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UART-RS232- Interface with FPGA

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UART Stands for Universal Asynchronous Transmitter Receiver. The function of UART is conversion parallel data (8 bit) to serial data. UART transmit bytes of data sequentially one bit at a time from source and receive the byte of data at the destination by decoding sequential data with control bits. As the entire processes require no clock input from source hence it is termed as asynchronous communication.

Baud Rate

In the UART communication data transmission speed is measured by Baud Rate. Baud rate describes the total number of bit sent through serial communication. It includes Start bit, Data byte, Parity bit and Stop bit. Transmitter and receiver need to be maintained in the baud rate. For example  FPGA transmit data at the baud rate of 9600 and at the receiving end PC need to be set with same baud rate using HyperTerminal or TeraTerminal.

Packet of Data in Serial Communication

Serial Communication consist of 2 lines Transmitter and Receiver pin.

Fig 1:Connection between FPGA and PC

Serial data communicate on FPGA side range in 0 to 3.3v. Logic 0 is represented by 0v. Logic 1 is represented by 3.3v

Fig 2: Voltage level of FPGA

On PC Side RS232 Port voltage range from -15v to +15v. Logic 0 is represented by +3v to 15v. Logic 1 is represented by -3v to -15v

Fig 3: Voltage level of PC

In order to communicate between FPGA and PC with different voltage level, MAX3232 Driver IC is required. It consists of 2 channel transmitter and Receiver.

Serial communication

The data communication of UART is made by 11 bit blocks.

Fig 4: Serial communication

The wave form showed the protocol of the UART. Here the β€˜0’ bit represent as start bit which is initiated the serial communication. The start bit must be β€˜0’. The next 8 bit’s are data bit. The LSB bit of data goes as first bit continue it sent other 7 bits. The 10th bit is a parity bit which is used to identify error in the communication. The parity bit is either 0 or 1 which is depending on the number of 1’s present in transmission. If even parity is used, the number of bit must be even. If odd parity is used, the number of bit must be odd. The speed of transmission is fixed which is measured by baud rate. The last bit is stop bit which must be β€˜1’.

Note: The parity bit is not necessary which is optional.

Transmission delay

The transmission rate is measured by bits per second. Each bit has a fixed time duration while transmission. The transmission delay for each bit 104.16 ΞΌs which is constant till the end of communication.
Example
The baud rate is 9600.
Transmission delay =1/9600 =104.16 ΞΌs.

RS 232 connector and cable

RS 232 connector is used to establish connection between FPGA and PC. It is either male or female connector. Here we use only female to female connector. RS 232 connector has only 9 pins, even though the only 3 pins are enough to make a transmission between PC and FPGA such as RD,TD and GND.

                                                                                                      Fig 5:RS232 connector                     

  Table 1:  

Pin noSignal
1Data carrier detect(DCD)
2DReceived data(RD)
3Transmitted data(TD)
4Data terminal ready(DTR)
5Signal ground(GND)
6Data set ready(DSR)
7Request to send(RS)
7Clear to send(CS)
8Ring indicator(RI)

RS232 interface using Max3232 Driver IC with FPGA

Fig 6: Schematic diagram of FPGA and MAX 3232

UART Placement in FPGA

In this article, 3 example codes are provided to demonstrate the UART Communication.

1st VHDL Code describes Transmitting data from PC HyperTerminal to FPGA and feedback to PC at 9600 Baud Rate.

This Code consists of Clock and Reset input. Clock running at 50MHz and Reset is assigned to Slide switch to enable or disable Serial Communication. Din and Do are transmit and receive of the FPGA. The Functionality of this code can be explained by 2 process statements. Process 1 involves transmission of data from PC to FPGA and stores them in array. Process 2 involves retransmission of data stores in FPGA array to PC.

VHDL Code for Serial Communication Transmitting data from PC to FPGA and feedback to PC at 9600 Baud Rate

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity uart is
port ( 	clk   : in std_logic;      -----system clock i/p
       	reset : in std_logic;      -----switch input
		 	din   : in  std_logic;     -----fpga receive i/p
		do    : out std_logic);   ------fpga transmit o/p
end uart;

