This is my concern using an I2C pull-up to 3.3V using a 5V processor:
From the Atmel ATmega328P datasheet... section 2.8, page 264. Vin High Min spec attached to this message as an image, full data sheet here:
iI2C input High voltage min: 0.7 * Vcc.
0.7 * 5.0 is 3.5V... nominal performing devices miss spec. 3.6V if the L7805 is at the top of it's specified output range (5.2V). Let the 3.3V regulator be on the low side and margins are even worse.
While we are at it, do we have any real control over the condition of the cable used? The electrical noise in the environment?
The pressure sensor at the end of the I2C line is measuring pressures going into a lung. Do you really want to skimp on a safety critical measurement on a life support device.?
Come on... Really?!?
Please someone tell me - what am I missing?
Bob
