smôl Power Board AAA Hookup Guide

Contributors: PaulZC
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Hardware Overview

In this section we'll cover what this board does and what is included on the smôl Power Board AAA.

What Does This Board Do?

What does this board do? Excellent question! Let's answer that right now.

The main job of the Power Board is to provide power for your smôl ecosystem. smôl is designed to be both small in size and small on current draw. The Power Board plays a critical role in reducing the current draw of smôl as much as possible. This board has a TPS61200 switching regulator on it and is designed to boost the voltage from one or two AAA or AA alkaline cells up to 3.3V. But it also comes with an ATtiny43 AVR microcontroller on it. This is an intelligent power board for the smôl ecosystem!

When you want your project to go into a low power or deep sleep state, it is standard practice to put your processor into the lowest power state it offers. But you may find that the current draw is still something like 100µA to 500µA depending on which processor you are using. Then there is the quiescent current drawn by the voltage regulator. The quiescent current is the current drawn by the regulator when it is on but has no load attached. Again, it varies from regulator to regulator but you may find this adds another 55µA to your current draw. Those little currents add up and reduce your battery life considerably.

With smôl, we took a new approach. The Power Board takes control and provides the 3.3V power for the whole smôl stack. When instructed to by the Processor Board, the on-board ATtiny microcontroller will turn off that power completely and then put itself into a low power sleep state for a pre-defined interval. Sleep intervals from a few 10's of milliseconds to several days are possible. We have used the fantastic Otii Arc Power Analyzer to help us reduce the sleep current to just 12µA. Yes, you read that right, 12 microamps! smôl makes it possible for your battery life to be measured in months not days!

Battery Connector

By far the biggest component on the board is the JST-PH connector for the battery.

Pictured is the Battery Connector

Battery Connector Polarity: Please make sure that you use one of our recommended battery holders or cables. Some battery holders use the same JST connector as ours but have the opposite polarity. Connecting one of these to your smôl power board will destroy it as the board has no reverse polarity protection. If you are going to use your own battery holder or cable, it is up to you to ensure it has the correct polarity. In the above photo, the positive pin is on the right.

ATtiny43U AVR Microcontroller

The smôl Power Board AAA is an intelligent power board. The on-board ATtiny43U microcontroller can monitor the battery voltage and other parameters. When the Processor Board requests it to, the ATtiny43 will turn off the smôl 3.3V power completely and place itself into deep sleep for a pre-defined interval. This is what makes it possible to reduce the sleep current to just 12µA.

Pictured is the ATtiny43 microcontroller

The ATtiny43U may seem a strange choice, but actually it is the perfect choice for this board. We use it because it has a built-in boost converter and can operate directly from voltages as low as 0.7V. It does not need a separate voltage regulator.

Our SparkFun smôl Power Board Arduino Library does all of the heavy lifting for you, making it easy to communicate with the ATtiny through simple function calls.

A Note About Temperature Sensing

The ATtiny43 has a built-in temperature sensor, which can be accessed via the SparkFun smôl Power Board Arduino Library. However, the quoted typical accuracy of the sensor is ±10°C. Yes, plus or minus ten degrees C. The sensor measurement requires external calibration to be useful.

Voltage Regulator

The smôl Power Board AAA has a TPS61200 switching regulator on-board. That's the same regulator we use on our LiPower - Boost Converter. It can deliver approximately 550mA when boosting 1.5V up to 3.3V, and approximately 1500mA when boosting 3.0V up to 3.3V. It can be switched off completely by the ATtiny43U microcontroller, avoiding the 55µA quiescent current draw.

Pictured is the TPS61200 switching regulator

The TPS61200 can operate in Buck mode too and regulate higher voltages down to 3.3V. However, we do not recommend operating the board in this mode as the ATtiny43U's VCC supply will also be at that same higher voltage, leading to unexpected behavior and complications. Never exceed 5.5V or you will damage the ATtiny43U.
Ye cannae change the laws of physics! The current draw from a single AAA or AA cell will be approximately three times what smôl is drawing at 3.3V. When powering the smôl ARTIC R2 satellite communication board you are likely to have to use two AA cells for best results.

FPC Connections

Like all of our smôl boards, the Power Board AAA is equipped with two 16-way 0.5mm-pitch Flexible Printed Circuit connectors. FPCs are used to link the smôl boards together in a daisy-chain.

The pin-out for the smôl Power Board AAA is as follows:

Connector Pin No.Signal NameFunctionNotes
1PROC_PWR_ENProcessor Power EnableThe power board pulls this signal low to disable the voltage regulator on the power board
23V33.3V Power Rail
3GNDPower Ground / 0V
4SCLKSPI ClockNot used
5COPISPI Controller Out Peripheral InNot used
6CIPOSPI Controller In Peripheral OutNot used
7CS0SPI Chip Select 0Not used
8CS1SPI Chip Select 1Not used
9CS2SPI Chip Select 2Not used
10GPIO0General Purpose Input / Output 0Not used
11GPIO1General Purpose Input / Output 1Not used
12SDAI2C DataUsed for ATtiny43U communication
13SCLI2C ClockUsed for ATtiny43U communication
14GNDPower Ground / 0V
153V33.3V Power Rail
16V_USBUSB Power Rail (5V)USB power is used to recharge the battery

The IN and OUT pin connections are identical on the smôl Power Board AAA. (That's not always true on smôl Peripheral Boards. Check the appropriate Peripheral Board Hookup Guide for full details.)

The order in which you connect smôl boards is important. However, because the Power Board AAA only uses I2C communication, it can be placed anywhere in the stack. Please see the smôl Hookup Guide for more details.