[emusan]

Hadn’t expected to see this here! I helped put this standard together while working at Opal Kelly.

I think we had some nifty ideas in there (using an analog voltage for peripheral addressing was one of my favorite). It has slowly seen more adoption as time passes. Would always love to see more adoption!

[slyfox125]

That sounds interesting (analog addressing). Could you expound?

[emusan]

With many of these types of standards (FMC and Syzygy) it becomes helpful to include some form of memory on the peripheral cards. With Syzygy it is required in order to store some helpful data about the card (product name, manufacturer) along with supported voltage ranges for an adjustable voltage rail from the carrier board. FMC used standard off the shelf I2C eeproms for this task, with some pins on the FMC connector used to indicate the address for the particular slot that the peripheral was attached to on the motherboard. We wanted to lose as few pins as possible for address lines on Syzygy, so we opted to instead use a microcontroller with an ADC for our peripheral flash memory. There was a single pin used to indicate the peripheral address. This pin had a fixed resistor to a power rail on the peripheral and the carrier would have a port-specific resistor to ground on the same pin. By reading the voltage across these resistors we defined 16 addresses with just a single pin. IIRC the microcontrollers we selected were almost the same cost as the I2C flash we would have used in their place!

It was a pretty neat hack.

Placing small microcontrollers on the peripherals also opened up other functions like using them for basic power supply sequencing.