Initialization/Application Information
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
24-9
Initiator mode—the I
2
C module cannot tell whether the bus is busy; therefore, if a START
condition is initiated, the current bus cycle can be corrupted. This ultimately results in the
current bus initiator of the I
2
C interface losing arbitration, after which bus operations
return to normal.
24.3.6 Clock Synchronization
Due to the wire AND logic on the SCL line, a high-to-low transition on the SCL line affects all
devices connected on the bus. The devices begin counting their low period when the initiator
drives the SCL line low. After a device has driven SCL low, it holds the SCL line low until the
clock high state is reached. However, the change of low-to-high in a device clock may not change
the state of the SCL line if another device is still within its low period. Therefore, synchronized
clock SCL is held low by the device with the longest low period. Devices with shorter low
periods enter a high wait state during this time. When all devices concerned have counted off
their low period, the synchronized SCL line is released and pulled high. Then there is no
difference between the device clocks and the state of the SCL line, and all the devices begin
counting their high periods. The first device to complete its high period pulls the SCL line low
again.
24.3.7 Handshaking
The clock synchronization mechanism can be used as a handshake in data transfer. Target
devices can hold the SCL low after completion of a 1-byte transfer (9 bits). In such cases, it halts
the bus clock and forces the initiator clock into wait states until the target releases the SCL line.
24.3.8 Clock Stretching
Targets can use the clock synchronization mechanism to slow down the transfer bit rate. After the
initiator has driven the SCL line low, the target can drive SCL low for the required period and
then release it. If the target SCL low period is greater than the initiator SCL low period, then the
resulting SCL bus signal low period is stretched.
24.4 Initialization/Application Information
This section describes some programming guidelines recommended for the I
2
C interface. It also
includes Figure 24-3, a recommended flowchart for the I
2
C interrupt service routines. The I
2
C
registers in this chapter are shown in big-endian format. If the system is in little-endian mode,
software must swap the bytes appropriately. The I
2
C controller does not guarantee its recovery
from all illegal I
2
C bus activity. In addition, a malfunctioning device may hold the bus captive. A
good programming practice is for software to rely on a watchdog timer to help recover from I
2
C
bus hangs. The recovery routine should also handle the case when the status bits returned after an
interrupt are not consistent with what was expected due to illegal I
2
C bus protocol behavior.
Содержание MSC8144E
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Страница 314: ...MSC8144E Reference Manual Rev 3 8 24 Freescale Semiconductor General Configuration Registers ...
Страница 414: ...MSC8144E Reference Manual Rev 3 10 14 Freescale Semiconductor MSC8144E SC3400 DSP Subsystem ...
Страница 452: ...MSC8144E Reference Manual Rev 3 11 38 Freescale Semiconductor Internal Memory Subsystem ...
Страница 520: ...MSC8144E Reference Manual Rev 3 12 68 Freescale Semiconductor DDR SDRAM Memory Controller ...
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