rs232 wake up, max232 powered down?

Guest · Posted: Sun Feb 01, 2009 11:59 pm

Hi List,

What would be the best way to wake up a PIC on RS232 cable connect, or start bit, or character receive, if the MAX232 chip is powered down to save power until the PIC is wake?

Thanks

Ttelmah · Guest

Think about it. If the MAX232 is powered down, how are you going to receive a character, or a start bit?.
The normal way to do this, is to have an opto-coupler, or resistive connection, to the DTR line, or RTS line, connecting to an input on the PIC. When whichever of these you want goes 'true', you enable your MAX232.

Best Wishes

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Guest · Posted: Mon Feb 02, 2009 2:48 pm

what would the resistive connection to DTR, or RTS look like? what value resistor would be ok?

Right now the RTS goes to -6.25v when I connect the cable. What If I get a case where the driver of the line is swinging this -10v to 10v or -15v to 15v? Will the same resistor work?

Ttelmah · Guest

Yes.
Something like 100K to the line, and 200K from the PIC pin to 5v. When disconnected, the PIC input will sit high (because of the 200K), when the line is connected, it'll pull down. The input protection diodes will prevent the line going more than 0.6v below the 0v rail.

Best Wishes