dmx512 transmitter

[mahmoud boroumand]

Posted on July 24, 2017 at 13:22

Hello 

I useing stm32f103c8(72mhz) and i want create DMX512 transmitter for  led lighing.

I have some problem.I config usart like this.

USART_InitStructure.USART_BaudRate = 250000;

USART_InitStructure.USART_WordLength = USART_WordLength_8b; USART_InitStructure.USART_StopBits = USART_StopBits_2; USART_InitStructure.USART_Parity = USART_Parity_No; USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None; USART_InitStructure.USART_Mode = USART_Mode_Tx; USART_Init(USART2, &USART_InitStructure);

when i make reset sequence and data with this code.

_delayus(20);///MarkBeforeBreak

USART_SendBreak(USART2);//Break

_delayus

(12);//MarkAfterBreak; USART_SendData(USART2,0);///start code

for(int i=0;i<512;i++)

{  

USART_SendData(USART2,255);//R

USART_SendData(USART2,255);//G

USART_SendData(USART2,255);//B

_delayus

(12);//MTBP

}

I have some problem with this code.

1-Break time is 44us but dmx need 92us at least how can solve this problem?

2-Is structure right for dmx led light?

Thanks

[Vangelis Fortounas]

Posted on July 24, 2017 at 14:30

Hi!!

for the first part:

by sending more (at least 3) consecutive breaks. Is it permited from protocol?

[Vangelis Fortounas]

Posted on July 24, 2017 at 15:24

Hi again

for the second part. .I am nnot familiar with this protocol but you have some UART issuesBefore you send a character or every character by USART_SendData(USARTX,y);must check before if DR txbuffer is empty by//  SendByte sequencewhile( USART_GetFlagStatus(USART2, USART_FLAG_TXE)==RESET);USART_SendData(USART2,data);to send two or more  consecutive breaks  must check before ' Break Character' flag //check before  the CR1 SBK flagwhile(USART2->CR1 & CR1_SBK_Set);USART_SendBreak(USART2);//Breakwhile(USART2->CR1 & CR1_SBK_Set);USART_SendBreak(USART2);//Break

[mahmoud boroumand]

Posted on July 25, 2017 at 13:06

hello thanks.

but when i use this code

while(USART2->CR1 & CR1_SBK_Set);

USART_SendBreak(USART2);//Break

break doesn't send at all

[Vangelis Fortounas]

Posted on July 25, 2017 at 16:18

Hello again

mahmoud

!

What you mean

doesn't send at all

?

You mean stucks to ''while loop''?

To observe if Uart tranmitts  clear manualy TC flag (transmit completed) just before transmit.

and observe it after send a break .

I took a look at dmx512 protocol.

It don't says ''send a break ''  but describes Break condition as LO state of line. (means keep IO pin low for xxx microsec)

So i think you need a different approximation for your project..

For Break and MAB and MTBFs   states you may use your TX pin as normal output and keep it low or hi  for some time. .

When you need to transmit serial data  Change it to Uart tx pin and transmit the data!

Regards

[ISHA]

The USART module available on STM32

microcontrollers has the ability to automatically generate a 12-bit long break signal,

corresponding to 48 µsec at 250 k baud. Unfortunately, this is too short for use in a DMX512

application as the protocol requires a minimum length of 92 µsec.

Figure 6 shows the alternative hardware method chosen to generate the longer break

signal. A 100 Ω resistor is connected in series with the microcontroller's USART transmit pin

and the other end of the resistor to an I/O pin. With this solution, the break time can be

varied, from 92 µsec to 176 µsec to meet the DMX protocol break time specification, when

sending a break signal, the I/O pin is driven low. Later, the I/O pin is tri-stated to allow

transmission from the USART to resume.