Baud Rate

spatlou

What is Baud Rats and how to set it in serial port device driver?

ece.harpreet

Definition Baud rate:
Fundamentally baud rate can be defined as the rate of modulation.
i.e., symbols transferred per unit time.
It may or may not be equal to bit rate that depends upon number of symbols the communicating devices would have agreed upon before starting data transfer (during hand-shaking).

and about serial port configuration... well i haven´t worked on it yet!

spatlou

In digital communications, symbol rate, also known as baud or modulation rate is the number of symbol changes i.e. waveform changes or signalling events made to the transmission medium per second using a digitally modulated signal or a line code.

spatlou

The symbol rate is measured in baud (Bd) or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second. Each symbol can represent or convey one or several bits of data. The symbol rate is related to, but should not be confused with, the gross bitrate expressed in bit/second.

spatlou

A symbol can be described as either a pulse (in digital baseband transmission) or a "tone" (in passband transmission using modems) representing an integer number of bits.

spatlou

A theoretical definition of a symbol is a waveform, a state or a significant condition of the communication channel that persists for a fixed period of time. A sending device places symbols on the channel at a fixed and known symbol rate, and the receiving device has the job of detecting the sequence of symbols in order to reconstruct the transmitted data.

spatlou

There may be a direct correspondence between a symbol and a small unit of data or the data may be represented by the transitions between symbols or even by a sequence of many symbols.

spatlou

spatlou

kashishgarg1992

guyzzzz do anybody think that why BAUD RATE values
110 300 600 1200 2400 4800 9600 14400 19200 28800 38400 56000 57600 115200

Standard baud rates supported by most serial ports:

were choosen not any other in between values like 5000 or 1000 or ....???????????????????

kashishgarg1992

Doubt regarding baud rate and bit rate relationship.....

If my baud rate is 9600 and my frame size=12 { 8data +1 start +2 stop +1 parity }.

by definition can i say baud rate is no of symbols transfer per sec = no of frames tarnsfer per sec ???????????

Also what would be the bit rate according to above scenerio??????

kartik.gupta

Implementation:

Voltages on port pins:

RX ->pin 2 -->-10.7V

TX ->pin 3 -->-10.7V

RTS->pin 7 -->-10.7V (bit 1 of Modem Control register is 0 initially)

CTS->pin 8--> 0V (read only pin) (bit 4 of Modem Status Register is 0 initially)

kartik.gupta

Baud rate setting for IC PC16550D:

To set the baud rate we need to access the divisor latch registers for which we have to set DLAB bit (no.7) of Line Control Register.

Calculate the divisor which needs to be feed in Divisor Latch Registers(DLL,0x3f8 and DLM, 0x3f9)

divisor = crystal freq/(16*baud rate)

Taking crystal freq of 1.8MHz as per our motherboard value of divisors for 3 std baud rates are:

9600:

addrbit=inb(LCR);

test_and_set_bit(7,&addrbit); //DLAB=>1

outb((unsigned char)addrbit,LCR);

outb(0x0C,DLL); //for 9600 baud rate divisor is 12

outb(0x00,DLM);

19200:

addrbit=inb(LCR);

test_and_set_bit(7,&addrbit); //DLAB=>1

outb((unsigned char)addrbit,LCR);

outb(0x06,DLL); //for 19200 baud rate divisor is 6

outb(0x00,DLM);

38400:

addrbit=inb(LCR);

test_and_set_bit(7,&addrbit); //DLAB=>1

outb((unsigned char)addrbit,LCR);

outb(0x03,DLL); //for 38400 baud rate divisor is 3

outb(0x00,DLM);

kartik.gupta

PC16550D Device Operation Modes:

->16450 mode

->Fifo polled mode

->Fifo interrupt mode

To prevent receiver buffer overrun for high data transmission, PC16550D introduced Fifo which were absent in 16450,

Without using interrupts and Fifos we need to use handshaking signals RTS and CTS for TX and RX ack

for proper synchronization.

kartik.gupta

Mechanism :

Transmitter side:

->First configure the LCR (0x3fb), FCR(0x3fa) and IER (0x3f9) registers.

//Configure Line Control Register

//enable the RBR,THR,IER

outb(0x03,LCR); //frame of 8 bit word length with one stop bit

//Disable Interrupt Enable Register

outb(0x00,IER);

//configure Fifo Control Register

outb(0x00,FCR); //enable 16450 mode (non-fifo)

-> Send character byte to THR register (0x3f8) and wait for TEMT bit 6 of Line Status register to set to 1.

-> Send ack to receiver by changing RTS bit of MCR(0x03fc) port pin 7 which is connected with CTS bit of MSR(0x03fe).

->Wait for receiver ack to send another byte.

Receiver Side:

-> Configure the registers as above .

-> Wait for sender ack by checking CTS bit of MSR(0x3fe) to get 1 during odd runs and to get 0 during even runs.

->Save data to user buffer

->Send ack to sender to get another byte

kartik.gupta

TX code snippet:

addrbit=inb(MSR)

ret=test_bit(4,&addrbit); //0 initially

while(buffsz)

{

outb(*(char*)(ldev->sqset->data[i]),THR);

while(!(inb(LSR) & 0x40)); //check TEMT bit,wait for TEMT to become 1(both THR and TSR to get empty),

//TEMT will be 0 when either of THR and TSR is having data

//when both THR and TSR empty ,set RTS to ack receiver

addrbit=inb(MCR);

test_and_change_bit(1,&addrbit); //for first run or odd runs, RTS=1(PORT volt--> 11V) will make CTS of DTE2 to 1

outb((unsigned char)addrbit,MCR); //for even runs RTS=0(PORT volt-->-11V) will reset the CTS of DTE2

if(i==register_no-1)

{

ldev->sqset=ldev->sqset->next;

i=-1;

}

i++;

buffsz=buffsz-1;

//wait for reader to send ack

if(count%2!=0) //for odd runs wait for 1

{

while(!ret)

{

addrbit=inb(MSR)

ret=test_bit(4,&addrbit);

}

else //for even runs wait for 0

{

while(ret)

{

addrbit=inb(MSR)

ret=test_bit(4,&addrbit);

}

count++;

}

addrbit=inb(MCR);

test_and_clear_bit(1,&addrbit); //reset the port bit to initial stage after writing

outb((unsigned char)addrbit,MCR);

kartik.gupta

RX code snippet:

addrbit=inb(MCR);

test_and_clear_bit(1,&addrbit); //reset the MCR bit to initial state

outb((unsigned char)addrbit,MCR);

while(data_byte!='\0')

{

//wait for transmitter ack

if(count%2!=0) //for odd count wait for CTS bit to become 1

{

while(!ret)

{

addrbit=inb(MSR);

ret=test_bit(4,&addrbit);

}

else //for even count wait for CTS bit to become 0

{

while(ret)

{

addrbit=inb(MSR);

ret=test_bit(4,&addrbit);

}

data_byte=inb(RBR);

ret2=copy_to_user(buff+nocsr,&data_byte,1);

addrbit=inb(MCR);

ret2=test_and_change_bit(1,&addrbit); //toggle the RTS bit to ack transmitter to send next character

outb((unsigned char)addrbit,MCR);

count++;

nocsr++;

}

return (nocsr);

Baud Rate

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