IntroductionFigure 1: PIC18F26K20 Connected to PC via MAX232A serial port interface can be used for serial communication of data (send and receive) where data is sent or received one bit at a time between a personal computer (PC) and various devices supporting this type of protocol like PIC microcontrollers, GPS, GSM modem etc. While other interfaces like Ethernet, FireWire, and USB all send data as a serial stream, the term “serial port” usually identifies hardware more or less compliant to the RS-232 standard, intended to interface with a modem or with a similar communication device. The Start bit informs the receiver that a word of data is about to be sent, this is to tell the receiver to synchronise its clock with the transmitter’s clock.
The frequency is very important here as well, that is why the transmitter and the receiver must have identical baud rate.After the Start bit, now each individual bits of data are sent starting the Least Significant Bit (LSB).When the entire data word has been sent, the transmitter may add a Parity Bit that the transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking.Then at last one Stop Bit is sent by the transmitter to indicate the end of transmission.When the receiver has received all of the bits in the data word, it may check for the Parity Bits (both sender and receiver must agree on whether a Parity Bit is to be used), and then the receiver searches for a Stop Bit.If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read. Common reason for the occurrence of Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted.The typical format for serial ports used with PC connected to modems is 1 Start bit, 8 data bits, no Parity and 1 Stop bit.The typical format for serial ports used with PC connected to modems is 1 Start bit, 8 data bits, no Parity and 1 Stop bit.This is basically in brief the software protocol of the UART.
How about the physical layer standards that actually defines the level of voltages and so on?There are quite a number of different standards that utilizes similar protocol. For instances, TTL level UART, RS-232, RS-422, RS-485 and etc.The TTL UARTMost microcontrollers with UART uses TTL (Transistor-transistor Logic) level UART. It is the simplest form of UART.Logic 1 is represented by 5V and logic 0 by 0V.Logic VoltageLow 0VHigh 5VVoltage level for TTL level UARTThe TTL level UART is commonly used in the communications between microcontrollers and ICs. Only 2 wires are required for the full duplex communications as illustrated in the picture below on figure 1.Figure 2: Serial Communication Between 2 deviceThe RS-232The RS-232 is a standard for serial binary data signals connecting between a Data Terminal Equipment (DTE) and a Data Communication Equipment (DCE).One of the significant differences between TTL level UART and RS-232 is the voltage level.
Valid signals in RS-232 are ±3 to – ±15V, and signals near 0V is not a valid RS-232 level.Logic VoltageLow +3 to +15VHigh -3 to -15VVoltage level for RS-232Besides voltage level, the RS-232 also has a few extra pins specifically designed for the communication between PC and modem. The pinouts of the DB-9 and their functions are shown below in table 1. UARTxReadTextPrototype: void UARTxReadText(char.Output, char.Delimiter, char Attempts);This function keeps on reading characters received via UART until the delimiter sequence is detected. The read sequence is stored in the parameter output; delimiter sequence is stored in the parameter delimiter.Parameters: Output: received textDelimiter: sequence of characters that identifies the end of a received stringAttempts: defines number of received characters in which Delimiter sequence is expected.If Attempts is set to 255, this routine will continuously try to detect the Delimiter sequence. Before using this function, the UART must be configured first using the UARTxInit fuction.Example: This example reads text until the sequence “END” is received, and send back what’s been received. Char message 8;UART1Init(4800); // initialize UART1 moduleUART2Init(4800); // initialize UART2 moduleRS485MasterInit; // initialize microcontroller as Master for RS485 communicationUARTSetActive(&UART1Read, &UART1Write, &UART1DataReady, &UART1TxIdle); // set UART1 activeRS485MasterSend(message,3,160); // send message through UART1UARTSetActive(&UART2Read, &UART2Write, &UART2DataReady, &UART2TxIdle); // set UART2 activeRS485MasterSend(message,3,160); // send through UART2.
