xref: /openbmc/u-boot/doc/I2C_Edge_Conditions (revision d9e268ed)
1I2C Edge Conditions:
2====================
3
4    I2C devices may be left in a write state if a read was occuring
5    and the CPU was reset. This may result in EEPROM data corruption.
6
7    The edge condition is as follows:
8	1) A read operation begins.
9	2) I2C controller issues a start command.
10	3) The I2C writes the device address.
11	4) The CPU is reset at this point.
12
13    Once the CPU reinitializes and the read is tried again:
14	1) The I2C controller issues a start command.
15	2) The I2C controller writes the device address.
16	3) The I2C controller writes the offset.
17
18    The EEPROM sees:
19	1) START
20	2) device address
21	3) START "this start is ignored by most EEPROMs"
22	4) device address "EEPROM interprets this as offset"
23	5) Offset in device, "EEPROM interprets this as data to write"
24
25    The device will interpret this sequence as a WRITE command and
26    write rubbish into itself, i.e. the "offset" will be interpreted
27    as data to be written in location "device address".
28
29Notes
30-----
31!!!THIS IS AN UNDOCUMENTED I2C BUS BUG, NOT A AMCC 4xx BUG!!!
32
33This reset edge condition could possibly be present in every I2C
34controller and device available. For boards where a I2C bus reset
35function can be implemented a i2c_init_board() function should be
36provided and enabled by #define'ing CONFIG_SYS_I2C_INIT_BOARD in your
37board's config file. Note that this is NOT necessary when using the
38bit-banging I2C driver (common/soft_i2c.c) as this already includes
39the I2C bus reset sequence.
40
41
42Many thanks to Bill Hunter for finding this serious BUG.
43email to: <williamhunter@attbi.com>
44
45Erik Theisen <etheisen@mindspring.com>
46Tue, 5 Mar 2002 23:02:19 -0500 (Wed 05:02 MET)
47