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Barometric Altitude and Temperature Sensor for DragonOSD V2
Details:, Micro barometric altitude and temperature sensor for the DragonOSD., Specifications:, 0.1hPA/ 0.1C resolution, Very important note from the developer:, , I2C is a very popular communication bus. It has two signal lines: clock (SCL) and data (SDA). In a normal I2C circuit, each of these lines is connected to each I2C component on the bus as well a pull-up resistor (usually somewhere between 4k and 10k). I2C doesn't to my knowlege stricly define what voltage that pull-up value is but each component has limits on what it can handle. For example, the BMP085 barometer maxes out at 3.6v., So there are a few possible fixes:, The one what we have been discussing in the last few posts: run ArduIMU on 3.3v throughout. You still get extra pull-ups but at least the pull-up is to a safe 3.3v. To do this, put 3.3v on 5V pin on the FTDI connector and on the 3.3V I2C bus connector., The other way to fix this is that an ArduIMU updated firmware could be developed that modifies these lines in twi.c to turn off the pull-ups:, , So ArduIMU users: before you add 3.3v sensors to your bus, check their tollerances and make this update if they can't handle 5V.,
Pin header is included as optional pass-through for other I2C devices.
Optional I2C pass-through
I2C Interface (3.3V)
BMP085 Bosch sensor
A warning to ArduIMU users that are using other 3.3v sensors on the I2C bus (e.g. BMP085-based baro/altimeter).
We have seen baros getting fried in this configuration and with the help of KrisW we have traced down the culprit. But first some background to get everyone up to speed.
The ATMega328 the ArduIMU uses has on-board pull-up resistors that can be be enabled through software. But if you power the ArduIMU via its power supply input (6v+) or by putting 5V on the servo line, the I2C bus is pulled up to 5v. THIS WILL DAMAGE I2C DEVICES THAT DO ARE NOT 5v TOLLERANT, EVEN IF THE I2C POWER LINE HAS ONLY 3.3V ON IT. (Ask KrisW--there's a package of two new baros in the mail to him today). If you put a voltmeter on it you won't see 5V because the digital data on the line will average out to somewhere closer to 4V on voltmeter.
// activate internal pull-ups for twi
// as per note from atmega128 manual pg204
sbi(PORTD, 0);
sbi(PORTD, 1);
But it's a messy fix because it would require forking off the Wire libraries in Arduino.
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Found 11 years, 7 months ago