CCS/TPS65381A-Q1: IGN signal for PowerLatch & Reset State

Hi Jack,

 The IGN_PWRL control bit is a method to keep the TPS65381 latched in a powered state even though the IGN pin goes low.  It is normally used for end of line programming of the system where there is direct control of the IGN pin.  Once the IGN pin is used to power up the device and the device is in DIAGNOSTIC state, the IGN_PWRL bit in SAFETY_FUNC_CFG may be set to 1 (note this register cannot be written to in any other state).  This allows the POST_RUN function, supporting the timing diagram that was attached to this thread and is figure 5-2 in the datasheet.  The text in this figure explains how the POST_RUN feature and IGN_POWL are used.    As shown in this timing diagram, if IGN_PWRL was set to 1 during DIAGNOSTIC state, even though the IGN signal goes low the device remains powered on (assuming VBATP and VBAT_SAFING supplies remain powered).  In the normal case for IGN is different, when IGN_PWRL is not used (cleared or 0), once IGN goes low the device will power off again to STANDBY state (see Figure 5-1).  When using IGN_PWRL the MCU has to determined when it is time to power down, it must set POST_RUN_RST to 1 (must also be done while in DIAGNOSTIC state).   As can be seen, when both IGN_PWRL and POST_RUN_RST were set to 1, the device will remain powered even though IGN goes low.  Since these bits can only be cleared in DIAGANOSTIC state RESET must be generated which will then transition to DIANGOSIC after RESET (see figure 5-16 for the state diagram).   Then to cause RESET the IGN pin must be transitioned from low to high while POST_RUN_RST is still set to 1, which will cause RESET and when DIAGNOSTIC state is re-entered the MCU should clear the IGN_PWRL.  If IGN is low again the device will go to STANDBY at that point because IGN is low and IGN_PWRL is cleared, if IGN remains high then the device will stay powered and the state diagram must be followed to either set the DIAG_EXIT_MASK bit to remain in DIAGNOSITC state (software debug) or set the DIAG_EXIT bit to go to ACTIVE state before DIAGNOSTIC state time out (all in Figure 5-16). 

 If the goal is to just wake up on a signal and keep the device powered on and then be able to command it off by the MCU, the CANWU pin may be used, it is a latched input.  Once a high is detected on CANWU and the device powers up it remains in a powered state until CANWU is low and then the command WR_CAN_STBY may be used to power down the device to STANDBY state (see figure 5-1 and 5-16).

 I hope that helps answer your question.

 Best Regards,

Scott