RS485MasterSend ( message, 3, 160 ); // send through UART2ExampleLet us create a simple project as shown on figure 1, it consists of a PIC18F26K20 connected to a PC via MAX232 voltage converter. 3 LEDs are connected to PORTB, Red LED on RB0, Yellow LED on RB1 and Green LED on RB2. If 1 received from the PC, the PIC switches ON the Red LED, 2 the Yellow LED is switched ON and 3 switches ON the Green LED.You can test this example using the mikroC PRO for PIC terminal, by clicking on Tools menu then USART Terminal or any other Serial Terminal. You can also use our serial terminal we created with.Remember to set correctly the PORT settings and select the correct PORT.
Watch the Video TutorialSPI stands for Serial Peripheral Interface. The SPI protocol was initially developed and proposed by Motorola for use in microprocessor and microcontroller based interface applications.In a system that uses the SPI bus, one device acts as the master and other devices act as slaves. The master initiates a communication and also provides clock pulses to the slave devices.SPI is implemented in the PIC microcontrollers by a hardware module called the Synchronous Serial Port or the Master Synchronous Serial Port (MSSP).
This module is built into many different PIC microcontroller devices. It allows serial communication between two or more devices at a high speed and is reasonably easy to implement The MSSP can operate either in or SPI mode.SPI is usually called a 4-wire full duplex synchronous serial communication, the functions of the 4 wires are as follow:SCLK (Serial Clock): This is the serial clock signal. It is generated by the master device and controls when d ata is sent and when it is read.
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The SCLK pin of master is connected to SCLK pin of slave.MOSI (Master Out Slave In) or SDO (Serial Data Output) This pin carries data out of a device (from master to slave).MISO (Master In slave Out) or SDI (Serial Data In) This pin carries data into a device (from slave to master).SS (Slave Select)or CS (Chip Select): This pin allows a master device to indicates to a slave that the master wishes to start an SPI data exchange between that slave device and itself. The signal is most often active low, so a low on this line will indicate the SPI is active and communication can take place, while a high will signal inactivity.This signal must be used when more than one slave exists in a system, but can be optional when only one slave exists in the circuit but it always a good practice to use it. Depending on the number of devices in the bus, we can have more than one chip Select lines, one per device in the bus.
Figure 1 hows the microcontroller connected to 3 SPI slaves (3 chip select lines).SPI is used for connecting peripherals to each other and to microcontrollers.There are many devices that support the SPI protocol and can easily communicate with a microcontroller via SPI: A/D converters, D/A converters, SD Cards, Liquide Crystal Displays (LCD), Real Time Clocks like the DS1306, serial display drivers like MAX7219, Serial EEPROM like 25LC256, etc.Depending on the number of slave devices connected, the master will have one dedicated pin for SS line for each slave. Figure 1 below shows an illustration of one master (microcontroller) controlling three slave devices.Figure 1: SPI BusRead also on Student Companion:Advantages of SPI:.
Full duplex communication. It’s faster than asynchronous serial and allowing large quantities of data to be transferred quickly.
SPI1Init ( ); //Note the x is replaced with 1. SPI2Write ( buffer );SPISetActiveThis function sets the active SPI module which will be used by the SPI routines and it is only available with microcontroller with two SPI modules. After using this function, they will be no need anymore to specify the SPI module to use during read or write operations.Prototype: void SPISetActive(char (.readptr)(char), void(.writeptr)(char))Parameters: readptr: SPIxRead handlerwriteptr: SPIxWrite handlerBefore you can use this function, the SPI must be initialized and communication established See SPIxInitAdvanced or SPIxInit.Example.
Table of Contents.This is a simple demo project showing how to setup an driven serial port with buffering using the existing mbed support without talking directly with hardware. There is also a project that talks directly to UART hardware registers and it is probably more efficient time wise and easier to use, but it is a bit more difficult to understand all of the low level hardware code details. Also there is a new project. Serial ports can transmit and receive data at the same time.
