UART

Let's begin with writing the main.c , msp.c, it.c from scratch to write hello-world using USART2.

Peripheral is collective name Uni Sync Asynch Receiver Transmitter but we use it in Asynch mode.

1. UART Essentials

Introduction: It is just a piece of hardware (pins on micro-controller) that converts Parallel data into Serial Data.

It is used in conjunction with protocols like RS-232, RS-434, USB, etc.

In synchronous mode, the clock is sent separately and for asynchronous, it is not. Rather, start and stop bits are sent.

2. Understanding UART Pins

1. It requires a minimum of two pins for bidirectional functionality when not using hardware flow control.
2. Rx pin continuously samples the data to detect Start bit.
3. For TX pin to work (send data), the Clear-To-Send (CTS) pin should be ground. (important debugging tip).

The communication is pretty sophisticated. The Request-To-Send (RTS) of one device is connected to Clear-To-Send (CTS) of another device. Whenever, the device 1 wants to reques the device 2 to send data, it makes its RTS low, so that the device-2 Tx is on.

3. UART Frame Formats

Different protocols have their different frame formats ( TCP, UDP, etc). The UART packet format is as follows.

4. Baud Rate

1. Baud rate is essentially the measure of how fast your data is being transmitted. It is given a SI unit of bits per second. If we invert the baud rate, we essentially get the amount of time that is required to send one bit.
2. The peripheral clock (UART) decides the maximum baud rate.

5. Synchronization Bits

1. 1.5 bits means one and half bit duration.
2. One start bit and 1 or 2 stop bits for high speed Baud Rates (Mbps).
3. Start bit: Idle state (High) to Low. Stop bit: Low to Idle State (High).

6. Parity Bits

1. Even Parity means the total number of 1 including both the payload and parity bit should be even. Odd, for odd parity.
2. It will discard the data and request for resend in the next frame.

7. UART Functional Block Diagram

8. Peripheral Clock

Although connected to APB1 and APB2 bus. If the controller is powered by 16 MHz RC Oscillator, then it will get only 16 MHz. (Max it will get is bus max).

9. UART Transmitter

The Transmit data register (TDR) is filled by the software. The shift register is loaded only when the stop bit is received from the last payload. The protocol

1. Program the M (8 or 9 bits) in USART_CR1 register for payload length,
2. Program the number of Stop bits in USART_CR2.
3. Select the desired baud rate in USART_BRR register.
4. Set the TE bit in USART_CR1 to enable the transmit block.
5. Enable the USART by writing the UE bit in USART_CR1.
6. If TXE flag is set, write the data byte to be send in the USART_DR register.
7. After writing the last data, you can disabled USAR peripheral after TC (Transfer Complete) = 1.

10. UART Reception

This is very interesting. It is same as transmitter till 5th.

6. If RXNE bit is set, it means the Frame1 is completely loaded in the shift register and can be read by the software into Receive Data Register (RDR).

7. The RXNE bit should be cleared on every complete RDR reception.

Instead of polling the Receive shift register, we can generate an interrupt for RXNE flag. The RX line keeps sampling the receiver line even when data is not there. We can some power over here by disabling the UART Rx after complete data reception.