RPi’s I2C bus maximum device loading/fan out?
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I have a RPi4 and I have been using the I2C bus with 4 MCP23017 gpio expanders and this works fine. My intention is to use 8 MCPMCP23017 devices, as they have only 3 bit addressing, this is the maximum on 1 I2C bus. My question is does the RPi’s I2C bus have a maximum device loading/fan out?pi-4i2cshareedit follow closeflag asked yesterdayRay2122 bronze badges
- 1I am sure it does. But I doubt you will get a more useful answer than try it and see for any particular configuration. The best source of GPIO information may be any compute module specs you can find. – joan yesterday
- Ah, let me see. (1) In my Rpi3B+ days with flat rate of I2C 100kHz, I once put 8 mcp23017s one bus, together with a couple of other I2C guys,but I found them not stable, with too frequent #121 I/O Errors, (2) Now with Rpi4B’s 5 I2C buses, and you can now (not for 3B+) lower I2C speed to much lower than 100Kz, so each MCP23017’s chip and associated wiring capacitance is lowered, resulting total impedance not that easily exceed the I2C hard limit of 400pF (3) of course you can use I2C mux, extenders, buffers, CAT 5 wiring etc, to push your limit. / To continue, … – tlfong01 21 hours ago
- I have blogged 150+ posts on MCP23017 a year ago. Perhaps you can skim them and see if any of my learning experiences useful: Refs – (1) Search found 178 matches: mcp23017 tlfong01 Searched query: mcp23017 tlfong01 – tlfong01, Rpi.org.forums, 2020aug1901 raspberrypi.org/forums/search.php?keywords=mcp23017+tlfong01 (2) MCP23017 Learning Notes – tlfong01, rpi.org.forums, 2019jan23 raspberrypi.org/forums/viewtopic.php?f=37&t=231984 (3) I2C I/O Port Expander Q&A – tlfong01, rpi.org.forums, 2019mar02 raspberrypi.org/forums/viewtopic.php?f=45&t=234869. – tlfong01 21 hours ago
- If you can use Rpi’s 5 SPI buses, you can also try MCP23S08, MCP23S17. Actually I once use MCP23017 GPIO pins to fake SPI CS pins, and found it OK. This way, you have (almost) unlimited number of SPI buses. You might like to see how I mux the SPI buses with the following examples: Refs: “Muxing SPI with MCP23S08/17, MCP3008/3201/3208 Examples”: (1) electronics.stackexchange.com/questions/512899/…, (2) electronics.stackexchange.com/questions/515225/…. – tlfong01 21 hours ago
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