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project:gsm:shield [2012/09/08 15:12]
jenda [brmbora anthem \o/]
project:gsm:shield [2013/01/12 06:23] (current)
jenda [Powering the phone]
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 The Darlington switch is actually ULN2003A. Single NPN transistor could be used too (peak current is about 200mA), FET probably too. Using Darlington has one disadvantage - the whole phone is at +0.6 (single silicon junction). Fortunately,​ our FTDI discriminates +0.6V as a correct LOG0. The Darlington switch is actually ULN2003A. Single NPN transistor could be used too (peak current is about 200mA), FET probably too. Using Darlington has one disadvantage - the whole phone is at +0.6 (single silicon junction). Fortunately,​ our FTDI discriminates +0.6V as a correct LOG0.
  
 + ==== Note on USB hubs and cables ====
 +
 +Some USB hubs have a diode between input and output V+. There is a voltage drop of about 0.4-0.5V. Just short the diode, we don't want this drop.
 +
 +Another voltage drop is on USB cable. Manufacturers tend to save copper, so they use very thin wires. I have seen a cable that had 2 ohms! This is drop of 0.8V at 400mA! The symptoms were simple - two or three phones worked, other didn'​t. Original FW was complaining about empty battery. Btw. osmocom Hello World FW measures the voltage too. Or just use your voltmeter...
  ==== Driving BTN_DRIVE and PWR_DRIVE ====  ==== Driving BTN_DRIVE and PWR_DRIVE ====
  
 
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