XR88C681_06 Datasheet, PDF (87/101 Pages) Exar Corporation – CMOS Dual Channel UART (DUART)

XR88C681
Receiver Operation During Multi-Drop Mode
When a channel has been programmed into the
Multi-Drop Mode, and the Receiver has been disabled (a
typical configuration), the Receiver will load a character
into the RHR and set the RXRDY indicator (and/or
interrupt) if the A/D bit is “1” (Address flag). However, the
character will be discarded if its A/D bit is “0” (Data flag).
Therefore, in response to the RXRDY indicator, the CPU
should then read the received character and determine if
the address that it represents matches that of the CPU. If
the Addresses do match, (indicating that it is the Target
Slave), then the CPU should enable the Receiver, in
preparation for the subsequent blocks of data.
Once the Receiver has been enabled the Receiver Serial
Data will be processed as in Normal Operation. The
received characters are accessible to the CPU by reading
the RHR. The state of the A/D flag bit is available at
SRn[5], the Status Register bit normally used to indicate
“Parity Error”. Therefore, in conjunction with receiver
each new character, the CPU should continue to monitor
SRn[5] in order to verify that it is a “0” (Data characters).
Once the “Target CPU” detects a new address character,
SRn[5] =“1”, it should compare this address with its own.
If the addresses do not match, then this CPU is not the
intended recipient of the next block of data, and now
should disable the Receiver. Figure 49 presents a flow
diagram depicting a recommended procedure for
handling received characters while in the Multi-Drop
Mode.
START
Receiver is Disabled
Channel is Commanded
into Multi-Drop Mode.
MR1n[4:3] = [1, 1]
No
Has
RXRDY Indicator
been asserted
?
Yes
Read in Address
Character from RHRn
No
Does
Newly Received
No
Reject Character
Address match
CPU Address
Receiver remains Disabled
?
Yes
Enable Receiver
(Write x2h to the
Appropriate Channel
Command Register)
Read in Data Character
from RHR.
Check SRn[5]
Is the
New Character a Yes
Data Character
SRn[5] = 1?
Figure 49. A Flow Diagram Depicting a Procedure That Can Be Used to Receive
Characters in the Multi-Drop Mode.
Rev. 2.11
87