The previous article explains data transfer. Also supports serial data transfer with frame size of 5, 6, 7 and 9 data bits.
The size of data frame can be adjusted according to application. For example, consider a system that needs to transmit only ASCII codes as data. In this case data frame size of 7-bits is sufficient because data length of ASCII code is equal to 7-bit.
This will makes system more efficient by saving time of one clock period in each data frame transmission. This article explains serial transfer of data with different frame size.
As shown in above picture, the proper output is received when numeric keys are pressed but when alphabetic keys are pressed some symbol appears. This unexpected output is received because we are working with 6-bit data frame size. If 6-bit data frame is selected, two most significant bits are masked to zero while reading from UDR. The ASCII code of ‘1’ is 0x31 (0011 0001).
If the two most significant bits are masked, 11 0001 remain which is also equal to 0x31. The size of ASCII codes for all numeric values is 6-bits so there will no problem in receiving and transmitting numeric values in 6-bit mode. But the ASCII value of ‘A’ is 0x41 (0100 0001). If two most significant bits are masked, 00 0001 remain which is equal to 0x01 so it will print some unusual symbol.
AVR Microcontroller The AVR is a type of controlling device manufactured by the Atmel Corporation in 1996. The AVR does not stand for anything, it is just a name. The AVR, and therefore, the device run very fast with a reduced number of machine level instructions (RISC). The AVR microcontrollers consist of special features compared with other microcontroller such as 6-sleep modes, inbuilt ADC, internal oscillator and serial data communication, etc.
The are available in different configurations of 8-bit, 16-bit, and 32-bit to perform various operations. AVR Microcontroller USART Serial Data Communication in AVR Microcontroller The USART stands for universal synchronous and asynchronous receiver and transmitter. It is a serial communication of two protocols. This protocol is used for transmitting and receiving the data bit by bit with respect to clock pulses on a single wire.
The has two pins: TXD and RXD, which are specially used for transmitting and receiving the data serially. Any AVR microcontroller consists of USART protocol with its own features. USART Communication in AVR Microcontroller The Main Features of AVR USART. The USART protocol supports the full-duplex protocol.
It generates high resolution baud rate. It supports transmitting serial data bits from 5 to 9 and it consists of two stop bits.
Data Transmission Mode Selection Step4: Start Bit and Stop Bit The start bit and stop bits are a way for sending and receiving the data serially. Generally any data fame consists of one stat bit and one stop bit, but the AVR microcontroller has one start bit and two stop bits for processing the data. The extra stop bit can be useful for adding a little extra receive processing time.
It is especially useful for high data transfer rates, whereas the data transfer speed is very high, so we don’t get proper data. Thus, we can increase the processing time by using two stop bits to get the proper data. Start Bit and Stop Bit The number of stop bits is selected by the USBS bit of UCSRC – the control status register.
The USBS=0, for one stop bit, and USBS=1, for two stop bits. Step5: Set the Character Size As in case with the sending and receiving the byte of data(8-bits) at a time, whether in a AVR microcontroller, we can choose a data frame format in each frame by the UCSZ bit of the UCSRC register. Data Frame Format Step6: Store the Received Data The AVR microcontroller consists of a UDR buffer register for transmitting and receiving data. The UDR is a 16-bit buffer register wherein 8-bits are used for receiving (RXB) the data and other bits are used for transmitting the data (TXB). Transmitting data buffer register will be the destination to UDR register for the written data on its location. Receiving data buffer register will be returning the content of the UDR register.
Step7: Transmitter and Receiver Enabling The transmitted and received data will be allowed by the RXC and TXC pins of the microcontroller which are set by the UCSRA register of the microcontroller. This flag bit set by the microcontroller for the data is completed by receiving and transmitting (TXC=RXC=1).
Double the Baud Rate We can double the transfer rate of the USART communication of the AVR effectively by the U2X –bit in the UCSRA register. This bit effects only on asynchronous operation. If we can set this bit (U2X=1), it will reduce the baud rate from 16-bit to 8-bit effectively doubling the transfer rate for synchronous communication. This is an advanced feature of the AVR microcontroller for speedy processing of the data. USART Program Every microcontroller is predefined with a specific IDE, and based on this IDE, or assembly language. The AVR microcontroller programming is developed by the AVR studio.
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The above design demonstrates the usage of UART to send and receive data via hyperterminal as well display the received data in 1 16×2 LCD. As we all know Microcontroller works in TTL logic which is not compatible with the PC so we have to employ a level converter IC MAX232, read more about the REGISTERS USED IN AVR SERIAL COMMUNICATION: In AVR there are five registers which are meant to use for Serial Communication such as UDR, UBBR, UCSRA, UCSRB, UCSRC. Lets see the functions of these registers briefly.
