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MC9S08QG4: BKGD/MS pin sharing with PTA4 output pin

Question asked by Paolo Messina on Oct 20, 2006
Latest reply on Feb 9, 2007 by ROB LUND
In our design (MC9S08QG4/8-pin), pin PTA4 is connected to the base of a transistor in a H-bridge with a 1k series resistor.

At power up, the pin is found at 1V and the MCU apparently starts in debug mode.

We tried to connect a pull up capacitor (1uF) from the same pin to Vcc, to keep the initial voltage above Vih before /Reset goes high.

Is this the correct solution and if so, how do we choose the capacitance value?
Otherwise, how can we guarantee that the MCU always starts correctly in user mode?

I haven't found any application notes about this matter and the reference manual does not provide much information either.

The strange thing about our solution is that if we try to shorten the time /Reset takes to go high, by using a smaller pullup resistor, so that MS is seen well above Vih, we can see the MCU starts up (because PTA4 goes low as initially expected), but then we can't communicate with the MCU via I2C.

Does anyone know what's going on?

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