SPRUIY4B February 2023 – May 2024 TMS320F2800152-Q1 , TMS320F2800153-Q1 , TMS320F2800154-Q1 , TMS320F2800155 , TMS320F2800155-Q1 , TMS320F2800156-Q1 , TMS320F2800157 , TMS320F2800157-Q1
The SCI can be configured to use the asynchronous timing mode using TIMING MODE bit in SCIGCR1 register.
The asynchronous timing mode uses only the receive and transmit data lines to interface with devices using the standard universal asynchronous receiver- transmitter (UART) protocol.
In the asynchronous timing mode, each bit in a frame has a duration of 16 SCI baud clock periods. Each bit therefore consists of 16 samples (one for each clock period). When the SCI is using asynchronous mode, the baud rates of all communicating devices must match as closely as possible. Receive errors result from devices communicating at different baud rates.
With the receiver in the asynchronous timing mode, the SCI detects a valid start bit if the first four samples after a falling edge on the LINRX pin are of logic level 0. As soon as a falling edge is detected on LINRX, the SCI assumes that a frame is being received and synchronizes to the bus.
To prevent interpreting noise as Start bit SCI expects LINRX line to be low for at least four contiguous SCI baud clock periods to detect a valid start bit. The bus is considered idle if this condition is not met. When a valid start bit is detected, the SCI determines the value of each bit by sampling the LINRX line value during the seventh, eighth, and ninth SCI baud clock periods. A majority vote of these three samples is used to determine the value stored in the SCI receiver shift register. By sampling in the middle of the bit, the SCI reduces errors caused by propagation delays and rise and fall times and data line noises. Figure 23-4 illustrates how the receiver samples a start bit and a data bit in asynchronous timing mode.
The transmitter transmits each bit for a duration of 16 SCI baud clock periods. During the first clock period for a bit, the transmitter shifts the value of that bit onto the LINTX pin. The transmitter then holds the current bit value on LINTX for 16 SCI baud clock periods.