Serial microcontroller interface

There is also a element FIFO and a bit shift register, which cannot be directly accessed by the programmer shown on the right side of the interactive tool. The receive shift register is 10 bits wide, but the FIFO is 12 bits, 8 bits of data and 4 error flags.

Again the receive shift register and receive FIFO are separate from those in the transmitter. The receiver obviously cannot start a transmission, but it recognizes a new frame by its start bit. The bits are shifted in using the same order as the transmitter shifted them out: start, b 0 , b 1 , b 2 , b 3 , b 4 , b 5 , b 6 , b 7 , and then stop. There are six status bits generated by receiver activity. There are other flags associated with the receiver. There are four status bits associated with each byte of data.

For this reason, the receive FIFO is 12 bits wide. The overrun error, OE , is set when input data are lost because the FIFO is full and more input frames are arriving at the receiver. An overrun error is caused when the receiver interface latency is too large.

The break error, BE , is set when the input is held low for more than a frame. Parity is a mechanism to send one extra bit so the receiver can detect if there were any errors in transmission. With even parity the number of 1's in the data plus parity will be an even number.

The PE bit is set on a parity error. Because the error rate is so low, most systems do not implement parity. We will not use parity in this class. The framing error, FE , is set when the stop bit is incorrect. Framing errors are probably caused by a mismatch in baud rate. The receiver waits for the 1 to 0 edge signifying a start bit, then shifts in 10 bits of data one at a time from the U0Rx line.

The internal clock is 16 times faster than the baud rate. After the 1 to 0 edge, the receiver waits 8 internal clocks and samples the start bit. Every 16 internal clocks it samples another bit until it reaches the stop bit. The UART needs an internal clock faster than the baud rate so it can wait the half a bit time between the 1 to 0 edge beginning the start bit and the middle of the bit window needed for sampling.

The start and stop bits are removed checked for framing errors , the 8 bits of data and 4 bits of status are put into the receive FIFO. The hardware FIFO implements buffering so data is safely stored in the receiver hardware if the software is performing other tasks while data is arriving.

An overrun occurs when there are 16 elements in the receive FIFO, and a 17 th frame comes into the receiver. In order to avoid overrun, we can design a real-time system, i.

If the latency is always less than bit times, then overrun will never occur. Observation: With a serial port that has a shift register and one data register no FIFO buffering , the latency requirement of the input interface is the time it takes to transmit one data frame. This section is intended to supplement rather than replace the Texas Instruments manuals. It is also good design practice to review the errata for your microcontroller to see if any quirks mistakes exist in your microcontroller that might apply to the system you are designing.

The specific port pins used to implement the UARTs vary from one chip to the next. To find which pins your microcontroller uses, you will need to consult its datasheet. For the other UARTs, the register names will replace the 0 with a 1 — 7.

For the exact register addresses, you should include the appropriate header file e. You need to enable the transmit and receive pins as digital signals. The alternative function for these pins must also be selected. The overrun error OE is set if data has been lost because the input driver latency is too long. BE is a break error, meaning the other device has sent a break. PE is a parity error however, we will not be using parity. The framing error FE will get set if the baud rates do not match.

The BUSY flag is set while the transmitter still has unsent bits, even if the transmitter is disabled. It will become zero when the transmit FIFO is empty and the last stop bit has been sent. If you implement busy-wait output by first outputting then waiting for BUSY to become 0 right flowchart of Figure Some UART registers. Each register is 32 bits wide. Shaded bits are zero. The baud rate divider is a bit binary fixed-point value with a resolution of 2 The baud rate is 16 times slower than Baud Let m be the integer part, without rounding.

More simply, we multiply 0. We did approximate the divider, so it is interesting to determine the actual baud rate. Assume the bus clock is 80 MHz. The error for this example is 0. This internal register is only updated when a write operation to LCRH is performed, so any changes to the baud-rate divisor must be followed by a write to the LCRH register for the changes to take effect.

Out of reset, both FIFOs are disabled and act as 1-byte-deep holding registers. Software that sends and receives data must implement a mechanism to synchronize the software with the hardware. In particular, the software should read data from the input device only when data is indeed ready. Similarly, software should write data to an output device only when the device is ready to accept new data. With busy-wait synchronization, the software continuously checks the hardware status waiting for it to be ready.

This operation is illustrated in Figure In a similar fashion, when the software wishes to output via the serial port, it first waits for TXFF to be clear, then performs the output. Flowcharts of InChar and OutChar using busy-wait synchronization. Polling before writing data is an efficient way to perform output. Be careful when using Port C to be friendly; the pins PC3-PC0 are used by the debugger and you should not modify their configurations.

I 2 C1SCL. I 2 C1SDA. I 2 C0SCL. I 2 C0SDA. I 2 C3SCL. I 2 C3SDA. I 2 C2SCL. I 2 C2SDA. PA0 and PA1 are hardwired to the serial port.

Program Let n be an unsigned bit integer. We will also need an index, i. To convert this string of 3 decimal digits into binary we can simply calculate. This second method could be used for converting any string of known and fixed length.

