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Guest
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rs232 wake up, max232 powered down? |
Posted: Sun Feb 01, 2009 11:59 pm |
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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 |
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Ttelmah Guest
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Posted: Mon Feb 02, 2009 4:20 am |
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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 |
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asmallri
Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia
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Re: rs232 wake up, max232 powered down? |
Posted: Mon Feb 02, 2009 7:02 am |
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Anonymous wrote: | 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 |
Maxim have variants of the MAX232 chip that power down their voltage generation circuitry (standby mode). They will power up on detection of valid rs232 levels. _________________ Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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Guest
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Posted: Mon Feb 02, 2009 2:48 pm |
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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? |
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Ttelmah Guest
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Posted: Mon Feb 02, 2009 3:09 pm |
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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.
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