9
8xC251Tx Hardware Description
3.2 Special Function Register Definitions
The following describes the special function registers associated with the second serial I/O port and their bit
definitions.
3.2.1 SCON1
Address: 9AH
Reset Value: 0000 0000B
Table 6. SCON1 Special F unction Register Definitions
Bit Number Bit
Mnemonic Function
7FE1SM0 Framing Error Bit 1:
To Select this function, set the SMOD0 bit in the BGCON register. Set by hard-
ware to indicate an invalid stop bit. Cleared by software, not by valid frames
Second Serial I/O Port Mode Bit 0:
To select this function, clear the SMOD0 bit in the BGCON register. Software
writes to bit SM0 and SM1 to select the second seria l I/O port operating
mode. Refer to SM1 bit for mode selections
6SM1 Second Serial I/O Port Mode Bit 1:
Software write to bit SM0 and SM1 (abo ve) to select the s erial port operating
mode.
SM0 SM1 Mode Description Baud Rate
0 0 0 Shift Register Fosc/12
0 1 1 8 bit UART variable
1 0 2 9 bit UART Fosc/32* or Fosc/64*
1 1 3 9 bit UART variable
* Select by programming the SMOD0 bit in the BGCON register
5SM2 Second Serial I/O Port Mode Bit 2:
Software writes to SM2 enable and disable the m ultiprocessor communi c a-
tion and automatic ad dress recognition features. This allows the second
serial I/O port to differentiate b etween data and comma nd frames and to rec-
ognize slave and br oadcast addresse s
4REN1 Receive Enable Bit 1:
To enable reception, set this bit. To enable tr ansmission, clear this b it
3TB8 Transmit Bit 8:
In modes 2 and 3, software w ri te s the n in th dat a bi t to be tr ans mit t ed to TB 8.
Not used in modes 0 and 1
2RB8 Receive Bit 8:
Mode 0: not used
Mode 1 (SM2 clear): Set or c leared by hardware to reflect the stop bit
received
Modes 2 and 3 (SM2 set): Set or clea red by hardware to reflect the ninth
data bit received
1TI1 Second Serial I/O Port Transmit Interr upt Flag Bit:
Set by transmitter after the la st data bit is transmitted. C leared by software
0RI1 Second Serial I/O Port Receive In terrupt Flag Bit:
Set by the receiver af ter the last data bit of a frame has been received.
Cleared by software