architecture Behavioral of uart is
type state is (ready,b0,b1,b2,b3,b4,b5,b6,b7,b8);   ----fsm for receive
signal ps : state := ready;
type state1 is (ready1,b01,b11,b21,b31,b41,b51,b61,b71,b81);  -----fsm for transmit
signal ps1 : state1 := ready1;
signal start,stop : std_logic := '0';
signal store : std_logic_vector(7 downto 0) := "00000000";  ----storing of recived value
begin
process(reset,clk)
variable i : integer := 0 ;
begin
if reset = '1' then
if clk'event and clk = '1' then
if ps = ready then     ----logic to detect the transmission start
start <= din;
end if;
if start = '0' then
ps <= b0;
elsif start = '1' then
ps <= ready;
end if;
------------------------------------------1
if ps = b0 then
store(0) <= din;        -----receive and store of each bit
i := i + 1;
ps <= b0;
if i = 5209 then    -----timing delay to receive bit
i := 0;
ps <= b1;
end if;
end if;
------------------------------------------2
if ps = b1 then
store(1) <= din;
i := i + 1;
ps <= b1;
if i = 5209 then
i := 0;
ps <= b2;
end if;
end if;
-----------------------------------------3
if ps = b2 then
store(2) <= din;
i := i + 1;
ps <= b2;
if i = 5209 then
i := 0;
ps <= b3;
end if;
end if;
----------------------------------------4
if ps = b3 then
store(3) <= din;
i := i + 1;
ps <= b3;
if i = 5209 then
i := 0;
ps <= b4;
end if;
end if;
---------------------------------------5
if ps = b4 then
store(4) <= din;
i := i + 1;
ps <= b4;
if i = 5209 then
i := 0;
ps <= b5;
end if;
end if;
---------------------------------------6
if ps = b5 then
store(5) <= din;
i := i + 1;
ps <= b5;
if i = 5209 then
i := 0;
ps <= b6;
end if;
end if;
---------------------------------------7
if ps = b6 then
store(6) <= din;
i := i + 1;
ps <= b6;
if i = 5209 then
i := 0;
ps <= b7;
end if;
end if;
--------------------------------------8
if ps = b7 then
store(7) <= din;
i := i + 1;
ps <= b7;
if i = 5209 then
i := 0;
ps <= b8;
end if;
end if;
--------------------------------------9
if ps = b8 then
stop <= din;
i := i + 1;
ps <= b8;
if i = 5209 then
i := 0;
ps <= ready;
end if;
end if;
-------------------------------------10
end if;
end if;
end process;
----------------------------------------------------------------------transmit
process(reset,clk)
variable i : integer := 0 ;
begin
if reset = '1' then
if clk'event and clk = '1' then
if ps1 = ready1 then
if start = '0' then     -----logic to indicate the communication to the partner
do <= '0';
i := i + 1;
ps1 <= ready1;
if i = 5209 then
i := 0;
ps1 <= b01;
end if;
elsif start = '1' then
ps1 <= ready1;
end if;
end if;
------------------------------------------1
if ps1 = b01 then
do <= store(0);     ----transmission of each bit
i := i + 1;
ps1 <= b01;
if i = 5209 then    ----timing delay for transmission of each bit
i := 0;
ps1 <= b11;
end if;
end if;
------------------------------------------2
if ps1 = b11 then
do <= store(1);
i := i + 1;
ps1 <= b11;
if i = 5209 then
i := 0;
ps1 <= b21;
end if;
end if;
-----------------------------------------3
if ps1 = b21 then
do <= store(2);
i := i + 1;
ps1 <= b21;
if i = 5209 then
i := 0;
ps1 <= b31;
end if;
end if;
----------------------------------------4
if ps1 = b31 then
do <= store(3);
i := i + 1;
ps1 <= b31;
if i = 5209 then
i := 0;
ps1 <= b41;
end if;
end if;
---------------------------------------5
if ps1 = b41 then
do <= store(4);
i := i + 1;
ps1 <= b41;
if i = 5209 then
i := 0;
ps1 <= b51;
end if;
end if;
---------------------------------------6
if ps1 = b51 then
do <= store(5);
i := i + 1;
ps1 <= b51;
if i = 5209 then
i := 0;
ps1 <= b61;
end if;
end if;
---------------------------------------7
if ps1 = b61 then
do <= store(6);
i := i + 1;
ps1 <= b61;
if i = 5209 then
i := 0;
ps1 <= b71;
end if;
end if;
--------------------------------------8
if ps1 = b71 then
do <= store(7);
i := i + 1;
ps1 <= b71;
if i = 5209 then
i := 0;
ps1 <= b81;
end if;
end if;
--------------------------------------9
if ps1 = b81 then
do <= '1';
i := i + 1;
ps1 <= b81;
if i = 5209 then
i := 0;
ps1 <= ready1;
end if;
end if;
-------------------------------------10
end if;
end if;
end process;
end Behavioral; 

UCF file

NET "clk"  LOC = "p55"  ;
NET "din"  LOC = "p60"  ;
NET "do"  LOC = "p63"  ;
NET "reset"  LOC = "p99"  ;

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