Rainbow Electronics AT89LP216 - Manual (Page 48)

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3621A–MICRO–6/06
AT89LP216 [Preliminary]
17.6 Framing Error Detection
In addition to all of its usual modes, the UART can perform framing error detection by looking for
missing stop bits, and automatic address recognition. When used for framing error detect, the
UART looks for missing stop bits in the communication. A missing bit will set the FE bit in the
SCON register. The FE bit shares the SCON.7 bit with SM0 and the function of SCON.7 is deter-
mined by PCON.6 (SMOD0). If SMOD0 is set then SCON.7 functions as FE. SCON.7 functions
as SM0 when SMOD0 is cleared. When used as FE, SCON.7 can only be cleared by software.
17.7 Automatic Address Recognition
Automatic Address Recognition is a feature which allows the UART to recognize certain
addresses in the serial bit stream by using hardware to make the comparisons. This feature
saves a great deal of software overhead by eliminating the need for the software to examine
every serial address which passes by the serial port. This feature is enabled by setting the SM2
bit in SCON. In the 9th bit UART modes, Mode 2 and Mode 3, the Receive Interrupt flag (RI) will
be automatically set when the received byte contains either the “Given” address or the “Broad-
cast” address. The 9th bit mode requires that the 9th information bit to be a “1” to indicate that
the received information is an address and not data.
The 8th bit mode is called Mode 1. In this mode the RI flag will be set if SM2 is enabled and the
information received has a valid stop bit following the 8th address bits and the information is
either a Given or Broadcast address.
Mode 0 is the Shift Register mode and SM2 is ignored.
Using the Automatic Address Recognition feature allows a master to selectively communicate
with one or more slaves by invoking the given slave address or addresses. All of the slaves may
be contacted by using the Broadcast address. Two special Function Registers are used to
define the slave’s address, SADDR, and the address mask, SADEN. SADEN is used to define
which bits in the SADDR are to be used and which bits are “don’t care”. The SADEN mask can
be logically ANDed with the SADDR to create the “Given” address which the master will use for
addressing each of the slaves. Use of the Given address allows multiple slaves to be recognized
while excluding others. The following examples show the versatility of this scheme:
Slave 0 SADDR = 1100 0000
SADEN = 1111 1101
Given = 1100 00X0
Slave 1 SADDR = 1100 0000
SADEN = 1111 1110
Given = 1100 000X
In the previous example, SADDR is the same and the SADEN data is used to differentiate
between the two slaves. Slave 0 requires a “0” in bit 0 and it ignores bit 1. Slave 1 requires a “0”
in bit 1 and bit 0 is ignored. A unique address for slave 0 would be 1100 0010 since slave 1
requires a “0” in bit 1. A unique address for slave 1 would be 1100 0001 since a “1” in bit 0 will
exclude slave 0. Both slaves can be selected at the same time by an address which has bit 0 = 0
(for slave 0) and bit 1 = 0 (for slave 1). Thus, both could be addressed with 1100 0000.