Data Transmission and Reception
11-42
yet. Another element, C, arrives and fills RSR. DRR is finally read, but not earli-
er than two and one half cycles before the end of element C. New data D over-
writes the previous element C in RSR. If RFULL is still set after the DRR is read,
the next element can overwrite D if DRR is not read in time.
Figure 11–24.
Serial Port Receive Overrun
D7
A1
A0
B7
B6
B5
B4
B3
B2
B1
B0
C7
C6
C5
C4
C3
C2
C1
C0
No RBR–to–DRR copy (B)
RBR–to–DRR copy (A)
No Read of DRR (A)
No RSR–to–RBR copy(C)
No Read of DRR(A)
CLKR
FSR
DR
RRDY
RFULL
Figure 11–25 shows the case in which RFULL is set but the overrun condition
is averted by reading the contents of DRR at least two and a half cycles before
the next element, C, is completely shifted into RSR. This ensures that a RBR-
to-DRR copy of data B occurs before the next element is transferred from RSR
to RBR.
Figure 11–25.
Serial Port Receive Overrun Avoided
A1
A0
B7
B6
B5
B4
B3
B2
B1
B0
C7
C6
C5
C4
C3
C2
C1
C0
No RBR–to–DRR copy (B)
Read of DRR (A)
RBR–to–DRR copy (A)
No Read of DRR (A)
RBR–to–DRR (B)
CLKR
FSR
DR
RRDY
RFULL