From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Frysinger Date: Sun, 11 Jul 2010 18:55:04 -0400 Subject: [U-Boot] [PATCH] i2c: fix SDA contention in read_byte() In-Reply-To: References: <1278396866-5571-1-git-send-email-thomas@wytron.com.tw> Message-ID: <201007111855.06308.vapier@gentoo.org> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de On Wednesday, July 07, 2010 00:45:42 Andrew Dyer wrote: > On Tue, Jul 6, 2010 at 1:14 AM, Thomas Chou wrote: > > We should not set SDA after TRISTATE, as it results in contention. > > > > Signed-off-by: Thomas Chou > > --- > > drivers/i2c/soft_i2c.c | 2 +- > > 1 files changed, 1 insertions(+), 1 deletions(-) > > > > diff --git a/drivers/i2c/soft_i2c.c b/drivers/i2c/soft_i2c.c > > index 847db76..344b7f8 100644 > > --- a/drivers/i2c/soft_i2c.c > > +++ b/drivers/i2c/soft_i2c.c > > @@ -305,8 +305,8 @@ static uchar read_byte(int ack) > > /* > > * Read 8 bits, MSB first. > > */ > > - I2C_TRISTATE; > > I2C_SDA(1); > > + I2C_TRISTATE; > > data = 0; > > for(j = 0; j < 8; j++) { > > I2C_SCL(0); > > -- > > I2C_TRISTATE is supposed to be persistent until I2C_ACTIVE is called, > so in the original code it should still be in effect when I2C_SDA(1) > is executed and there should be no contention. This patch causes the > code to actively drive SDA high at the same time the addressed device > might be driving it low, causing contention until the I2C_TRISTATE > takes effect. > > In some sense the code is misleadingly written, as it is not allowed > in the spec to actively drive a '1' on the bus, a chip is only > supposed to drive 'z' or '0', and the platform is supposed to provide > the pullup current. This comes more into play if the i2c bus supports > clock stretching or arbitration among multiple masters. how do you propose we get i2c gpio working ? it works fine under Linux. we cannot tristate a pin (set the gpio to an input) and then turn around and attempt to drive it (set the gpio to an output with a specific value). that is what the code currently does. our end goal is simple: have i2c gpio bitbanging work under u-boot like under linux. we dont really care about the exact way we get there. this patch was just one idea. -mike -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 836 bytes Desc: This is a digitally signed message part. Url : http://lists.denx.de/pipermail/u-boot/attachments/20100711/63b1adc8/attachment.pgp