expandergpioi2craspberry pi
I'm quite new to electronics and just trying to do my first Raspberry Pi based system: one diode, one pressure sensor and LCD 1602 with keypad shield (the one for an Arduino board). The way I'd like to connect all the devices is using I2C (the Raspberry Pi doesn't have much GPIO).
And here comes the question: How can I possibly connect non-I2C devices to the I2C bus? I heard about different expanders, like the MCP23017, will that solve the problem?
There are standard RS422/RS485 transceiver ICs available
Here you'' find many pages of RS485 related ICs at Digikey
You can use standard asynchronous serial data using RS485 if desired. "Rolling your own" multistation code should not be too hard [tm], but it's an oft invented wheel - there will be many people already offering software (much for free) that does this.
The CAN bus lives on top of RS485, but is probably overkill for what you want.
AN-218 Implementing a RS-485 Multi-drop Network
This is targeted at ZBASIC but is liable to be of direct relevance to your application.
RS485 Multidrop Wiring Diagram & RS232 - RS485 comparison
They say -
RS485 is sometimes termed as RS485 Multidrop LAN since it can connect several devices in a LAN network environment. These devices are all connected to a single pair wire. Transmit and receive share the same two wires.
Officially the RS485 specification allows only 32 nodes (devices) on the LAN. However, I.C. manufacturers have developed RS485 drivers capable of allowing 128 to 255 nodes on an RS485 LAN. We use these next generation RS485 drivers in our products. This means that you can use our Converters and Remote I/O devices in more expansive situations.
There are specialized ICs for I2C level translation. PCA9306 (down to 1.8V),TCA9517 (down to 0.9V), FXMA2102 (down to 1.65V). IC level shifter would be my choice.
@Laszlo had mentioned AN97055 by Philips (now NXP).
Excerpt from that app note (p.13).
3. CHARACTERISTICS OF THE MOS-FET’S.
The requirements for the most important characteristics of the MOS-FET’s, used as bi-directional level shifter in an I2C-bus system with max. 6V and min. 2.7V levels, are listed below. The values are intended as an indication and may be adapted for other supply voltages, other logic levels and/or other applications.
Type : N-channel enhancement mode MOS-FET.
Gate threshold voltage : VGS(th) min. 0.1V max. 2V
On resistance : RDS(on) max. 100 Ohm @ ID = 3mA, VGS = 2.5V
Input capacitance : Ciss max. 100 pF @ VDS = 1V, VGS = 0V
Switching times : ton toff max. 50 ns.
Allowed drain current : ID 10 mA or higher.
Best Answer
It is possible that the question represents a misunderstanding or miscommunication of the kind of connection desired: Not something to be connected to the I2C bus itself, but to be controlled via some intermediary I2C to GPIO device, which further is connected on the I2C bus.
The idea of non-I2C devices being connected to an I2C bus is meaningless. The I2C protocol has specific requirements, for the I2C master to address the slave device, and for communicating data in either direction.
The LCD mentioned in the question, for instance, works on a different set of communication protocols, using multiple GPIO pins, and (from the description) most likely does not recognize I2C. There are however I2C LCD modules available, which incorporate I2C slave functionality on board - either through an LCD controller or a dedicated microcontroller with I2C slave communication code. Instead of the basic LCD module mentioned, such an I2C display module would be required for use on an I2C bus.
I2C bus expanders cannot magically create I2C functionality on a connected device. What I2C bus expanders can do, however, is provide additional GPIO pins, which are then controlled by code from the Raspberry Pi or microcontroller through the expander, which itself responds to I2C commands.
Perhaps that is what is desired - in which case, sure, it will work but not as described: A GPIO-driven device such as the LCD module or an LED can be wired up to the expander's GPIO pins, and the expander is the device that is connected to the I2C bus.