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UM10601
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
Preliminary user manual
Rev. 1.0 — 7 November 2012
203 of 313
NXP Semiconductors
UM10601
Chapter 16: LPC800 I2C-bus interface
16.7 Functional description
16.7.1 Bus rates and timing considerations
Due to the nature of the I
2
C bus, it is generally not possible to guarantee a specific clock
rate on the SCL pin. The clock can be stretched by any slave device, extended by
software overhead time, etc. In a multi-master system, the master that provides the
shortest SCL high time will cause that time to appear on SCL as long as that master is
participating in I2C traffic (i.e. when it is the only master on the bus, or during arbitration
between masters).
Rate calculations give a base frequency that represents the fastest that the I
2
C bus could
operate if nothing slows it down.
16.7.1.1 Rate calculations
SCL high time (in I
2
C function clocks) = ( 1) * (MSTS 2)
SCL low time (in I
2
C function clocks) = ( 1) * (MST 2)
Nominal SCL rate = I
2
C function clock rate / (SCL high time + SCL low time)
16.7.2 Time-out
A time-out feature on an I
2
C interface can be used to detect a “stuck” bus and potentially
do something to alleviate the condition. Two different types of time-out are supported.
Both types apply whenever the I
2
C block and the time-out function are both enabled,
Master, Slave, or Monitor functions do not need to be enabled.
In the first type of time-out, reflected by the EVENTTIMEOUT flag in the STAT register, the
time between bus events governs the time-out check. These events include Start, Stop,
and all changes on the I
2
C clock (SCL). This time-out is asserted when the time between
9
MONRESTART
Monitor Received Repeated Start.
0
0
No start detect. The monitor function has not detected a
Repeated Start event on the I
2
C bus.
1
Repeated start detect. The monitor function has
detected a Repeated Start event on the I
2
C bus.
10
MONNACK
Monitor Received Nack.
0
0
Acknowledged. The data currently being provided by the
monitor function was acknowledged by at least one
master or slave receiver.
1
Not acknowledged. The data currently being provided by
the monitor function was not acknowledged by any
receiver.
31:11 -
Reserved. Read value is undefined, only zero should be
written.
NA
Table 186. Monitor data register (MONRXDAT, address 0x4005 0080) bit description
Bit
Symbol
Value Description
Reset
value