AN2633 Freescale Semiconductor / Motorola, AN2633 Datasheet - Page 3

• Manufacturer Part Number: AN2633
• Description: LIN Drivers for SLIC Module on the MC68HC908QL4
• Manufacturer: Freescale Semiconductor / Motorola

SLIC Module Operation, Features, and Benefits
True Autobauding
up to 120 kbps with
No Software
Changes
MOTOROLA
The SLIC module on the QL4 represents a level of hardware support for LIN
slave applications that offers unparalleled performance. This increased
performance enables LIN slave node designers to do much more with smaller
devices than possible with UART-based solutions.
The SLIC module automatically performs two very important—but distinctly
separate—functions while establishing communication on the LIN bus:
Automatic LIN message frame synchronization (autosynchronization) is the
ability to detect an idle bus and correctly determine when a LIN message frame
header has begun.
In many standard UART solutions, this operation can prove problematic,
because it is possible to erroneously detect a 0x00 data character as a break
symbol, even when within LIN timing specifications. This can happen if the
driver software uses the standard UART break-detection circuitry without
actually measuring the length of the break symbol. (The ESCI on many
Motorola HC08 MCUs contains a feature controlled by a bit called the LINR bit
which prevents this error.)
Autobauding is the ability to derive the LIN bus speed from the synchronization
byte in the header. Autobauding is possible only if the header is synchronized
to the beginning of the message frame. The SLIC handles autobauding and
sets up to transmit or receive the rest of the message frame at this speed.
Then, the SLIC receives the identifier for the message frame and checks the
parity bits to ensure the data integrity of the identifier byte. No software
intervention is required until this point, when the ID is presented to the
application or driver software. The software performs a lookup and then
decides what to do for this message frame.
Because LIN was designed to use inexpensive RC oscillators, the SLIC module
was designed to allow an input clock tolerance of about ±50% and ensure that
the accuracy of LIN communication is ±1% or less. This wide range of clock
accuracy means that in any LIN slave application, it is not necessary to trim the
internal oscillator to establish and maintain LIN communications. (Motorola
internal RC oscillators, such as the one on the HC908QL4 device, typically
come from the factory with a guaranteed tolerance of ±25% before trimming.)
This also means that ROM devices are perfectly suited to be used without any
nonvolatile memory at all, because they would never need to store a trim value.
LIN Drivers for SLIC Module on the MC68HC908QL4
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Freescale Semiconductor, Inc.
For More Information On This Product,
Autosynchronization
Autobauding
Go to: www.freescale.com
SLIC Module Operation, Features, and Benefits
AN2633/D
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