Stm32 Serial Communication Protocols
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Single Master to Single Slave: basic SPI bus example. The Serial Peripheral Interface bus ( SPI) is a interface specification used for short distance communication, primarily in. The interface was developed by in the mid 1980s and has become a. Typical applications include cards and. SPI devices communicate in mode using a architecture with a single master. The master device originates the for reading and writing.
Multiple slave devices are supported through selection with individual (SS) lines. Sometimes SPI is called a four-wire serial bus, contrasting with, and serial buses. The SPI may be accurately described as a synchronous serial interface, but it is different from the (SSI) protocol, which is also a four-wire synchronous serial communication protocol. SSI Protocol employs and provides only a single channel. A typical hardware setup using two to form an inter-chip To begin communication, the bus master configures the clock, using a frequency supported by the slave device, typically up to a few MHz.
What Is Serial Communication
The master then selects the slave device with a logic level 0 on the select line. If a waiting period is required, such as for an analog-to-digital conversion, the master must wait for at least that period of time before issuing clock cycles.
During each SPI clock cycle, a full duplex data transmission occurs. The master sends a bit on the MOSI line and the slave reads it, while the slave sends a bit on the MISO line and the master reads it. This sequence is maintained even when only one-directional data transfer is intended. Transmissions normally involve two shift registers of some given word size, such as eight bits, one in the master and one in the slave; they are connected in a virtual ring topology. Data is usually shifted out with the most-significant bit first.
On the clock edge, both master and slave shift out a bit and output it on the transmission line to the counterpart. On the next clock edge, at each receiver the bit is sampled from the transmission line and set as a new least-significant bit of the shift register. After the register bits have been shifted out and in, the master and slave have exchanged register values.
If more data needs to be exchanged, the shift registers are reloaded and the process repeats. Transmission may continue for any number of clock cycles.
When complete, the master stops toggling the clock signal, and typically deselects the slave. Transmissions often consist of 8-bit words. However, other word sizes are also common, for example, 16-bit words for touch screen controllers or audio codecs, such as the TSC2101 by, or 12-bit words for many digital-to-analog or analog-to-digital converters. Every slave on the bus that has not been activated using its chip select line must disregard the input clock and MOSI signals, and must not drive MISO. Clock polarity and phase. A timing diagram showing clock polarity and phase.
Red lines denote clock leading edges, and blue lines, trailing edges. In addition to setting the clock frequency, the master must also configure the clock polarity and phase with respect to the data. Motorola SPI Block Guide names these two options as CPOL and CPHA respectively, and most vendors have adopted that convention. The is shown to the right. The timing is further described below and applies to both the master and the slave device. CPOL determines the polarity of the clock. The polarities can be converted with a simple.
CPOL=0 is a clock which idles at 0, and each cycle consists of a pulse of 1. That is, the leading edge is a rising edge, and the trailing edge is a falling edge. CPOL=1 is a clock which idles at 1, and each cycle consists of a pulse of 0.
That is, the leading edge is a falling edge, and the trailing edge is a rising edge. CPHA determines the timing of the data bits relative to the clock pulses. It is not trivial to convert between the two forms. For CPHA=0, the 'out' side changes the data on the trailing edge of the preceding clock cycle, while the 'in' side captures the data on (or shortly after) the leading edge of the clock cycle. The out side holds the data valid until the trailing edge of the current clock cycle. For the first cycle, the first bit must be on the MOSI line before the leading clock edge.
An alternative way of considering it is to say that a CPHA=0 cycle consists of a half cycle with the clock idle, followed by a half cycle with the clock asserted. For CPHA=1, the 'out' side changes the data on the leading edge of the current clock cycle, while the 'in' side captures the data on (or shortly after) the trailing edge of the clock cycle. The out side holds the data valid until the leading edge of the following clock cycle. For the last cycle, the slave holds the MISO line valid until slave select is deasserted. An alternative way of considering it is to say that a CPHA=1 cycle consists of a half cycle with the clock asserted, followed by a half cycle with the clock idle. The MOSI and MISO signals are usually stable (at their reception points) for the half cycle until the next clock transition.
