UART Communication

UART stands for universal asynchronous receiver and transmitter. UART is an asynchronous half duplex communication protocol. It is called universal because of configurable parameters such as speed, frame format and can be configured to meet the needs of a given communication requirement. There is no clock signal between transmitter and receiver. Both transmitter and receiver may work on different clock speed.  Two devices communicate with each other through three signals TX, RX and GND as shown in figure below:

Device1 Tx will connect with Device2 RX and Device1 RX will connect with Device2 TX and both devices GND will be common.  It is half duplex communication protocol which share the same register for data transmission and register. So when using UART it should be kept in minds that you should transmit and receive simultaneously. At transmitter side UART done parallel to serial conversion of the data which is received from the CPU and at receiver end UART perform serial to parallel conversion on the data received through serial communication.

Frame Structure

In UART data is transferred in a proper frame structure. UART frame consist of data bits and control bits. UART frame consists of following data:

·        Start bit

·        8 or 9 data bit

·        Parity Bit (optional)

·        1 or 2 stop bit

Figure below show the data frame with 8 data bit 1 start bit and 1 stop bit:

The parity bit in UART data frame is optional.  The parity can be even or odd. The UART peripheral calculates the number of 1s present in the transmission. If the parity is configured to even and the number of 1’s is even then the parity bit is set zero. If the number of 1s is odd, the parity bit is set to a 1 to make the count even. If the parity is configured to odd, and the number of 1s is odd, then parity bit is set to 0. Otherwise it is set to 1 to make the count odd. At receiver end UART check remove this bit from the data. START bit always will be 0 and STOP bit always will be 1.

When there is no communication TX pin remains high. When Communication starts Transmitter pulls the TX pin low (START bit). After data bit STOP bit will be transmitted. Every Bit will be of certain duration. This depends on the communication Baud rate selected.

After getting START bit (Sense LOW on RX pin) receiver samples in the middle of every period and save data to UART DATA REGISTER and also give information to CPU by setting flag in register or by interrupt or CPU if enabled.  Receiver also verifies the Parity Bit and STOP bit. If there is any framing or parity receiver notifies to CPU by setting error bits in UART STATUS register.

Figure show below the transmission of Char ‘D’ without parity bit and 1 stop bit.

Char ‘D’ Decimal equivalent is 68 and hex decimal equivalent is 0x44.

Binary equivalent will be B”01000100”.

What is BAUD RATE?

BAUD rate defines the data communication rate in UART communication. Baud rate specified in bps(bits per second). So baud defines how many bits may be transferred in one second from one device to another device. For example if Baud rate defines as 9600 bps. It means 9600 bits can be transferred from one device to another device. For proper communication receiver and transmitter should be configured to same baud rate. Otherwise receiver will receive abrupt data.

One bit period at 9600 baud rate will be = 1/9600 sec = 104.16 usec

1 Byte with 1 start bit and 1 stop bit transmission time will be = 10*104.16 usec                                                                                            = 1.04 ms

So it takes 1.04 ms to transmit one byte from one device to another device.

Communication in Microcontroller

“In this blog we will discussed about wired communication modules in microcontroller, types of communications such as parallel, serial, synchronous or asynchronous communication.”

Main function of communication modules in microcontroller is for interacting with other microcontroller, peripherals, PC or other modules. These communications modules are inbuilt in microcontrollers. Communication protocols are the rules and regulation which both devices follow to communicate with each other. For example if two microcontrollers wants to communicates with UART it requires that both microcontroller should configure for same baud rate, no of stop bits and data bit to be communicated.

Parallel Communication

In parallel communication two devices communicates through several lines by communicating more than one bit at a time. In parallel communication transmitter will uses serial in and parallel out shift registers. The receive data can be directly read through 8-bit GPIO Register.

Parallel communication used for short distance communication. Example of parallel communication can be found in LCD interfacing through microcontroller. Parallel communication requires large numbers of electrical conductors wires and expensive to setup it. In parallel communication number of conductor wires increases as increase in numbers of bits to be transmitted at one time.

Serial Communication

In serial communication two devices communicates through single wire by communicating single bit at a time.  Data bits are transferred sequentially. At transmitter side there will be a parallel in serial out shift register and at receiver there will be serial in parallel out shift register.  In serial communication data is communicated in packet format by adding start and stop bits in the data.

Data is transmitted from one device to another device through single electrical conductor. Serial communication used in long distance communication and less expensive then parallel communication. Examples of serial communications protocol are as UART, USARTSPI, and I2C etc.

