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Pullups/downs on RX when using a bootloader

 
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ringo



Joined: 10 Feb 2004
Posts: 8

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Pullups/downs on RX when using a bootloader
PostPosted: Thu Jul 22, 2004 12:35 pm     Reply with quote
I use a bootloader on a pic18f452 and occasionally it seems to “lose the bootloader” when the board is not connected to the RS232 line. I have to pull the chip out of the board and reload the bootloader. I suspect it may be because I don’t have any pull resistors on the RX line. Any idea if I would need to pull this up or down to prevent unwanted garbage from messing with the bootloader?
Thanks
Ringo
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Thu Jul 22, 2004 1:10 pm     Reply with quote
Quote:
I use a bootloader on a pic18f452 and occasionally it seems to “lose the
bootloader” when the board is not connected to the RS232 line.

Normally you would have a MAX232-type chip connected to the
RX and TX pins on the PIC. The MAX232 has pull-down resistors
built-in on its RX pins (which connect to the PC). Since the MAX232
receiver is an inverter, this puts a logic high level on the PIC's RX
pin. That's a safe state. (A start bit is a logic low level).
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