UDR: UDR Register UDR or USART Data Register is meant for writing and receiving the data through the UART. In this there are two shift registers referred to as Transmit Shift register and Receive Shift register with each having a separate buffer register. When the data is written to UDR it will be transferred to Transmit Data buffer register and when we read the contents of the Receive Data buffer register is returned. UBRR: In AVR the baud rate of the UART is programmable and it is achieved by means of this UBRR register. It is 16 bit register classified into lower UBRRL and higher UBRRH out of which 12 bit is usable The formula governing the relation between the value of UBRR and Oscillator is Baud Rate = (Oscillator Frequency / (16( UBRR Value +1)) So for a 8MHz oscillator frequency and 9600 baud rate the value need to be loaded in the UBRR will be UBRR = (8MHZ /16(9600))-1 =(500KHz/ 9600) – 1 = 51 ( equivalent hex 33) UCSRA: UCSRA Register UCSRB: UCSRB Register UCSRC Register: UCSRC Register STEPS TO PROGRAM UART:.
Load the hex value in the UBRR Register for the Baud rate you are about to use. Set the bits in the registers UCSRA, UCSRB & UCSRC based on your usage requirement. For Transmission Place the data in the UDR register and check for the appropriate flag to set in the UCSRA regsiter. Clear the Flag for further transmission. For receiving the data, wait for the Receive flag to set in the UCSRA register and then read the UDR register to obtain the received data for processing or display.
Clear the Flag for further data reception.
Avr Microcontroller Simulator
One of the distinguishing characteristics of beginner-friendly microcontroller platforms– Arduino, PICAXE, and a few dozen others– is that they neatly wrap up and hide the nuts-and-bolts details of interfacing with the hardware. Like everything else, it’s a blessing and a curse. The benefits are clear: A new user who has just acquired an Arduino can plug it in, blink an LED, and have a working demonstration of two-way serial communication in just a few minutes. The drawbacks are a little harder to see.
When you just use one line of initialization that calls a “library,” it’s easy to overlook exactly what’s involved: how many lines of code have invisibly been added to your program? What memory structures have been allocated? What interrupts are now going to disrupt program flow and timing?
There’s also a portability issue. We often hear from people who got started with Arduino but now want to explore other AVR microcontroller systems, and don’t know how or where to start the migration process. In what follows we discuss a minimal setup for serial communication with AVR microcontrollers, and give two example implementations, on an ATmega168 and on an ATtiny2313. While this fundamental “AVR 101” stuff, we’re approaching the problem (this time) from the migration standpoint.
Suppose that you had an Arduino based project, where you relied on serial communication– using the library functions–between that hardware and your computer. From there, how would you migrate to a stand-alone AVR microcontroller with similar functionality, or even to a different microcontroller?
The answers, of course, are (1) that you have to hook things up correctly and (2) it’s nice to have some simple and lightweight set of routines as a good starting point. To begin with, we need a good working cross-platform example of two-way communication between the host computer and the Arduino. We chose as the starting point the sketch (example program) “Serial Call-Response” by Tom Igoe. This is actually a two-part program, where one half runs on your computer and the other half on the microcontroller. What we’re doing is to create a functional clone of the microcontroller program, that can still talk to the host-side program (which is still running in Processing on your computer). Let’s look at the chips: We’ve drawn the ATmega168 and ATtiny2313 here, with the pinouts for power supply and the serial port transmitter (TX) and receiver (RX) pins highlighted. These are some but (probably) not all of the pins that you’ll need.
For example, if you want to program the AVR in circuit, it’s often best to do that with an ISP programmer, and our article about explains how to hook one up. Back to the serial ports. If you look in the datasheets for these chips, you’ll find each serial port labeled as a “USART,” for Universal Synchronous and Asynchronous serial Receiver and Transmitter. Now, the microcontroller USART pins are designed to operate at logic levels– between 0 and 3-5 V. This means that they can’t directly hook up to RS-232 without a level adaptor. They also aren’t set up for directly hooking to USB, so an adapter of some sort is usually needed unless you’re talking directly to another microcontroller. Our adapter of choice is the FTDI USB-TTL cable: This cable has an integrated circuit built in that translates between a microcontroller’s serial port (USART) and USB.
