Datasheet

142
AT8xC51SND1C
4109E–8051–06/03
Interrupt The Serial I/O Port handles 2 interrupt sources that are the “end of reception” (RI in
SCON) and “end of transmission” (TI in SCON) flags. As shown in Figure 109 these
flags are combined together to appear as a single interrupt source for the C51 core.
Flags must be cleared by software when executing the serial interrupt service routine.
The serial interrupt is enabled by setting ES bit in IEN0 register. This assumes interrupts
are globally enabled by setting EA bit in IEN0 register.
Depending on the selected mode and weather the framing error detection is enabled or
disabled, RI flag is set during the stop bit or during the ninth bit as detailed in Figure 110.
Figure 109. Serial I/O Interrupt System
Figure 110. Interrupt Waveforms
ES
IEN0.4
Serial I/O
Interrupt Request
TI
SCON.1
RI
SCON.0
RXD D0D1D2D3D4D5D6D7
Start Bit 8-bit Data Stop Bit
RI
SMOD0 = X
FE
SMOD0 = 1
a. Mode 1
b. Mode 2 and 3
RXD D0D1D2D3D4D5D6 D8
Start bit 9-bit data Stop bit
RI
SMOD0 = 1
FE
SMOD0 = 1
D7
RI
SMOD0 = 0