|The main battery supply is switched off, however the LED blinker program continues to operated from USB power. The added breadboard circuit controls the brightness of the LED.|
Experiment 3 - Running on USB Trickle Power
Last time we disabled BOE (Brown Out Enable) so that our simple prop board could run on low power. We then asked the question, "How low can we go?" Exploring and experimenting led to a discovery.
First we ran the LED blinker program. Then we shut off the 3.0 volts battery power supply expecting the program to shut down and the LED to go off. However, in a darkened room, the LED was dimly continuing to blink, and the program stayed in memory continuing to run properly.
waitcnt(clkfreq*2 + cnt)
The program is modified to slow down the LED blinker so the meter can settle to get a good reading of voltage drop across the LED.
Run the program with the power supply connected. Remove the power supply. With the USB port connected, the prop chip has a 2.32 voltage high from P15 when the circuit's power supply is off and the USB cable remained connected.
Tests were performed on an iMac with an unpowered four port HUB. When the power supply is on, the prop chip has a 3.0 volt high from P15. The simple way to increase the LED brightness is to use a 5 volt LED or a high intensity LED, carefully controlled without the dropping resistor. When switching back to the full 3.0 volts power supply, the resistor must be reconnected beforehand.
A transistor circuit can also boost the power to the LED. The illustration shows an experiment. The shown circuit is found in Parallax' book, "What's a Microcontroller."
When the main power is shut off, the program continues to run and the LED continues to blink. When the USB is disconnected, there is no power to the prop circuit and the program is lost. With USB power attached again, without the mains power supply, the prop is not detected and the program will not reload.
One idea is to place the amplifier circuit in between the battery power supply and the prop circuit to see if the programs will continue to load on USB power only. The most simple approach - substitute a 5V LED and the LED dropping resistor can be removed to increase the LED brightness. When switching back to the full 3.0 volts power supply, the resistor must first be reconnected. Or simply route the USB +5 Volt line to become the prop's power supply.
LINKS TO PROPELLER SIMPLE SPIN BOARD EXPERIMENTS
Experiment One - Bare Bones Circuit
Experiment Two - Disabling BOE for Low Power Operations
Experiment Three - Running on USB trickle power
Experiment Four & Five - Improving Reliability