Hot-pluggable bus design review

Question

I am currently designing a hot-pluggable bus system where a module is connected to its respective slot via a first mate/last break spring-loaded connector. I am using PJON, in specific PJDL on the link layer, for the actual communication which is designed to be hot-pluggable. The modules are rated for a maximum power consumption of 5 W. The pinout of the connector is as follows:

Pin#	Label	Mating Order	Description
1	+5V	1st (or 2nd?)	Pre-charge supply voltage
2	+5V	3rd	Supply voltage
3…6	ADR[0…3]	3rd	4-bit module address
7	DAT	3rd	PJON bus
8	PD	3rd	Presence detect
9	GND	1st	Ground

Slot connector schematic:

Module connector schematic:

When plugging the module into the slot, pin 1 (+5V) and GND are mating first, pre-charging the module circuitry via pre-charge resistor R2. As soon as all non-staggered pins have mated as well, the module's MCU is supplied with power from pin 2 (+5V) and starts to read out the 4-bit module address from the ADR[0…3] pins set via jumpers on the slot. It then initializes the PJON bus on the DAT pin and finally draws PD to HIGH (pulled to GND on the slot via R1 if not powered), signaling readiness to the main controller MCU.

I would like to ask for some advice on the general soundness of the design, but the following questions in particular:

1. How do I dimension the pre-charge resistor R2? I suppose it depends on the total decoupling capacitance of the module?
2. Do I need three-level staggering in the sense that the GND pin 9 (1st) mates before the pre-charge supply voltage pin 1 (2nd) followed by all other pins (3rd)? A three-level staggered connector is not readily available from this manufacturer, but I could set back the receptive tip of pin 1 a bit so it mates after common ground has been established (pin 9). What could happen if supply voltage mates before/at the same time as ground?
3. Should I add current-limiting resistors or any other kind of protective circuitry to the ADR, DAT and PD pins?
4. I am aware that there are dedicated hot-plug ICs, but they look quite complicated and would significantly increase my BOM. Is there are simple solution available for my use case or is this overkill considering the relatively low maximum power consumption (5 W per module) in my design?

Answer 1

How do I dimension the pre-charge resistor R2? I suppose it depends on the total decoupling capacitance of the module?

The way that I did it when I developed a hot swappable bus was find the amount of inrush current used and size the resistor around that. I measured what the average reasonable time it would take to plug in the hot swappable connector, and then tried to have the resistor small enough so the rail would come up enough. These values are probably best measured though I simulated my bus and estimated but it's not as good as actually measuring it you can tune the resistor later when you build the hardware.

Do I need three-level staggering in the sense that the GND pin 9 (1st) mates before the pre-charge supply voltage pin 1 (2nd) followed by all other pins (3rd)?

Yes with hot swapping the power pins must connect first before the data pins, the worst case scenario is if VCC connects and then data then there's a current pathway that's not involving ground and it can blow transistors out. I once had a connector that I inherited that was not hot swappable and people were damaging products because they were hot swapping it and the data pins were the first thing that blew out

Should I add current-limiting resistors or any other kind of protective circuitry to the ADR, DAT and PD pins?

Buffers are sometimes nice to have but not completely necessary, another thing you could do is either with a micro or with a buffer turn off the digital output pins until the connector is connected but this is more of a nice thing

I am aware that there are dedicated hot-plug ICs, but they look quite complicated and would significantly increase my BOM. Is there are simple solution available for my use case or is this overkill considering the relatively low maximum power consumption (5 W per module) in my design?

If cost is a factor I wouldn't use them, I would use other simple methods

Answer 2

The value of R2 will depend upon the time between the first stage contacts mating and the third. The time for pre-charge will depend upon the insertion speed and the ~0.75mm difference in the pin heights so the time may be pretty short.

From my interpretation of the connector drawing there is no second stage - there are only two pin heights shown in the drawing.

At minimum there should be weak pull-up/pull-down resistors on ADR, DAT and PD pins to ensure that they have not picked up any static potential when the card is not plugged in. It is not obvious that series resistors are needed. Maybe diodes to the supply rails would provide added protection against potentials beyond the supply rails.

One advantage of hot-plug ICs that I have used is that they provide added protection for scenarios where the card is only partially inserted or the card is rocked that provided intermittent contact during insertion.