Hi Terry On 06/06/18 13:53, Terry Coles wrote:
Pros and Cons of I2C and 1-wire Interfaces: This is to communicate with multiple devices connected to a Raspberry Pi. Two fundamental elements are relevant. I2C (literally Inter-Integrated Circuit) is only good for short distances, but has the advantage of allowing addressing as part of the protocol and sensor design. 1-Wire has longer range, but the devices arrive with their unique identifier hard coded into them by the manufacturer. After last night, I was unsure whether we would be able to implement a level sensor with 1-wire capability, since they tend to be very expensive and aimed at specific industries and therefore expensive. Our current level sensors are using hall-effect devices with a bit per level, so we clearly want to multiplex that data into a serial bus of some kind. This appears fairly easy with I2C, but not so easy (because of the unique ID requirement) with 1-wire. The jury's still out.
With I2C you are typically limited to just a small number of identical devices on the same bus, as they come preprogrammed with a 7-bit identifier, with the ability to change perhaps 1 or 2 of those bits (but if there are no spare pins available on the device, it won't be possible to select any address bits). 1-Wire devices come with a 64-bit identification code (8 bit family code, 48 bit serial number, 8 bit CRC). This allows multiple identical devices to hang off the same bus and be uniquely identified. Identification of all devices on the bus is via a search routine implemented in the master.
One problem with 1-Wire is that it requires time-critical pulse generation and measurement. The DS2282-100 IIC to 1-Wire bridge allows you to avoid time-critical stuff on the master processor.
In terms of multiplexing several Hall effect sensors, you could use a PISO shift register (eg 74HC165) and clock the bits out as with SPI.
For longer distances, I'd suggest a single chip micro to read the Hall devices and send the data over asynchronous serial (RS232, RS485 etc). An MC9S08SH8 is available in DIL package and a USBDM programmer can be bought on Ebay for around £20.
Cheers Tim -- Next meeting: Bournemouth, Tuesday, 2018-07-03 20:00 Meets, Mailing list, IRC, LinkedIn, ... http://dorset.lug.org.uk/ New thread: mailto:[email protected] / CHECK IF YOU'RE REPLYING Reporting bugs well: http://goo.gl/4Xue / TO THE LIST OR THE AUTHOR