Serial handshaking lines to start and stop the data flow are typically not used or connected, so using interrupts along with buffering of character data is typically required to avoid loss of characters at high baud rates on serial ports. UARTs have an internal FIFO buffer to hold perhaps 16 characters or so, but it is just not large enough. The other important advantage is that using interrupts saves processor time, since code does not spin in I/O wait loops constantly checking slowly changing I/O status bits.Two are used to hold serial data with an in and out pointer.
Mikroc Serial Interrupt Protocol Download
One buffer is used for TX data buffering and one for RX data buffering. In is the location where new data is added to the buffer and out is the buffer location where data is removed. When in and out are incremented and reach the end of the buffer they wrap around using mod (i.e.,%) buffer size. The buffer empty condition is in=out and the buffer full condition is (in + 1)% buffer size = out.Using attach, you can specify the interrupt handler function for serial receive and serial transmit. In general, interrupt routines should not call any library functions unless you know that they are. Typically, they are not reentrant.
Most hardware I/O devices cannot be in use in the main program and also used by an interrupt routine without problems. Mutual exclusion may also be needed on such hardware devices by disabling interrupts in the main program whenever the I/O device is being actively used.Formatted IO using sprintf and sscanfGetc and putc will work with interrupt routines, but in many cases formatted I/O using printf or scanf would be more desirable.Printf in its current version cannot currently be used with any serial interrupt routines that use getc. Even a printf to a different serial port such as USB that does not have an interrupt handler will lock up, if one serial port has interrupt code using getc.
This is likely some reentrancy issue with the two functions. For buffering, sprintf needs to be used in any case. Fortunately, it is possible to use sprintf to print to a buffer first and then copy the formatted data into the TX buffer using another function. In this code, the function is called sendline. Sscanf is used in a similar manner with readline in this short demo program.Critical Sections using Enable and Disable InterruptsWhen accessing and modifying global or shared data with more than one process (i.e., main and interrupt routine) mutual exclusion is needed to avoid errors due to data inconsistency. This can happen when switching back and forth between processes while a new global or shared variable value is being changed in a register and has not yet been written back to memory. With an OS, would normally be used to solve this problem.
On a single processor system with no OS, mutual exclusion for critical sections can be provided by disabling interrupts. In the case here, we only want to disable interrupts briefly from the UART on the serial port being used. The volatile keyword in C on variable declarations helps this problem somewhat by making operations on that variable atomic (i.e., non-interruptible), but does not solve all of the possible problems here with the buffer array. In the mbed compiler, interrupts can be disabled with disableirq; and enabled with enableirq; (i.e., prefix is two underscores).
Mikroc Serial Interrupt Protocol Pdf
In the mbed C compiler, the functions NVICEnableIRQ and NVICDisableIRQ are used to disable and enable individual interrupts. In the demo code, UART1 interrupt hardware is used (i.e., UART1IRQn). UART1 connects to p9 and p10. This and the explain the UART and interrupt control hardware in more detail. Contains interrupt assignments and names. There is even a book just on.Running the Demo CodeThe test program below sets up two interrupt routines (Rx,Tx) with two circular buffers.
It is setup for a loopback test, and serial out must be tied back to serial in with a jumper wire from mbed p9 to p10. In an infinite loop, the main program increments a counter, prints out the counter in hex, decimal, and octal on three lines with sprint and sendline. It then reads back three lines using sscanf and readline. If there are any errors reading back data, led4 turns on and stays on. In the main program, while sending data led3 is on and while reading data led3 is off. Led1 and led2, indicate code activity in the two interrupt routines.
So if all is well, led1, led2, and led3 are dimly lit and led4 is off. A demo using serial interrupts with buffering in a loopback testLast commit 09 Dec 2012 byThe mbed's four LEDs display activity from the serial interrupt demo code. The dim state shows that the code is constantly sending and receiving data in the main program (LED3) and that it is also spending some time in both of the serial interrupt routines (LED1 and LED2). LED4 is off indicating no errors in reading back thousands of data values using interrupts and buffering.On many processors, the divide required for the mod (i.e.,%) operation takes more execution time than the other basic integer operations. If the buffer size is restricted to a power of two, the mod operation could be replaced by a simple bitwise AND (i.e., &) operation that masks off the lower bits and it would have a faster execution time. Tools and hardware to setup and debug serial connectionsA voltage level conversion circuit or is required to connect mbed to an RS-232 serial port. The serial port on a PC uses RS-232 voltage levels +/-3 to +/-15V (i.e., not TTL 0-5V logic levels).
