Hi All
I need additional GPIO's to control various Relays etc.
My primary PCB has both UART and I2C output ports, therefore i can either connect any additional PCB with an AVR or a GPIO IC such as TCA9534A.
What is the best way to create additional GPIOS?
Use a device like TCA9534A or an AVR?
I might be wrong but using multiple TCA9534A would be the simplest way?
clawson 
You need a clearer specification like budget, board area, EXACT expansion requirements like IO current limits, voltage tolerance etc.
If it really is just IO then port expanders will likely be cheapest but if there's any chance you might need access to "extra" peripherals then the micro may give you more "expansion possibilities" but it may cost more. If you are building 3 the cost may not matter. If you are building 3,000,000 every last $0.01 counts hugely!
I use shift registers on a daily basis.
I need additional GPIO's to control various Relays etc.
https://www.ebay.com/itm/1609273...
Multi-relay board interfaces using only two i/o pins, spi (two wire interface), multiple boards can be daisy chained if needed!
OR I2C solid state relay module:
https://www.amazon.com/Channel-R...
https://www.amazon.com/IOT-Elect...
or magnetic relay module:
https://www.amazon.com/Channel-E...
Jim
awneil 
What is the best way to create additional GPIOS?
As always, it depends on your definition of "best".
You need to specify the criteria for your particular project - without knowing the criteria, "best" is meaningless.
Have you tried drawing up a list of the pros & cons of each approach?
For using an AVR, remember to factor-in the cost of developing & maintaining the code, and programming the devices.
using multiple TCA9534A would be the simplest way?
"simplest" is also rather subjective ...
Try a 74HC595 ...very cheap, or a HEF4894 for more outputs (12)
awneil shift registers .
74HC595 ... or a HEF4894
But OP only mentioned UART and I2C as available - unclear if they could be "repurposed" ... ?
I am looking to get about 50-60 PCBS made. I aim to control some Relays and Triac's. If current is limited, i can always use a transistor circuit. The voltage can be either 3.3V or 5V.
74HC595
74HC595 Looks like a good choice. I guess i could make my UART pins, simple GPIOS to set the output instead.
74HC595.
AVR is not a recent product. On The Beginning, It Was 8 Bits. It was easy, then, to do out PORT, reg for access to HC373/374, 20-pin/8-bit luxury that normal AVR user can not afford.
Fortunately, HC595 can be adapted to SPI. This chip, HC595 is perhaps the most used one by AVR fans (and I challenge other opinions).
gchapman AVR is not a recent product.
AVR Dx 64 pin are available though most are long lead (iow a few have a reasonable lead-time)
PORTMUX | I/O Ports and Pinouts | Migration from the megaAVR® to AVR® Dx Microcontroller Families
AVR Dx 64 pin are available though most are long lead
One could cut them down a bit to fit the pads.
Sorry everyone - I'll get my coat.
74HC595 looks promising, but can it be operated from 2 GPIO pins? If not, I guess i need to look for something in the following range:
I2C General-Purpose I/Os | Products | I2C | TI.com
gchapman So, you do not like to drive it by SPI, which btw. does engage 4 wires. You can then search the net for Two Wires LCD and One Wire LCD.
Its just that my Primary PCB which will be controlling that has for i2C and UART port which is accessible.
74HC595 looks promising, but can it be operated from 2 GPIO pins? If not, I guess i need to look for something in the following range:
You easily can use 2 pins, clock and data, The latch sig can be made using an RC from one of the 2 signals...In fact, with enough cleverness you can get it to 1 wire, with some tradeoffs.
Of course, if you have I2C , why mess around?
Even better...send your uart to another AVR...the you can tell that avr to turn on various ports, have timers (turn on PA3 for xx ms & PB5 for xxx ms, PWM, etc, you can even use the adc (turn on PB2, when ADC <= xxxx counts) ...anything you can imagine
Turn on PA3 whenever PA2 is low while PA6 is high
Even better...send your uart to another AVR...the you can tell that avr to turn on various ports, have timers (turn on PA3 for xx ms & PB5 for xxx ms, PWM, etc, you can even use the adc (turn on PB2, when ADC <= xxxx counts) ...anything you can imagine
This. My experience has been that it is often better to have lots of smaller parts doing less, in terms of both programming simplicity and power usage, than one humongous part with everything but the kitchen sink included.
"do this and tell me when you're done" is a very handy way to program in an event driven style.
Neil
awneil Even better...send your uart to another AVR...
Which takes us right back to the original question!
What is the best way to create additional GPIOS?
Use a device like TCA9534A or an AVR?
74HC595 looks promising, but can it be operated from 2 GPIO pins? If not, I guess i need to look for something in the following range:
You easily can use 2 pins, clock and data, The latch sig can be made using an RC from one of the 2 signals...In fact, with enough cleverness you can get it to 1 wire, with some tradeoffs.
Of course, if you have I2C , why mess around?
Even better...send your uart to another AVR...the you can tell that avr to turn on various ports, have timers (turn on PA3 for xx ms & PB5 for xxx ms, PWM, etc, you can even use the adc (turn on PB2, when ADC <= xxxx counts) ...anything you can imagine
Turn on PA3 whenever PA2 is low while PA6 is high
Seems to be a good idea to have a bit more flexibility using AVR, we use ATMEGA48A on few other boards, so i aim to use the same AVR . Unless there would be better to anther AVR, my aim is to keep the cost down and have much uniformity on all my BOMs. I will be controlling some Relays and possibly some Triacs, so i guess i just need a static ON/OFF. Where would we need to use timer?
clawson Unless there would be better to anther AVR, my aim is to keep the cost down
To give you an idea (though these are not identically specified devices) a mega48P is listed by M'chip as $1.75-1.90 for 100-199 while a tiny427 (24 rather than 28 pins) is $0.67-0.75 - so less than 1/2 price. You would have to weigh up a comparison of all the peripherals though.
gchapman my aim is to keep the cost down and have much uniformity on all my BOMs.
Choose parts with a lead-time history (IIRC, there are such services as that data is at the distributors)
Continual calculation of risk, risk evaluation, risk plan, risk mitigation; be innovative though can be difficult (qualification, verification, validation)
P.S.
If possible, choose AVR that are wafer fab'd at Microchip.
How to search for Microchip PCNs
PSP Letter (Preferred Supply Program)
Lead Time | World's Largest Inventory of Microchip Products
edit :
one of several (many?) AVR wafer fab PCN :
Tempe Fab 2, Gresham Fab 4 | AVR Freaks
That's True
I found SAMD21 has lower price then a existing XMEGA we are using.
I will need to do a comparison.
Where would we need to use timer?
Wherever you need a timer, that's where...turn a pin on, off, or maybe on for xx ms
Thanks I will get down to some planning and designing.
