November 2004, Issue 172

Wi-Fi Sunlogger

CompactFlash Wi-Fi

I wanted to use Wi-Fi (802.11b) to communicate with the outside world. I was hoping to take advantage of a widely implemented wireless standard that supports TCP/IP communications. To do this, I used a Linksys Wi-Fi Compact-Flash module, which is based on the Intersil PRISM chipset. This card was designed to be used mainly in PDAs. Although low power is desired, this proves consumption isn’t critical, because most PDSAs are charged at the end of the day in the cradle. Wi-Fi is a good solution for PDAs because it’s reasonably low-powered, has a medium speed, and has good range. Compare this to Bluetooth, which has a potentially high speed but suffers from low range and high power consumption.

The first CompactFlash card I used consumes approximately 300 mA at 3.3 V when transmitting and 200 mA when actively receiving. It consumes approximately 90 mA when in Standby mode. Clearly, this wasn’t going to work because it would drain a 1,200-mAh battery in about 13 h, which isn’t enough for the system to survive a day when solar power is low.

My first attempts at reducing power consumption when not transmitting involved adding a MOSFET transistor to cut off power from the battery. If there is no power from the battery, then no power should be consumed by the card. Well, doing this, I still had a power consumption of approximately 8 mA by the CompactFlash card, which I tracked down to having some of the outputs to the card actually driven to a high state (VCC). This caused a current path through the protection diodes of any inputs on the CF to ground, because VCC on the card was now floating. Changing the output state in Idle mode to all low reduced the current consumption to what I expected.