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Re: [avr-chat] Atmega165 USI I2C (TWI)

From: Joerg Wunsch
Subject: Re: [avr-chat] Atmega165 USI I2C (TWI)
Date: Fri, 22 Jul 2005 23:07:28 +0200 (MET DST)

Grant Elliott <address@hidden> wrote:

> 3) Removed the __x from the declaration of
> USI_TWI_Start_Transceiver_With_Data.  Honestly, I don't even know
> what __x does, but AVR GCC didn't like it.

Even the IAR docs does not document that.  I grepped their supplied
header files, and could not find any plain __x declarations but __x__z
declarations.  This looks like it might somehow magically trigger
special handling for some of the pointer registers (X and Z).  I'd
suggest you bug avr -at- atmel -dot- com for that, after all, they
wrote that code. ;-)

> 4) Commented out the lines that set SCL and SDA as outputs.  With
> them present, the lines don't pull high as expected after
> initialization.  Further, an attempt to use the transceiver stalls
> because the lines never change state.

Hmm, SCL and SDA aren't plain outputs (all the time) but are normally
supposed to be active-low (i.e. open drain) outputs that work against
external pull-up resistors.  I didn't look at their code, but these
pins IMHO need to be changed between output and input frequently if
there's no open-drain hardware implementation available.

cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)

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