This one from comes with the conversion circuit on a breakout board with the typical DB9 serial connector. It also has LEDs that indicate serial data flow.mbedSparkfun RS232 SMD adapter3.3VVccGndGndP9 - TXRXP10- RXTXIf you hook up a serial port and data is not flowing, a serial cable or a null modem connector is often required so that TX is connected to RX, and RX is connected to TX at each end.
Serial cables can be wired straight through or they may include the null modem signal swap and unfortunately they typically are not labeled. Here is a small DB9 mini null modem connector from. If a null modem is required, you can't just swap the TX and RX pins to the mbed chip since the voltage level conversion circuits only work in one direction.
Small are also available when the cables are the wrong sex for the connector. Serial devices that work when the cable is plugged directly into the back of a PC will need both a null modem and a gender changer when used with mbed and the RS232 breakout boards.
A single could be used to minimize all of the connector clutter, if you can find one.A few serial devices also require the optional hardware handshake signals for. If you need to use handshaking, here is a serial breakout from Pololu that also includes the conversion circuits for the serial handshake lines. The wiring table does not include connections that might be needed to RS-232 handshake lines, but the default no connection may be OK for some devices.
Mbed's APIs do not support all hardware handshake options. In some, cases pulling the handshake line high or low can get a device talking that is waiting on a handshake signal to send data. Realterm shows the state of the handshake lines and they can also be forced high or low on the PC side to experiment with a device that needs a handshake (lower right in image of Realterm that follows in next section).
A few devices even pull power off of a handshake line to power the serial device (see for such an example).mbedPololu RS232 adapterVU=5vVCCGNDGNDRX(10)TXTX(9)RXA terminal emulation program that runs on a PC is a handy way to test serial ports and code. As seen below includes more features to debug serial port hardware than most. It is seen running the demo code. It has a status display of the connector signals in the lower right.
Data can be displayed in ASCII, or hex for binary data transmissions. It also displays non-printable ASCII control codes, so you can see exactly what data is being sent, even if it is a character that does not print. If your PC does not have a serial port, small are available that can be used with a terminal emulation program. A is inside the cable and an OS device driver is typically required.
03.10.11 #2Comment by.The SMB380 digital 3-axis accelerometer has 10-bit acceleration output resolution. For complete read the acceleration data output, you need two read cycles because the 10-bit data is split into 8-bit MSB and 2-bit LSB. The 16-bit protocol mention in datasheet is the protocol used to read/write the SMB380 accelerometer where the first 8-bit contain a read/write command (0 – writing, 1 – reading) follow by 7-bit address and the last is the 8-bit acceleration data, therefore you could use it with Microchip PIC18LF4520 microcontroller SPI peripheral. 08.01.15 #5Comment by chasxmd.This is a really nice blog entry. I was poking around because I noticed my /CS wasn’t working as I expected and found this.
I happened to be using the same PIC and similar code. You have:// Start Data transmissionSSPBUF = data;// Wait for Data Transmit/Receipt completewhile(!SSPSTATbits.BF);// CS pin is not activePORTCbits.RC6 = 1;In my program my CS output (RC6 for yours) goes high before my second bit is on the way (reviewed by logic analyzer). In fact my main loop has made it back to the top before the byte is sent. The SSPSTAT register specification mentions.BF is only used for receive only have you confirmed that your chip select (RC6) actually is waiting to go high until the last bit of the byte is on it’s way? Mine appears that (!SSPSTATbits.BF) is true before the byte is finished.I’m not looking for support or help, I was just wondering if you had actually ever checked that?Thank you.