simple bus level-shift / I2C mosfet level converter Fairchild BSS138

Posted on Feb 5, 2014

While doing some background reading about the Philips I2C bus I came across an application note written by Herman Schutte (of Philips Semiconductors Systems Laboratory in Eindhoven). Mr. Schutte described an elegant mechanism for interfacing sections of I2C-bus which are operating at different voltages through the use of MOSFETs and pull-up resist

simple bus level-shift / I2C mosfet level converter Fairchild BSS138
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ors. I had also been looking at a number of options for interfacing a 5V LCD panel to a microcontroller system that I wanted to run at 3V and so decided to try applying this idea to the data and control lines in my circuit. Although I was building the circuit by hand I decided to use a surface mount package for the MOSFETs. As three-terminal devices they are relatively easy to solder as long as you can hang onto them. The image above on the right shows some of the 11 MOSFETs in use on the 0. 1" proto board. I used the Fairchild Semiconductor BSS138 (datasheet) N-channel enhancement mode MOSFET which is available in the standard SOT-23 package from folks like Digi-Key. Below on the right are wave forms for the circuit using three different values for the pull-up resistors: 100k, 10k and 4. 7k ohms. These values were picked from what I had close at hand. The circuit functions with any of them in fact. A basic drawing for the circuit I`m using is here. (or here for browsers without JavaScript. ) As the schematic shows, the circuit is remarkably simple and straight-forward. I have this circuit in-use with an ATMEL AT90S8515 (at 3. 6864Mhz) controlling a Seiko L203400J000 20x4 character LCD module. The 8515 is connected to a 3V supply while the LCD requires 5V. The control lines, RS, RW and E and the data lines D0-D7 are each level-shifted with the BSS138 MOSFETs. The level-shifting imposes no directionality on the bus so both...

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