AT89LP428 datasheet(90/149 Pages) ATMEL

90 / 149 page

3654A–MICRO–8/09

AT89LP428/828

16.6

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.

16.7

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 for Modes 1, 2 or 3. In the 9-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 “Broadcast” address. The 9-bit mode requires that the 9th information bit be a “1”

to indicate that the received information is an address and not data.

In Mode 1 (8-bit) 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.

Automatic Address Recognition is not available during Mode 0.

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.

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