BBB PCB: Details on increasing C24 and ORing PORz with SYS_RESETn


We are building a custom board, based on BBB. We are looking at the following revisions that look different from the official AM335x-evm design:

Rev A6A: 3) “Changed C24 to a 2.2uF capacitor. This extends the reset signal to solve an issue where some boards would not boot on power up.”

In our custom board, we use nRESETin_out as output to reset external peripherals, and having this larger cap prevents the line from going low during warm reset, as per this TI forum post:
Our custom board will not use a nRESETin_out with a push button, therefore, do we still need it, and if yes, how does adding this capacitor improve sitara processor boot success on POR?

Rev A6: 1) “Based on notification from TI, in random instances there could be a glitch in the SYS_RESETn signal from the processor where the SYS_RESETn signal was taken high for a momentary amount of time before it was supposed to. To prevent this, the signal was ORed with the PORZn (Power On reset).”

What subsystems/peripherals were impacted by this glitch? We suspect that this change relates to having to satisfy the 25ms nRST requirement of the LAN8710A PHY. We are thinking of using an alternative approach, by using GPIO to drive nRST pin of LAN8710A, as suggested on TI forums: In which case, do we still need this fix ?


Here is the relevant answer from TI:

TI is suggesting that there is still a bug on BBB PCB:
“Note: The BeagleBone Black should have a resistor in series with the AM335x WARMRSTn terminal (named NRESET_INOUT on BeagleBone Black schematic symbol) to limit the current when these two outputs are driven to opposite logic levels. Without the series resistor, the output of U16 could be driven low while the WARMRSTn terminal is driven high for a short duration as power is sequenced. You may not need this circuit if the components connected to the WARMRSTn output do not have an issue with a short duration high level pulse driven from WARMRSTn.”

There is not an issue here. This is just Paul’s opinion. He has lots of opinions.