Multiplex 80 analog signals w/ SX1509 IO Expander & CH74HC4067 Analog Mux

Sensors
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I ordered the SX1509 HERE I see I can configure each pin one line of code at a time given the Arduino Library provided in hookup guide. I’m going to use all the pins as outputs, is it possible to configure all 16 pins simultaneously with one command / a single I2C write?

Background: I need to read 80 analog signals. My thinking is to use several analog multiplexors and address those using I2C IO expanders as follows…

Two SX1509 chips gives me 32 Digital IO pins, divide that by four and I can address eight CD74HC4067 analog multiplexors. This would give me up to 128 analog channels, more than enough for what I need to do.

However, going through the Arduino library for the chip I don’t see a method / function to configure output on all IO pins simultaneously. I only see code to do one pin at a time. This would create some grief for me in addressing the Analog Multiplexors.

On another note, it would be more than ideal if there was a I2C addressable analog Mux. Was wishful thinking but hoped the SX1509 would provide a signal path but I don’t see one.

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Reading the datasheet, I see I can set all outputs in one I2S write by writing my own code so I can set all bits in RegData Addresses (0x10 & 0x11). The hookup guide provided library masks each pin in register and writes one at a time.

Does anybody know how these IO pins stabilize / latch? Do they do so as soon as the corresponding register bit for each IO is set or once entire register is written as a whole?

PS. I see the comment in library code. Next round is on me!