If Data were a string of 9 decimal digits we could put the above function into a loop. If the length were variable, we can replace the for-loop with a while-loop.

If Data were a variable length string of ASCII characters terminated with a null character 0 , we could convert it to binary using a while loop, as shown in Program A pointer to the string is passed using call by reference. In the assembly version, the pointer R0 is incremented as the string is parsed. R1 contains the local variable n , R2 contains the data from the string, and R3 contains the constant The example, shown in Program All input characters are echoed.

If there is a minus sign, then set a flag. The most commonly used triggering is edge triggering and there are two types: rising edge low to high transition on the clock and falling edge high to low transition.

Depending on how the receiver is configured, up on detecting the edge, the receiver will look for data on the data bus from the next bit. Since both the clock and data are sent by the Master or transmitter , we need not worry about the speed of data transfer. What makes SPI so popular among other Synchronous Serial Communication protocols or any serial communication for that matter is that it provides a high speed secured data transfer with reasonably simple hardware like shift registers at relatively less cost.

Because of its simple interface, flexibility and ease of use, SPI has become a standard and soon other semiconductor manufacturers started implementing it in their chips. In SPI protocol, the devices are connected in a Master — Slave relationship in a multi — point interface. In this type of interface, one device is considered the Master of the bus usually a Microcontroller and all the other devices peripheral ICs or even other Microcontrollers are considered as slaves.

In SPI protocol, there can be only one master but many slave devices. Refer the data sheet for exact information. Let us first see a simple interface between a single master and single slave that are connected using SPI protocol and then we will explain about the 4 wires. Hence, MOSI pins on both the master and slave are connected together. MISO pins on both the master and slave are ties together.

Since the clock is generated by the Master, the flow of data is controlled by the master. For every clock cycle, one bit of data is transmitted from master to slave and one bit of data is transmitted from slave to master.

This process happen simultaneously and after 8 clock cycles, a byte of data is transmitted in both directions and hence, SPI is a full — duplex communication. If the data has to be transmitted by only one device, then the other device has to send something even garbage or junk data and it is up to the device whether the transmitted data is actual data or not.

This means that for every bit transmitted by one device, the other device has to send one bit data i. If the slave wants to transmit the data, the master has to generate the clock signal accordingly by knowing when the slave wants to send the data in advance. I expect I need to disable interrupts and reset pointers etc, but an example of receiving strings using the ISR would be very helpful.

Awesome Tutorial. I have come across a very long way to this. So far best Tutorial. UART explained very elaborately. OR another tutorial may be :. Thank you. Hi Kareem! Nice question. You can also google this subject to get an idea how reading a uart bit 16 times is useful. It helps uart module to make sure that a specific received bit is a 0 or 1. Greatings Khaled. First of thank you for this wonderful website.

Hi Edward. Thx for your feedback. I actually still in doubt if that is the issue in your test setup. HI Khaled Thank you for your time. In the uart tut you refer to your spi tut, however, when I follow the link — Page not Found- I see that on the Tut homepage, clicking on SPI points to the same link with — Page not Found-.

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Previous Tutorial. Tutorial Next Tutorial. Component Name. Buy On Amazon. Add Add. Resistors Kit. Capacitors Kit. Jumper Wires Pack. LM Voltage Regulator 5v. Crystal Oscillator. PICkit2 or 3 Programmer. Add Add Add. I highly recommend using the hardware UART if you are only using one serial device and your microcontroller supports one. The Beaglebone Black and Raspberry Pi also contain serial communication hardware. Arduino boards communicate with the PC via USB cable or to other serial devices using the serial pins.

Each arduino board has at least one serial port and is at digital pins 0 and 1 Tx and Rx respectively. Using these pins while loading a program to the board causes an error because it's the same pins for USB connection.

This is why it's a bad idea to connect the arduino's serial pins directly to a PC's serial port. The USB port which is also serial also uses different voltage levels and protocol. The AnalogReadSerial example is one of the best starting points in understanding how to use the arduino's serial port.

This example reads analog voltage at pin A0 and stores it to the variable sensorValue. The sensorValue is now visible in the serial terminal via the Serial.

If you want your arduino to receive data, you'll be using the Serial. For more information on arduino's serial functions, check out the official documentation Arduino Serial. Running out of pins to use is common in microcontroller programming.

This is especially true for the Arduino UNO which has only one serial port. The GSM module in fact uses software serial to communicate witht the arduino board. You can assign whichever pins are available for use as Tx and Rx pins! The above code assigns the serial pins to digital pins 10 and Once there is s software serial object, you can now use the functions of the library which are the basically the same functions as with hardware serial.

The only way to implement it is through software. A numeric item will send the ascii representation of that number. Software serial in assembly is longer work. Here's how to send a single character "A" using a single pin as transmitter:. Here I created a delay routine to keep the state of the pin for about us approx. I also needed to keep the line idle for a longer time so that the receiving device can distinguish the start bit and the consequently the rest of the data bits.

And so I created another delay delay2 which lasts for around 65 ms. The character "A" is 41 in hexadecimal. The LSB is sent first and then the rest of the bits follow.