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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
436 of 808
NXP Semiconductors
UM10360
Chapter 19: LPC17xx I2C0/1/2 interface
possible. This is achieved by setting the STO flag while the STA flag is still set. No STOP
condition is transmitted. The I
2
C hardware behaves as if a STOP condition was received
and is able to transmit a START condition. The STO flag is cleared by hardware
.
9.6.4 I
2
C-bus obstructed by a LOW level on SCL or SDA
An I
2
C-bus hang-up can occur if either the SDA or SCL line is held LOW by any device on
the bus. If the SCL line is obstructed (pulled LOW) by a device on the bus, no further serial
transfer is possible, and the problem must be resolved by the device that is pulling the
SCL bus line LOW.
Typically, the SDA line may be obstructed by another device on the bus that has become
out of synchronization with the current bus master by either missing a clock, or by sensing
a noise pulse as a clock. In this case, the problem can be solved by transmitting additional
clock pulses on the SCL line
. The I
2
C interface does not include a dedicated
timeout timer to detect an obstructed bus, but this can be implemented using another
timer in the system. When detected, software can force clocks (up to 9 may be required)
on SCL until SDA is released by the offending device. At that point, the slave may still be
out of synchronization, so a START should be generated to insure that all I
2
C peripherals
are synchronized.
9.6.5 Bus error
A bus error occurs when a START or STOP condition is detected at an illegal position in
the format frame. Examples of illegal positions are during the serial transfer of an address
byte, a data bit, or an acknowledge bit.
The I
2
C hardware only reacts to a bus error when it is involved in a serial transfer either as
a master or an addressed slave. When a bus error is detected, the I
2
C block immediately
switches to the not addressed slave mode, releases the SDA and SCL lines, sets the
interrupt flag, and loads the status register with 0x00. This status code may be used to
vector to a state service routine which either attempts the aborted serial transfer again or
simply recovers from the error condition as shown in
Fig 95. Simultaneous repeated START conditions from two masters
SLA
A
W
SLA
S
18H
08H
A
DATA
28H
08H
OTHER MASTER
CONTINUES
other Master sends
repeated START earlier
S
retry
S
P