Motorola MPC8260 - Manual (Page 591)

MOTOROLA Chapter 20. SCC UART Mode 20-7
Part IV. Communications Processor Module
Receive commands are described in Table 20-3.
20.8 Multidrop Systems and Address Recognition
In multidrop systems, more than two stations can be on a network, each with a speciÞc
address. Figure 20-2 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.
Table 20-3. Receive Commands
Command Description
ENTER HUNT
MODE
Forces the receiver to close the RxBD in use and enter hunt mode. After a hardware or software reset,
once an SCC is enabled in the GSMR, the receiver is automatically enabled and uses the Þrst BD in the
RxBD table. If a message is in progress, the receiver continues receiving in the next BD. In multidrop
hunt mode, the receiver continually scans the input data stream for the address character. When it is not
in multidrop mode, it waits for the idle sequence (one character of idle). Data present in the Rx FIFO is
not lost when this command is executed.
CLOSE RXBD Forces the SCC to close the RxBD in use and use the next BD for subsequent received data. If the SCC
is not in the process of receiving data, no action is taken.
Note that in an SCC UART controller,
CLOSE RXBD functions like ENTER HUNT MODE but does not need to
receive an idle character to continue receiving.
INIT RX
PARAMETERS
Resets the receive parameters in the parameter RAM. Should be issued when the receiver is disabled.
Note that
INIT TX AND RX PARAMETERS resets both Tx and Rx parameters.