SERIAL COMMUNICATION INTERFACE (SCI)
MOTOROLA
PORT C
6 - 55
;
MAIN PROGRAM
*
;*************************************************
ANDI
#$FC,MR
;Re-enable interrupts
MOVE
#>$41,X:(R3)
;Move a byte to the transmit buffer
MOVE
R0,X:(R3)
BSET
#12,X:SCR
;and enable interrupts so it
;will be transmitted
SEND
JMP
SEND
;Normally something more useful
;would be put here.
END
;End of example.
Figure 6-28 SCI Asynchronous Transmit/Receive Example (Sheet 3 of 3)
6.3.8
Multidrop
Multidrop is a special case of asynchronous data transfer. The key difference is that a pro-
tocol is used to allow networking transmitters and receivers on a single data-transmission
line. Interprocessor messages in a multidrop network typically begin with a destination ad-
dress. All receivers check for an address match at the start of each message. Receivers
with no address match can ignore the remainder of the message and use a wakeup mode
to enable the receiver at the start of the next message. Receivers with an address match
can receive the message and optionally transmit an acknowledgment to the sender. The
particular message format and protocol used are determined by the user’s software.
These message formats include point-to-point, bus, token-ring, and custom configura-
tions. The SCI multidrop network is compatible with other leading microprocessors.
Figure 6-29 shows a multidrop system with one master and N slaves. The multidrop mode
is selected by setting WDS2 equals one, WDS1 equals one, and WDS0 equals zero. One
possible protocol is to have a preamble or idle line between messages, followed by an ad-
dress and then a message. The idle line causes the slaves to wake up and compare the
address with their own address. If the addresses match, the slave receives the message.
If the addresses do not match, the slave ignores the message and goes back to sleep. It
is also possible to generate an interrupt when an address is received, eliminating the need
for idle time between consecutive messages and addresses. It is also possible for each
slave to look for more than one address, which allows each slave to respond to individual
messages as well as broadcast messages (e.g., a global reset).
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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