(That’s in fact the same chip that the Arduino has on board for the same reason.) Here’s how to hook it up to one of the microcontrollers: Three pins on the connector are relevant for us: TX, RX, and ground. (Ground so that your computer and microcontroller board have a common voltage reference.) The TX (transmit) pin of your microcontroller goes to the RX (receive) pin of the cable/computer, and vice versa. In practice you can do this with a 5 pin header with two teeth cut out, where the fifth pin is connected to ground. This can be done on a target board like we’ve done here or on a breadboard– it’s a quick way to hook things up without laying out a special board just for the serial port. The serial port pins on the ‘2313 (above) and ‘168 (below) are in the same place, so you can hook this up identically on either board. Now that we have the physical connections ready, we need the software. You can download our example code for the ATmega168 is (5 kB.ZIP file), and the code example for the ATtiny2313 is (5 kB.ZIP file).
Both source code examples contain a C source file and a makefile; they can be compiled and loaded onto the microcontrollers from AVR-GCC or compatible systems; if you’ve never put code onto an AVR before, you might find our list of helpful. The source code is meant to be a minimal implementation of bidirectional communication that can talk to the “Serial Call-Response” sketch. The two implementations, for the two different chips, are mostly identical. We begin by setting several pins to be outputs, using the “data direction registers” defined in the datasheet. The only one of these that’s actually important is pin D1, the TX pin, on each board.
List Of Microcontrollers
We also have a place to put an indicator LED on each board, and a few other outputs for optional debugging. The baud rate is implemented using a non-obvious but relatively compact scheme taken from the datasheet. It takes as inputs the CPU rate and the desired baud rate. The CPU clock frequency is 16 MHz from an external crystal oscillator on the ‘168 example and 8 MHz from the internal RC oscillator on the ‘2313 example. (If desired those can be changed with the fuse settings.
You know about the AVR, right?) Communication is implemented through four serial functions: serialCheckRxComplete (which checks whether there is serial data ready to read), serialCheckTxReady (which checks if the serial port is ready to transmit new data), serialRead (which reads a byte of data from the serial port), and serialWrite (which transmits one byte from the serial port). These functions work directly with the hardware; buffering and interrupts are not implemented. This makes the program lean and mean, with a tiny footprint. Working without interrupts and buffers means that you need to poll (i.e., check for new incoming data) at least as often as new data could appear– e.g., about 1 kHz or faster for 9600 baud. It also means that incoming serial data will never interrupt time-critical functions that are executing. And the memory footprint is much, much lower. This takes mere bytes of RAM to implement– making it quite practical on the ATtiny2313 which only has 128 bytes of RAM.
(For comparison: the Arduino serial library usually allocates 128 bytes of RAM as a RX buffer alone.) So, there are tradeoffs– but these lightweight functions work well enough, and should provide another helpful stepping stone to AVR, from either above or below. An interrupt itself does not significantly affect the program size; an interrupt routine is roughly equivalent in terms of footprint to adding a function call in a main loop.
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However, (1) the way that serial interrupts are often used.does. tend to grow the memory footprint, and (2) a program is not 'lean and mean' in my opinion when it’s bloated by unnecessary interrupts. There are two.primary.
reasons (that I can see) that you might want to use a serial interrupt. First, you have time-critical incoming data, and you want to respond to it ASAP. Obviously, a serial interrupt that parses and responds to data can be very fast and memory efficient– 'lean and mean' indeed, if the main point of the program is to respond to incoming serial data quickly. The second reason is if you cannot (for any number of possible reasons) make a commitment in your program to check for incoming data at a sufficiently high rate, and you want a routine to receive and store serial data until you get around to looking at it.
The latter is a good description of the Arduino serial library. If you have the resources to manage a (potentially) large serial input buffer, and don’t mind the footprint, this is also a fine method, but it’s never lean or mean. As for unnecessary interrupts, I think that it’s important to avoid them, especially in example code. In one of our AVR applications, we need to respond very quickly when an input level goes high or low. To do this we use an interrupt, of course. But here’s the problem: Our pin-change interrupt routine cannot begin if there’s already an interrupt running.
That means that if a serial interrupt routine were executing, we’d have to wait until it finished to respond to our important signal. In contrast, our serial polling routine has no trouble running the port at full speed, while still allowing our CPU to respond quickly when our signal changes value. In this context, the footprint of a serial interrupt routine would be.huge.– completely defeating our ability to respond to signals in real time.
Oskay drwho(at)evilmadscientist.com.
