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:
|1||+5V||1st (or 2nd?)||Pre-charge supply voltage|
|3…6||ADR[0…3]||3rd||4-bit module address|
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:
- How do I dimension the pre-charge resistor R2? I suppose it depends on the total decoupling capacitance of the module?
- 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?
- Should I add current-limiting resistors or any other kind of protective circuitry to the ADR, DAT and PD pins?
- 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?