i will try to understand by writing some code. Program suppose to send three bytes to computerAt this level, the UART breaks down into 3 separate functions:
Init
Transmit a character
Receive a character
The PDF shows one way to do the 3 functions. There is room for improvement but hopefully you get the basic idea.
// CONFIG
#pragma config FOSC = HS // Oscillator Selection bits (HS oscillator)
#pragma config WDTE = OFF // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF // Power-up Timer Enable bit (PWRT enabled)
#pragma config BOREN = ON // Brown-out Reset Enable bit (BOR enabled)
#pragma config LVP = OFF // Low-Voltage (Single-Supply) In-Circuit Serial Programming Enable bit (RB3 is digital I/O, HV on MCLR must be used for programming)
#pragma config CPD = OFF // Data EEPROM Memory Code Protection bit (Data EEPROM code protection off)
#pragma config WRT = OFF // Flash Program Memory Write Enable bits (Write protection off; all program memory may be written to by EECON control)
#pragma config CP = OFF // Flash Program Memory Code Protection bit (Code protection off)
// End of configuration
#include <xc.h>
#define _XTAL_FREQ 20000000
#define Baud_rate 9600
void Initialize_UART(void) //Initializing UART module for PIC16F877A
{
TRISC6 = 0; // TX Pin set as output
TRISC7 = 1; // RX Pin set as input
/**Initialize SPBRG register for required
baud rate and set BRGH for fast baud_rate**/
SPBRG = ((_XTAL_FREQ/16)/Baud_rate) - 1;
BRGH = 1; // for high baud_rate
SYNC = 0; // Asynchronous
SPEN = 1; // Enable serial port pins
TXEN = 1; // enable transmission
CREN = 1; // enable reception
TX9 = 0; // 8-bit reception selected
RX9 = 0; // 8-bit reception mode selected
}
void UART_send_char(char message) //Function to send one byte of date to UART
{
while(!TRMT); // Wait till the transmitter register becomes empty
TXREG = message; //load the char to be transmitted into transmit reg
}
// Sends null-terminated ASCII string
void UART_send_string(char* p)
{
char c;
while((c = *p) != '\0')
{
UART_send_char(c);
p++;
}
}
void main(void) // START of Main Function
{
Initialize_UART(); //Initialize UART module
UART_send_string("1"); //pass first byte only
UART_send_string("2"); // pass second byte only
UART_send_string("3"); // pass third byte only
}
Not sure what you are asking but yes, the code will send 123 to the UART at 9600 baud. The signals you show are generated by the hardware and you don't need to worry about them unless you have problems or just want to see them with an oscilloscope.1) Do we send three bytes like shown above ?
UART_send_string("123"); // does the same thing as 3 calls.
My error. I should have said TXIF, not TRMT. Change your code to use TXIF. Sorry.2) When TRMT become enabled and disable during transmission of three bytes ?
void UART_send_char(char c) //Function to send one byte of data to UART
{
while(!TXIF); // Wait till the transmitter register becomes empty by polling TXIF (not TRMT)
TXREG = c; //load the char to be transmitted into transmit reg. This clears TXIF automatically.
}
My question is, will I need start bit and start bit every time if I send more than one byteNot sure what you are asking but yes, the code will send 123 to the UART at 9600 baud.
The start/stop bits are hardware programmed UART synchronization events (start-ready receive buffer time interval/stop-buffer store or execute function time interval) for the encapsulated data for the transmit to receive mechanism (originally a mechanical buffer but mainly electronic today).My question is, will I need start bit and start bit every time if I send more than one byte
View attachment 197128
As you can see I have used the start bit and stop bit three times.
Either I should use start and stop bit only once As given in the below diagram
View attachment 197131
No, YOU won't need a start and stop bit every time. You are not bit banging, you are using a hardware peripheral that knows how to do it. The asynchronous protocol used by the UART does need start and stop bits but the UART does it automatically for each byte sent. All you have to do is write the character to TXBUF and everything else is done for you.My question is, will I need start bit and start bit every time if I send more than one byte
Hi JohnInTXNo, YOU won't need a start and stop bit every time. You are not bit banging, you are using a hardware peripheral that knows how to do it. The asynchronous protocol used by the UART does need start and stop bits but the UART does it automatically for each byte sent. All you have to do is write the character to TXBUF and everything else is done for you.
NO. The PIC UART sends one 8 bit character with start and stop bits for each byte you write to TXREG. To send more than one byte, write each byte to TXBUF when TXIF==1. That's ALL you need to know. You are confusing UART with I2C. Don't. They are completely different protocols.This means that my second diagram is correct. This indicates that communication has started and we can send one or more bytes.
Communication will stop after the last byte
The UART always sends 8 bits. Period. You can send a 4 bit value but it must be padded to 8 bits. 0000dddd.My other doubt is that if we want to send only 4 bits of data, then how can we do this via uart?
NO. You are thinking bit-banging again.I think we will need to use bit wise operator to send only 4 bits, Am I right ?
@JohnInTX ThanksWhy are you making this so hard? Configure the UART then write each character to TXREG. You wrote the code to do that above. Do you not understand what you wrote??
1 GB?? You do know that 1GB will take almost 13 days to send at 9600 baud?I think MCU will read 1 byte from sensor and it will load 1 byte to TXREG when it completes it will load next byte to TXREG up to 1GB data.
We do not need to store the sensor data inside MCU
Please take a look at diagram in link https://de.wikipedia.org/wiki/Universal_Asynchronous_Receiver_TransmitterIf the sensor data comes in bursts you can use a buffer to hold the data from the sensor while you transmit to the UART.
Some sort of flow control between the PIC and the sensor so that the PIC can tell the sensor not to send anything more until the previous data is sent.
That's right. Your UART is configured for 9600,N,8,1This is confusing me there is parity bit after 7 bit but we don't do that
I saw the datasheet of pic16f877a It has inbuilt USART not UART so USART can operate in both mode Asynchronous/SynchronousThat's right. Your UART is configured for 9600,N,8,1
USART stands for Universal Synchronous / Asynchronous Receiver Transmitter. This means that the USART in the PIC can run in one of two modes: either synchronous, which uses a clock and data (like SPI) or asynchronous which does not use a separate clock.Does this mean that clock signals are sent with every single bit in USART?
I understand UART but I do not understand exactly what happen in USARTAt your level, for communicating with a terminal or PC, you would want to confugure the USART for asynchronous mode so I dropped the 'S' from the name.
Good question, though!
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