In this tutorial we are going to establish a serial communication between two ATMEGA8 microcontrollers. The communication established here is UART (Universal Asynchronous Receiver Transmitter) type. By this serial communication data can be shared between two microcontrollers, which is required in various embedded systems.
Components Required Hardware: ATMEGA8 (2 pieces), power supply (5v), AVR-ISP PROGRAMMER, 100uF capacitor (connected across power supply), 1KΩ resistor (two pieces), LED, Button. Software: Atmel studio 6.1, progisp or flash magic. Circuit Diagram & Explanation Let us understand the serial communication in AVR microcontrollers. Here ATMEGA sends data to the other ATMEGA in serial. It has other mode of communication but for easy communication we are choosing.
The RS232 pin of first ATMEGA8 is connected to RXD pin of second ATMEGA8. The data communication established is programmed to have:. Eight data bits. Two stop bits. No parity check bit. Baud rate of 2400 BPS(Bits Per Second).
Asynchronous communication (No clock share between two ATMEGA8) So we have two set registers for two ATMEGA8 differently, where one acts as TRANSMITTER and other acts as RECEIVER. Now for the RS232 interfacing between two ATmega microcontrollers, the following features must be satisfied for the TRANSMITTER and RECEIVER: 1. The TXD pin (data receiving feature) of first controller must be enabled for TRANSMITTER and RXD pin of second controller must be enabled for RECEIVER. Since the communication is serial we need to know whenever the data byte is received, so that we can stop the program until complete byte is received. This is done by enabling a data receive complete interrupt. DATA is transmitted and received to controller in 8bit mode.
So two characters will be sent to the controller at a time. There are no parity bits, one stop bit in the data sent by the module. The above features are set in the controller registers; we are going to discuss them briefly, DARK GREY (UDRE): (TRASMITTER SIDE) This bit not set during startup but it is used during working to check whether transmitter is ready to transmit or not.
See the program on TRASMITTER SIDE for more details. LIGHT GREY (RXC): (RECEIVING SIDE) This bit not set during startup but it is used during working to check whether receiver is ready to receive data or not. See the program on RECEIVING SIDE for more details. VOILET (TXEN): (TRASMITTER SIDE) This bit is set for enabling transmitter pin on TRASMITTER SIDE. RED (RXEN): (RECEIVING SIDE) This bit represents receive data feature, this bit must be set for the data from the module to be received by the controller, it also enables RXD pin of controller.
BROWN (RXCIE): This bit must be set for getting an interrupt after successful data reception. By enabling this bit we get to know, right after 8 bit data receive. We are not going to use this bit here so it is left alone. PINK (URSEL): This bit must be set before enable other bits in UCSRC, after setting other needed bits in UCSRC; URSEL must be disabled or put to zero. We are not going to use this bit here so it is left alone. YELLOW(UCSZ0,UCSZ1,UCSZ2): (RECEIVING SIDE & TRASMITTER SIDE)These three bits are used for selecting the number of data bits we are receiving or sending in a single go.
The communication between two ATMEGA is established as eight bit communication. By matching the communication with table we have, UCSZ0, UCSZ1 to one and UCSZ2 to zero. We have to set these on both receiving and transmitting side.
ORANGE (UMSEL): (RECEIVING SIDE & TRASMITTER SIDE)This bit is set based on whether the system is communicating asynchronously (both use different clock) or synchronously (both use same clock). Both the controllers do not share any clock. Since both of them use internal clock of their own. So we need to set UMSEL to 0 in both controllers.
GREEN (UPM1, UPM0): (RECEIVING SIDE & TRASMITTER SIDE) These two bits are adjusted based on bit parity we are using in communication. The ATMEGA is programmed to send data with no parity, as the data transmission length is small, we can clearly expect no data loss or error. So we are not setting any parity here. So we set both UPM1, UPM0 to zero or they are left, because all bits are 0 by default. BLUE (USBS): (RECEIVING SIDE & TRASMITTER SIDE) This bit is used for choosing the number of stop bits we are using during communication. The communication established here is of asynchronous type, so for getting more accurate data transmission and reception, we need to use two stop bits, Hence we set USBS to ‘1’ in both controllers.
The baud rate is set in controller by choosing the appropriate UBRRH. The UBRRH value is chosen by cross referring baud rate and CPU crystal frequency. So by cross reference UBRR value is seen as ‘25’, and so the baud rate is set. As shown in circuit a button is connected on transmitter side.
When this button in pressed a eight bit data is sent by TRANSMITTER and this data is received by RECEIVER. On receiving this data successfully it toggles the LED connected to it ON and OFF, which shows successful data transfer between two controller.