Multiplexer like component? - Page 1
Forum section: electronics / projects

author: ryanmills
author_rank: Unknown
timestamp: March 03, 2022, 11:36:49 pm
content: Forgive me I have no idea what to call this type of component.I have a lack of digital I/O pins, 4-6 pins open but I need to drive 10 to 16ish to control a bank of transistors, near zero current, just simple on/off bits. Multiplexer is the obvious choice however I need to turn on multiple pins in any number of combinations. I have both SPI and I2C available, just need a simple device with pin registers and enough outputs. But I have no idea what to search for, is something like this made and if so, what might it be called?

author: jbb
author_rank: Super Contributor
timestamp: March 03, 2022, 11:49:18 pm
content: “IO port expander” might get some hits

author: ryanmills
author_rank: Unknown
timestamp: March 04, 2022, 12:04:45 am
content: Quote from: jbb on March 03, 2022, 11:49:18 pmAhh yes, found exactly what I was looking for, thank you!

author: Cerebus
author_rank: Super Contributor
timestamp: March 04, 2022, 12:53:31 am
content: Quote from: ryanmills on March 03, 2022, 11:36:49 pmGiven you're trying to drive transistors, you might find it useful to know that those port expanders/shift registers also come in open collector/open drain variants. Using one might allow you to kill two birds with one stone, and drive whatever those transistors are connected to directly - obviously it's going to depend on the exact specifics of what you're doing. There are even variants with internal flyback diodes intended for driving relays.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2022, 10:48:21 am
content: For additional outputs one can use shift registers (e.g. HC595 or 4094). There are even ready made shift registers with powerful drivers tpic6c595, that may include the transistors.

author: ryanmills
author_rank: Unknown
timestamp: March 04, 2022, 07:17:53 pm
content: Quote from: Kleinstein on March 04, 2022, 10:48:21 amOhh that tpic6c595 is not quite what I need on this project but I have another its perfect for. Thank you!

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2022, 07:34:25 pm
content: If you need more current than the 100mA per channel TPIC6C595 can handle, in the same family there are also TPIC6B595 (150mA) and TPIC6A595 (350mA).If you are really stuffed for pins, look at I2C I/O expanders, which if you are already using I2C bus chips in your design can get you extra I/O pins for no increase in MCU pins used, or if not, only need two MCU pins. The down side is they are typically slower than SPI or clocked SIPO or PISO I/O expanders.

author: ryanmills
author_rank: Unknown
timestamp: March 04, 2022, 11:12:24 pm
content: Quote from: Kleinstein on March 04, 2022, 10:48:21 amQuote from: Ian.M on March 04, 2022, 07:34:25 pmWhat would you call the reverse of a port expander or shift register? A component with 10 something inputs with a bus that I can poll and see what pins are high/low?

author: langwadt
author_rank: Super Contributor
timestamp: March 04, 2022, 11:26:25 pm
content: Quote from: ryanmills on March 04, 2022, 11:12:24 pmthe same, it is just parallel in - serial out instead of serial in - parallel out

author: ryanmills
author_rank: Unknown
timestamp: March 04, 2022, 11:37:31 pm
content: Gotcha, thank you!

author: jbb
author_rank: Super Contributor
timestamp: March 05, 2022, 02:35:16 am
content: On inputs via IO port expander…Remember you’ll need to poll the port expander to see what the inputs are. Some port expanders have an interrupt output line, which you can connect to the main processor to say ‘hey, something changed!’. This can be super helpful.Also, some port expander chips offer special extras like a built-in multiplexed keypad scanner. Could be helpful…