If both Serial and parallel channels for 1-byte communication are operated on same clock than Parallel communication speed will be 8 time faster than serial communication.

Serial communication can be Synchronous or Asynchronous.

Synchronous communication

In Synchronous communication transmitter and receiver work on the same clock signal.  This clock signal is provided by the master device who initiates the communication.  Receiver will detect the data according to timing of clock signal. Data transfer rate in Synchronous communication is faster as compare to asynchronous communication.

In synchronous communication there will be one master device and all devices are slave device. To start communication master will first generate start communication frame. In synchronous communication all the slave devices are addressed or selected through control signal from master device. Example of Synchronous communication protocols are SPI and I2C.

Asynchronous Communication

In asynchronous communication there is common clock signal between transmitter and receiver. To start communication it is required that sender and receiver should be configured on the same communication parameters such and data communication speed etc. In asynchronous communication bits are grouped together and control bit such as Start and stop bit also added with data to be transfer. To properly receives data receiver will oversampling the data.

Asynchronous communication is slow due to increase in extra control bit with every byte of data. If we are transmitting 8 bit data with two control bit start and stop bits than there will be 25% overhead.  Example of asynchronous communication is UART.

Continued...........

GPIO Pins in Microcontroller

GPIO Stands for General Purpose Input and Output pins in microcontroller. Peripherals in microcontrollers such as UART, ADC, SPI, DAC etc. will interact with the external environment through GPIO pins. To communicate with external environments core uses GPIO pins. For example to transmit or receive data from devices core will manages TX and RX pins for that purpose.  Depending on the operation required GPIO pins can be configured in PUT or OUT put mode. For example if controller giving any command to external environment GPIO pin will be in OUT put mode and if controller getting data for manipulation GPIO pin will be in INPUT mode.

In microcontroller there are different modes for INPUT and OUT put. These mode can be selected through peripheral registers in microcontroller.  

GPIO Input Mode

GPIO input mode can be selected by writing GPIO data direction and control registers. GPIO input mode is of following types:

High Impedance Mode

This mode is also called Tri-State or Floating point mode. In this mode pin status is not deterministic until external source is applied. By Default controllers pins are in floating point mode. Advantage of using High Impedance State is that controller pin can be pulled to high or low with less current. It is used in ADC sense when it required keeping ADC sense signal to be low.

Pull Up/Pull down Mode

In Pull up mode controllers pin will remain high until it is forced to ground by external source. In this mode a week pull up resistor is connected to VDD (controller supply) internal to pin microcontroller. In pull down mode controllers pin remains low until external supply is not applied. In this mode a week pull down resistor to ground. Pull up and pull down is used to ensure a defined state of input pin. This mode is used for external interrupt pins, ADC etc.

All GPIO input pins uses the concept of hysteresis to prevent spurious changes in state when an input value changes. Hysteresis is implemented on GPIO by setting two thresholds for a digital input which are VIH and VIL typically enumerated in the “Electrical Characteristics” section of microcontroller datasheets. VIH is the input-high voltage and represents the minimum voltage that must be applied to drive the pin high. VIL is the maximum input-low voltage needed to drive the pin low. This means if the input voltage is greater than VIL and less than VIH, the value at the pin depends on whether the past input value was above VIH, in which case the pin is high, or below VIL, which means the pin is low.

GPIO OUT MODE

GPIO input mode can be selected by writing GPIO data direction and control registers. In Microcontroller output mode is of following types:

PUSH –PULL MODE

Push-Pull Output Configuration uses two Mosfet or transistors. One is N-Channel and another is P-Channel Mosfet. The Upper switch will be ON when the output has to be driven high. It will connect controller pin directly with VDD. Lower switch will be ON when output has to be driven LOW. In this mode Pin State is deterministic. It will have any one state either High or Low Push Pull configuration will have the capability to Source or Sink the Current.  

OPEN DRAIN MODE

In this mode, the physical GPIO pin behaves same way as drain of MOSFET. The drain is open to connect it to any external element. The source is grounded and gate of internally driven. It has two states: either High-impedance or Low. The output pin is driven low by firmware, the voltage at pin is zero. When the output pin is driven high by firmware, the voltage can be either high or low

. It is mostly used for I2C bus where external pull-up resisters are used widely. It can be also used in Interrupt line where it is connected to multiple interrupt sources.