium ion charging