SCI Wake from Stop3 Mode

[dvona]

In my application, I need the ability to wake the MC9S08QG8 from stop3 mode on reception of a character over the SCI. Now, I understand that the SCI is disabled in stop3 mode (actually, it is in "standy mode") according to the datasheet. However, the micro only needs to communicate at 300 baud, which translates to roughly a 3ms bit time. With 16 times oversampling the sample time is roughly 200us.

So, since the bit sampling time is relatively slow, I'm thinking maybe I could tie the receive side to another pin enabled as a KBI that could wake the micro on a falling edge.

Since the data sheets says that in stop 3 mode, all states are saved, I would think that if I can wake quickly enough from stop 3, the SCI might still have enough time to detect the falling edge on the SCI receiver.

So my question, has anyone ever tried this or see a reason why it absolutely wouldn't work? Any other ideas how I can trigger a wakeup even via the SCI receiver?

Thanks,

[rocco]

I have not used the QG8, but I believe the uart is similar to the one I use in the GP32.

In the GP32 uart, the receiver synchronizes to the leading edge of the start bit. All other bits are then sampled, based on the startbit's leading edge. If the uart is sound asleep during this edge, I suspect the uart will ignore it and synchronize to the next edge (after it wakes) and give you a framing error.

This is just my speculation, but it's based on a few years of fighting with this uart.