The beauty of these operators is that the x and y in the above examples can be any combination of function, RS232 serial stream, variable, string and more. One of the key features of this new feature in the CCS C Compiler is the way it automatically handles the conversion based upon the data types of the variables passed. Tutorial para usar la comunicacion serial en los microcontroladores PIC con el compilador CCs.

Introduction to Serial communication with PIC16F877 microcontroller

In this tutorial we will study the communication component – USART (Universal Synchronous Asynchronous Receiver Transmitter) located within the PIC. It is a universal communication component (Synchronous/Asynchronous), which can be used as transmitter or as receiver. We will look at:

We will show how to set USART in order to allow communication between PIC to PIC or between PIC to a personal computer. We will start with the definition of media concepts. There are two options to differentiate when speaking about transmission of information on the transmission lines:

In order to understand what serial communication is, and emphasize the difference between serial communication and parallel communication, let’s take a look at the following example:
We have a multi-bit word, and we want to transmit it from one computer to the second computer.

Using the serial communication:

When using the serial communication we transmit the multi-bit word bit after bit (when at any given moment only one bit will pass).

Transmitting the word 10011101 using serial communication.

Using the parallel communication:

When using the parallel communication, however, the number of bits will be transmitted at once from one computer to the second computer.

Transmitting the word 10011101 using parallel communication.

In addition to the serial and parallel communications, there are 2 types of communication we will explore:

Synchronous communication

When using the synchronous communication – the information is transmitted from the transmitter to the receiver:

in sequence
bit after bit
with fixed baud rate
and the clock frequency is transmitted along with the bits

That means that the transmitter and the receiver are synchronized between them by the same clock frequency. The clock frequency can be transmitted along with the information, while it is encoded in the information itself, or in many cases there is an additional wire for the clock.

This type of communication is faster compare to the asynchronous communication since it is 'constantly transmitting” the information, with no stops.

Asynchronous communication

When using the asynchronous communication - the transmitter and the receiver refraining to transmit long sequences of bits because there isn't a full synchronization between the transmitter, that sends the data, and the receiver, that receives the data.

In this case, the information is divided into frames, in the size of byte. Each one of the frame has:

“Start” bit marks the beginning of a new frame.
“Stop” bit marks the end of the frame.

Frames of information must not necessarily be transmitted at equal time space, since they are independent of the clock.

Enabling Serial Communication

To communicate with external components such as computers or microcontrollers, the PIC micro uses a component called USART - Universal Synchronous Asynchronous Receiver Transmitter. This component can be configured as:

a Full-Duplex asynchronous system that can communicate with peripheral devices, such as CRT terminals and personal computers
a Half-Duplex synchronous system that can communicate with peripheral devices, such as A/D or D/A integrated circuits, serial EEPROMs, etc.

To enable the serial communication with PIC micro we must set different parameters within two registers: (click the links for the explanation of each bit)

An example of 8-bit transmission:

Let’s assume that we need to transmit the following information: 10110010. This information will be stored inside TXREG register, which acts as a temporary buffer storage of information prior to transmission.
The bit TX9 will be zero (TX9=0) - which determines that the transmission will be 8-bit transmission, so there is no need to address TX9D bit, which stores the ninth bit of information.

The information before the transmission looks like this:

Transmitting 8 bit data

Now, let’s define the receiver side to receive 8 bit information. To do so, the register RX9 will be zero (RX9=0). The received information will be stored in the RSR register, which acts as a temporary buffer storage.

The received information will look like this:

Receiving 8 bit data

An example of 9-bit transmission:

Suppose we want to transmit the following information: 110010110. This information is in the size of 9-bit, so there is not enough space to store all the information in the TXREG register . Thus, we will store the low 8-bit in the register TXREG and the MSB in the TX9D bit.

We will set the TX9 = 1 - enabling transmission of 9-bit data. It is important to note, that first we need to store the 9th bit and only later other 8-bits. This is important because the information of 8 bits may be transmitted immediately once being inside the TXREG register. As a result the transmitted information will be incorrect.

The information before the transmission will look like this:

Transmitting 9 bit data

Now, let’s define the receiver side to receive 9 bit information. To do so, the register RX9 will be set (RX9=1). The received, lower 8-bit information, will be stored in the RSR register, which acts as a temporary buffer storage. The higher bit information (MSB) will be stored in RX9D.

The received information will look like this:

Receiving 9 bit data

Now let’s continue the explanation. Each transmission is transmitted in the particular rate (BAUD). The baud rate is measured in units of bps (bit per second) or kbps (kilo bit per second ).

Calculating the value being placed in the SPBRG register

Let’s assume we want to transmit using the BAUD rate of 1200bps. This is done by setting the system clock to the value needed. To do so, we need to “write” a hexadecimal number to the SPBRG register. The value written to the SPBRG register set the clock cycle to the value we want for the BAUD rate.

The size of SPBRG register is 8-bit. As discussed previously, in asynchronous mode, the baud rate of transmission of the information can be set to high speed or to low speed. The rate selection, as already seen, is made by the BRGH bit in TXSTA register:

1 = High speed
0 = Low speed

For each baud rate we need to calculate the value being placed in the SPBRG differently:

SPBRG = (Fosc / (16 x Baud rate)) - 1, BRGH = 1 High Speed

SPBRG = (Fosc / (64 x Baud rate)) - 1, BRGH = 0 Low Speed

The following outlines how the value which is placed in the SPBRG register is being computed, in the case of a high baud rate and low baud rate.

For example:
We want to calculate the hex value that will be placed the register SPBRG, to get the baud rate of 1.2kbps with low speed. The formula SPBRG = (Fosc / (64 x Baud rate)) - 1was chosen since, its describing the calculation needed for transmission in Low Speed:
SPBRG = (4MHz / (64x1200)) -1 = 51.08

Because it is not possible to write a number with a decimal point to the register, we take only the whole part of the number and place inside the register SPBRG = 51.
The following tables are the BAUD RATES FOR ASYNCHRONOUS MODE BRGH=0 and BRGH=1.

BRGH=0

BRGH=1

USART transmit block diagram

Comunicacion Serial Rs232 Pic Ccs 2017

USART transmit block diagram

The information we want to transmit is loaded into the 8-bit register - TXREG. If you want to transmit a 9-bit data, the 9th bit is loaded into TX9D. At the same time, the information above is being