Power Sequencing in controllers

As design engineers, we daily deal with various embedded system components. The range starts from a simple micro controller to a complex FPGA. If we call all of them as controllers, as controlling and monitoring is their primary functionality, for all these controllers, power supply is the crucial parameter which determines the controller performance. Take any modern day controller, you have not less than 3 rails. this all to support the core and the external interfaces. For the controller to be electrically compatible with a specific peripheral, these all rails are necessary. So, now when we turn on a system, any of the system rail can come up anytime based on the rise time of respective power circuitry. this uneven coming up of power rails is not desirable. For all these controllers, there is a specific power sequencing that is required. So, power sequencing is nothing but which defines the sequence at which the different power rails of the board are to be brought up.

Important points w.r.t power sequencing:

1. Power sequencing is always necessary for the controller as well as the peripheral to power on in a known state.
2. Every controller will have a RESET signal. The digital circuit inside the controllers start their functionality only if RESET is released. The general practice is to release the RESET signal only after the power rails have settled.
3. If there are several power rails on the board and one rail has come up, there is a specific time interval before which the next rail should come up. That means it is not preferable to maintain long gaps between rails.
4. Some designs use supervisor circuit to keep the RESET signal low till the desired level is achieved.
5. Any improper power sequence leads to latch up condition drawing high current leading to core failure.
6. When I/O power comes up before Core power supply, the GPIO lines tend to be in undefined state which is not desirable. in this state, most of the external peripherals may not initialize properly. So, it is preferred to power the core before I/O section. However, check out the controller datasheet and follow the same.
7. Sometimes, we see that after several power ons the controller tend to fail, if this is the case in your board, check out the power sequencing.
8. Power down sequencing also to be followed. This sequence will be mentioned by the respective datasheets.
9. Power sequencing is also required to avoid any bus contention by the controller and the connected peripherals.
10. Several power sequencing chip sets are available in the market for various applications.