HCS08QG8 stop2 crazy !!!!!

[marcom]

Hi gang,

I have developed a simple board with HCS08QG, with mixed line/battery power supply.
When line power supply is not and only battery supply is present, micro will enter in STOP2 mode. A RTI will wake up it after 1.024s, then it will go in STOP MODE2, then it will again waked up by RTI: this loop will over when line power supply will come again.
I sense the current sink in STOP2 mode through a battery series resistor (560R), all is as expected (I see 1mV across 560R, with short spike, 1.024 sec period).

All is perfect, the problem will come when i try to connect the board with the world...

I have a very simple RS232 interface board (standard max232 with 5 cap) to interface micro with PC serial port.

If I connect the board to micro board, all is ok.

When I connect PC RS232 (tx rx ground) to this interface board, immediately I see that the voltage across current sense resistor 560R R23 jump to a 2 Volt in theorical STOP2 Mode !!!!!

But if I get power supply to the board with a wall power supply, micro is again wake, but across 560R I see ever 2 Volt, and I see 2 V after disconnect the power supply !!!!

The only mode, to see a nA sink current, is to disconnect the power supply, the pc from RS232 board, the connet the power supply, say hallo to micro, and then disconnect again the power supply!!! Wonder, now some nA of current sink.

I try to cut tx and rx line, the problem seems to arise from ground line connection !!!!

It seems, that connect an external ground is source of a crazy behavior, I don't understand nothing !!!!

Thanks

Marco

[yb]

Hi marcom,

Try with a "standby-able" RS232 driver like MAX3222.

This chip has a standby pin which reduces the current consuption very low.

Your Rx and Tx pins will be in high Z state.

Yvan

[bigmac]

Hello Marco,

You do not say the location of the current sense resistor.  If it is in the ground lead, this could be problematic, especially when it has a 2 volt drop.  The resistor is probably best located in the Vdd lead, so you are measuring only the MCU current (and not that of the RS232 interface).

Further, a 2 volt drop when the MCU is operational is likely too much, and may cause disruption.  Perhaps you should place a Schottky diode in parallel with the resistor, to limit the maximum voltage drop to about 0.4 volts.

Regards,

Mac