MC33908 WkUpSrc register read always returns 0.

[bhanukandimalla]

We are using MPC 5643 with MC33908L(SBC). After intializing the SBC and SPI, I am trying to read the SBC's wake up src register where one of the IO_0 is setup as wkup src. The IO_0 is configured as rising edge wakeup detection. (Init IO_WU1 is properly set as x00040. So after intializing, All the read SPI command to the SBC are successful. But the WkUpSrc read is always returning 0. The SBC deglitch time seems to be 70us. The switch that drives the IO_0(from 0 to 24v) has a 1uF Cap. When the switch is turned out, I expect the wakeup Src register to be set for IO_0. But SBC is returning 0. Any idea what is going wrong here. Thanks in advance!!

[TomasVaverka]

Hello Bhanu,

Is the MC33908 in the LPOFF mode? The wake-up by IO is detected when a rising (WU_0_1:0 = 0b01) or a falling (WU_0_1:0 = 0b10) edge event happens at the IO pin and the device is in LPOFF mode. It means a transition from low to high or high to low must happen when the device is in LPOFF mode to validate the wake-up event.

Best regards,

Tomas

[bhanukandimalla]

Tomas,

Thank you for getting back. Yes, I made sure SBC enters LP mode. Then I turn on Battery key(max 24V) which is connected to IO_1. I checked the Mode Register WU bit and it is 0 indicating no wakeup is detected. I checked the data sheet of SBC chapter 9 (Typical applications) Vbat connected to IO_0 and has no capacitor. But to counteract the abuse of our key_switch our guys added a 1uF cap. Does this cause SBC not to detect? I made sure all the SPI requests to SBC are successful. All the configuration values are good for a rising edge IO_0 wakeup. I am attaching a picture of our IO_0 mechanism. Do you think there is a problem in the detection with this circuit?

Thanks,

Bhanu.