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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)
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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)
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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;
} |
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