MOTOROLA
Chapter 23. SCC UART Mode
23-7
Part V. The Communications Processor Module
23.8 Multidrop Systems and Address Recognition
In multidrop systems, more than two stations can be on a network, each with a speciÞc
address. Figure 23-2 shows two examples of this conÞguration. Frames made up of many
characters can be broadcast as long as the Þrst character is the destination address. The
UART frame is extended by one bit to distinguish an address character from standard data
characters. Programmed in PSMR[UM], the controller supports the following two
multidrop modes:
¥
Automatic multidrop modeÑThe controller checks the incoming address character
and accepts subsequent data only if the address matches one of two user-deÞned
values. The two 16-bit address registers, UADDR1 and UADDR2, support address
recognition. Only the lower 8 bits are used so the upper 8 bits should be cleared; for
addresses less than 8 bits, unused high-order bits should also be cleared. The
incoming address is checked against UADDR1 and UADDR2. When a match
occurs, RxBD[AM] indicates whether UADDR1 or UADDR2 matched.
¥
Manual multidrop modeÑThe controller receives all characters. An address
character is always written to a new buffer and can be followed by data characters.
User software performs the address comparison.
Figure 23-2. Two UART Multidrop Configurations
23.9 Receiving Control Characters
The UART receiver can recognize special control characters used in a message-based
environment. Eight control characters can be deÞned in a control character table in the
UART parameter RAM. Each incoming character is compared to the table entries using a
mask (the received control character mask, RCCM) to strip donÕt cares. If a match occurs,
the received control character can either be written to the receive buffer or rejected.
Tx
Rx
1
Tx
Rx
2
Tx
Rx
3
Tx
Rx
4
Tx
Rx
Tx
Rx
Tx
Rx
Tx
Rx
SLAVE 2
SLAVE 3
SLAVE 1
MASTER
UADDR1
UADDR2
PAODR
Choose Wired-Or Operation in the Port A
Open-Drain Register to Allow Multiple Transmit
Pins to be Directly Connected
+ V
R
Two 8-Bit Addresses can be Automatically
Recognized in Either ConÞguration
+ V
R
Содержание MPC860 PowerQUICC
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