MicroSetup

Microcontroller Setup

The microcontroller USART peripheral requires setting up so that data can be transmit and received. The USART needs to be activated so that the associated pins are used for communications (and not as general purpose Input/Output). The data format and speed (baudrate) also need configuring to be the same as the PC. All the setup information can be found in the PIC datasheet.

UART Registers

The PIC 16F627 has 3 registers that require setting up for communications

SPBRG (Baud rate generator) - this register is used to set up the baudrate (speed) of the UART.

TXSTA (Transmit status and control) - this register is used to set up the UART for transmission and also to see when bytes have been transmit

RCSTA (Recive status and control) - this register is used to set up the UART for receiving and also for checking to see in an error occurred whilst recieving a byte.

In our example we are going to transmit and receive at 9600 baud. Our data will have 8 data bits and each byte will have a start and one stop bit.

Baudrate (SPBRG)

The baudrate is dependant on the microcontroller clock speed, as we are using the internal oscillator in our example this will be 4MHz. There are two formulas for calculating the baudrate depending on whether the High Baud Rate bit is selected (BRGH).

BRGH = 0 (low speed)

BaudRate = Fosc / (64(X+1))

X = (Fosc / 64xBaudRate) - 1

X = (4000000 / 64x9600) - 1

X = 5.51

choosing X = 6 yields:

BaudRate = 8929 (7.00 % error)

BRGH = 1 (high speed)

BaudRate = Fosc / (16(X+1))

X = (Fosc / 16xBaudrate) - 1

X = (4000000 / 16x9600) - 1

X = 25.04

choosing X = 25 yields:

BaudRate = 9615 (0.16 % error)

As the baudrate needs to be within 5% (max), then high speed (BRGH = 1) must be used.

Transmit Register (TXSTA)

The Transmit Register requires setting to 00100100 (binary) which is 0x24 (hex)

Bit	Name	Setting	Deception
7	CSRC	0	We are running asynchronous so we don't have a clock
6	TX9	0	8-bit transmission
5	TXEN	1	Enable the Device for Transmission
4	SYNC	0	Asynchronous Mode
3	-	0	-
2	BRGH	1	High Speed Baud Rate required
1	TRMT	0	This is a READ only bit
0	TX9D	0	No 9 bit transmission or parity.

Receiving Register (RCSTA)

The Recieve Register requires setting to 10010000 (binary) which is 0x90 (hex)

Bit	Name	Setting	Deception
7	SPEN	1	Enable the serial port
6	RX9	0	8-bit reception
5	SREN	0	not used for asynchronous
4	CREN	1	Continuous Receive
3	ADEN	0	not used for asynchronous
2	FERR	0	This is READ only bit
1	OERR	0	This is READ only bit
0	RX9D	0	This is READ only bit

Initialisation Code

As well as the UART registers there is one other set up that is required. The PORT Pins must be set up as inputs for the pins to be associated with the UART peripheral. The pins associated with the UART are on PORT B, RB1 (RX) and RB2 (TX). The following code extract configures the UART.

void uart_initialisation(void)
{
    TRISB |= 0x6; //Set RB1 and RB2 as inputs
    SPBRG = 25;   //Setup Baudrate to 9600
    TXSTA = 0x24; //Enable Transmission
    RCSTA = 0x90; //Enable Reception
    return;
}