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Serial Communication Interface (SCI)
Table 8-4. SCI Status Register (SSR) Bit Definitions (Continued)
Bit
Number
4
Bit
Name
OR
Reset
Value
0
Overrun Error Flag
Description
Set when a byte is ready to be transferred from the receive shift register to the receive
data register (SRX) that is already full (RDRF = 1). The receive shift register data is not
transferred to the SRX. The OR flag indicates that character(s) in the received data
stream may have been lost. The only valid data is located in the SRX. OR is cleared
when the SCI status register is read, followed by a read of SRX. The OR bit clears the
FE and PE bits; that is, overrun error has higher priority than FE or PE. A hardware
RESET signal, a software RESET instruction, an SCI individual reset, or a STOP
instruction clears OR.
3
IDLE
0
Idle Line Flag
Set when 10 (or 11) consecutive ones are received. IDLE is cleared by a start-bit
detection. The IDLE status bit represents the status of the receive line. The transition of
IDLE from 0 to 1 can cause an IDLE interrupt (ILIE).
2
RDRF
0
Receive Data Register Full
Set when a valid character is transferred to the SCI receive data register from the SCI
receive shift register (regardless of the error bits condition). RDRF is cleared when the
SCI receive data register is read.
1
TDRE
1
Transmit Data Register Empty
Set when the SCI transmit data register is empty. When TDRE is set, new data can be
written to one of the SCI transmit data registers (STX) or the transmit data address
register (STXA). TDRE is cleared when the SCI transmit data register is written. Either a
hardware RESET signal, a software RESET instruction, an SCI individual reset, or a
STOP instruction sets TDRE.
In Synchronous mode, when the internal SCI clock is in use, there is a delay of up to 5.5
serial clock cycles between the time that STX is written until TDRE is set, indicating the
data has been transferred from the STX to the transmit shift register. There is a delay of
2 to 4 serial clock cycles between writing STX and loading the transmit shift register; in
addition, TDRE is set in the middle of transmitting the second bit. When using an
external serial transmit clock, if the clock stops, the SCI transmitter stops. TDRE is not
set until the middle of the second bit transmitted after the external clock starts. Gating
the external clock off after the first bit has been transmitted delays TDRE indefinitely.
In Asynchronous mode, the TDRE flag is not set immediately after a word is transferred
from the STX or STXA to the transmit shift register nor when the word first begins to be
shifted out. TDRE is set 2 cycles (of the 16 × clock) after the start bit; that is, 2 (16 ×
clock) cycles into the transmission time of the first data bit.
0
TRNE
1
Transmitter Empty
This flag bit is set when both the transmit shift register and transmit data register (STX)
are empty, indicating that there is no data in the transmitter. When TRNE is set, data
written to one of the three STX locations or to the transmit data address register (STXA)
is transferred to the transmit shift register and is the first data transmitted. TRNE is
cleared when a write into STX or STXA clears TDRE or when an idle, preamble, or
break is transmitted. When set, TRNE indicates that the transmitter is empty; therefore,
the data written to STX or STXA is transmitted next. That is, there is no word in the
transmit shift register being transmitted. This procedure is useful when initiating the
transfer of a message (that is, a string of characters).
DSP56311 User’s Manual, Rev. 2
8-16
Freescale Semiconductor
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