project-wearwulf-power
| index | WikiPages | AdditionalFiles |
Wearwulf will be battery powered via an external battery connector, with the facility to allow the hotswapping of battery packs. This same connector will allow an external regulated power supply to be used, for development purposes.
Wearwulf will need to operate with a number of voltages:
| Volts | Used by |
|---|---|
| 1.8V | Philips LPC2106(core) & CoolRunner II(core) |
| 2.0V | Kopin 320M CyberDisplay |
| 3.3V | Philips LPC2106(io) & CoolRunner II(io) & Geek Port |
| 9.0V | Kopin 320M CyberDisplay |
In order to conserve power the display must be capable of being switched off, i.e. the processor should be able to switch off the 2.0V and 9.0V power supplies at will. The remaining components, the processor, mass storage device etc, use relatively little power, and most have low power shutdown modes.
The ability to swap power supplies on the hoof, allows continuous use, with one battery charging while another is in use. (Not including the battery in the main unit frees the owner to choose the size and type of battery that suits them.) Hot swapping will require some form of energy store to retain the processor's internal state while the supplies are changed over.
Finally, to assist the owner, some form of fuel gauge is required to indicate the remaining battery life, so that the user can keep a spare battery to hand.
The »Linear Technology LT1761 series are micropower low dropout regulators capable of supplying upto 100mA of current, and can be switched off by a logic low on their shutdown pin.
Available in 1.8V, 2.0V and 3.3V versions, the regulators are available in tiny SOT-23 packages, and the only additional components required are a small input and output capacitor.
The »Linear Technology LT3464 series adjustable step-up regulator will be used to provide the 9.0V supply for the Kopin 320M CyberDisplay. Like the LT1761 it can be switched off by applying a logic low to its shutdown pin.
Wearwulf will need some form of energy store to maintain the LPC2106's state while the batteries are changed. If a capacitor is used, it will be necessary to ensure that it charges at a rate sufficient to start the regulators reliably. Regardless of the type of energy store, some mechanism will be needed to determine that the batteries are low, or removed, so that the processor can switch off the power consuming components such as the display.
A manual reset button should be provided to ensure that the Wearwulf can be restarted quickly in the advent of a software error. Without such a button, the batteries would have to be removed, and the hotswap energy store depleted before the system would reset.
Wearwulf is intended to be used 24-hours a day, and therefore has the ability to hotswap batteries. However, the user needs to know the state of the batteries so that they can plan to change them. Having the batteries fail without notice is not acceptable, even without data loss.
Wearwulf will use a »Atmel ATmega8L microcontroller to test the batteries' voltage to give an indication of teh power left. The microcontroller will be accessed as an SPI slave, and will be reprogrammable, allowing any additional resources (ADCs, GPIOs etc) to be made available to the LPC2106 processor as the Geek Port.
An approximate power budget has been created to give an indication of how long battery life might be expected to be. Pessimistic estimates have been used where-ever possible, and it is assumed that all components are continuously operating, including writing to the MMC card!
| Resource | Imax@1.8V | Imax@2.0V | Imax@3.3V | Imax@9.0V | Total |
|---|---|---|---|---|---|
| Philips LPC2106 | 50mA | 0mA | 5mA(guess) | 0mA | 55mA |
| CoolRunner II | 2mA | 0mA | 5mA(guess) | 0mA | 7mA |
| Atmel ATmega 8L | 0mA | 0mA | 5mA | 0mA | 5mA |
| Kopin CyberDisplay | 0mA | 2mA | 0mA | 3mA | 5mA |
| Video Driver | 0mA | 0mA | 12mA x2 | 0mA | 24mA |
| LED Backlight | 0mA | 0mA | 5mA | 0mA | 5mA |
| MMC Card | 0mA | 0mA | 35mA | 0mA | 35mA |
| Serial Port | 0mA | 0mA | 1mA(unloaded) | 0mA | 1mA |
| Lost in PSUs | 3mA | 1mA | 5mA | 1mA | 10mA |
| TOTAL | 55mA | 3mA | 85mA | 4mA | 147mA |
The 1.8V & 3.3V supplies are more than capable of supplying these currents, with the 3.3V supply being the most heavily taxed when writing to the MMC card. Assuming NiMH batteries with capacities of 1300mAh - 1800mAh, then a roughly estimated lifetime would be between 8.5 and 12 hours, even using these unrealistically pessimistic assumptions.
We should expect to get about 8 hours of continuous operation from a set of recharagable NiMH batteries - equivalent to a working day. The use of standard batteries will ensure that they can be readily, and cheaply, replaced.
(Powered by PWP Version 1.4.2)