my microcontroller is not booting

[PW_CTL]

Hi All,

I have an strange problem with my last design. I am using a KA4 in a simple standby circuit. The KA4 has an indepent voltage regulator and reset of the rest of the circuit, and is used to manage the power and the resets of the other chips at the board.

The problem is that if the rest of the board is powered, the KA4 is working correctly, but If I place the transistors to cut the power in the rest of the board, the KA4 is not booting (the outputs are in a undefined state, ~1.2V). I also cannot acceed with the programmer. If I remove the transistors and place 0R resistors, I can program the KA4 again, and it works perfectly.

I´ve checked the reset signal with the oscilloscope, and it´s perfect. Exactly the same in both cases (power and reset are absolutely independent of the ret of the circuit). The power is also clean and stable in both cases.

Any idea?

[bigmac]

Hello,

I am not very familiar with the intricacies of the 'RS08KA4, but I presume that what you are saying is that the reset pin is capable of producing an output pulse under the conditions of an internal reset occurring, but will be a reset input at other times.

If this be the case, when the external circuitry is powered down, the reset (input) would be pulled continuously to a sufficiently low voltage by the intrinsic diodes at the reset pins of the peripheral devices, so as to cause a continuous MCU reset condition.

A possible work-around is to place an isolating schottky diode between the reset pin of the MCU, and the peripheral reset pins, with the cathode connected to the MCU.  If none of the peripherals have an intrinsic pullup resistor at their reset pin, you will also need to add an external resistor to minimize the effect of leakage current through the diode.  This should result in the MCU still able to supply a reset pulse to the peripheral devices, but with the MCU reset pin isolated from the external circuitry.

Regards,

Mac

[Lundin]

While I haven't used RS08, in my experience most Freescale parts are picky about what decoupling capacitor you have chosen for the reset pin. The manual's recommendation for S08 is 100nF. If you have a larger cap than that, you can experience some weird behavior.