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SFR explanation for PIC16F873(16)
TXSTA ( Transmit Status and Control Register ) 98h
This register controls USART transmit function.

The value in the parenthesis is in the condition immediately after the turning on.

CSRC : Clock Source Select bit
Asynchronous mode
Don't care

Synchronous mode
1:Master mode ( Clock generated internally from BRG )
0:Slave mode ( Clock from external source )

TX9 : 9-bit Transmit Enable bit
1:Selects 9-bit transmission
0:Selects 8-bit transmission

TXEN : Transmit Enable bit
1:Transmit enable
0:Transmit disable

SREN/CREN of RCSTA register overrides TXEN in SYNC mode

SYNC : USART Mode Select bit
1:Synchronous mode
0:Asynchronous mode

BRGH : High Baud Rate Select bit
Asynchronous mode
1:High speed
0:Low speed

Synchronous mode
Unused in this mode

TRMT : Transmit Shift Register Status bit ( Read only )
1:TSR empty
0:TSR full

TX9D : 9th bit of transmit data. Can be parity bit.