SPI master and slave devices may well sample data at different points in that half cycle. This adds more flexibility to the communication channel between the master and slave. Mode numbers The combinations of polarity and phases are often referred to as modes which are commonly numbered according to the following convention, with CPOL as the high order bit and CPHA as the low order bit: For 'Microchip PIC' / 'ARM-based' microcontrollers (note that NCPHA is the inversion of CPHA): SPI Mode Clock Polarity (CPOL/CKP) Clock Phase (CPHA) Clock Edge (CKE/NCPHA) 0 0 0 1 1 0 1 0 2 1 0 1 3 1 1 0 For PIC32MX: SPI mode configure CKP,CKE and SMP bits.Set SMP bit,and CKP,CKE two bits configured as above table. For other microcontrollers: Mode CPOL CPHA 0 0 0 1 0 1 2 1 0 3 1 1 Another commonly used notation represents the mode as a (CPOL, CPHA) tuple; e.g., the value '(0, 1)' would indicate CPOL=0 and CPHA=1. Independent slave configuration.
Daisy-chained SPI bus: master and cooperative slaves Some products that implement SPI may be connected in a configuration, the first slave output being connected to the second slave input, etc. The SPI port of each slave is designed to send out during the second group of clock pulses an exact copy of the data it received during the first group of clock pulses. The whole chain acts as a communication; daisy chaining is often done with shift registers to provide a bank of inputs or outputs through SPI. Such a feature only requires a single SS line from the master, rather than a separate SS line for each slave. Other applications that can potentially interoperate with SPI that require a daisy chain configuration include, and.
Valid communications Some slave devices are designed to ignore any SPI communications in which the number of clock pulses is greater than specified. Others do not care, ignoring extra inputs and continuing to shift the same output bit. It is common for different devices to use SPI communications with different lengths, as, for example, when SPI is used to access the scan chain of a digital IC by issuing a command word of one size (perhaps 32 bits) and then getting a response of a different size (perhaps 153 bits, one for each pin in that scan chain).
Interrupts SPI devices sometimes use another signal line to send an interrupt signal to a host CPU. Examples include pen-down interrupts from touchscreen sensors, thermal limit alerts from temperature sensors, alarms issued by real time clock chips, and headset jack insertions from the sound codec in a cell phone. Interrupts are not covered by the SPI standard; their usage is neither forbidden nor specified by the standard.
Example of bit-banging the master protocol Below is an example of the SPI protocol as an SPI master with CPOL=0, CPHA=0, and eight bits per transfer. The example is written in the C programming language. Because this is CPOL=0 the clock must be pulled low before the chip select is activated. The chip select line must be activated, which normally means being toggled low, for the peripheral before the start of the transfer, and then deactivated afterward. Most peripherals allow or require several transfers while the select line is low; this routine might be called several times before deselecting the chip. Retrieved 2015-01-28.
Retrieved 3 September 2015., pp. 80, 84. ^ Not to be confused with the SDIO(Serial Data I/O) line of the half-duplex implementation of the SPI bus, sometimes also called '3-wire SPI-bus'. MOSI (via a resistor) and MISO (no resistor) of a master is connected to the SDIO line of a slave. IEEE 1149.1-2013. with support of custom serial protocols, Byte Paradigm., Freescale Semiconductor.
Such as with the, or McSPI, used in Texas Instruments OMAP chips. Such as the SPI controller on like the at91sam9G20, which is much simpler than TI's McSPI. National Semiconductor Application Note AN-452. National Semiconductor Application Note AN-579. ^ (PDF) (data sheet).
12 August 2016. Retrieved 2017-02-10. ^ (PDF) (data sheet). 11 February 2011. Retrieved 2017-02-10. NXP community forums. December 2014.
Retrieved 2016-02-10. (PDF) (Data sheet). Retrieved 2017-02-10. Patterson, David (May 2012). (PDF) (Application note). Retrieved September 21, 2016.
Pell, Rich (13 October 2011). ^ (PDF) (Report). Revision 1.0.
January 2016. Document number 327432-004. Retrieved 2017-02-05. (PDF) (Report). Revision 0.6.
Document Number 327432-001EN. Retrieved 2017-02-05. Retrieved April 15, 2015. External links Wikimedia Commons has media related to.
What's Wrong with Serial Ports? A common serial port, the kind with TX and RX lines, is called “asynchronous” (not synchronous) because there is no control over when data is sent or any guarantee that both sides are running at precisely the same rate. Since computers normally rely on everything being synchronized to a single “clock” (the main crystal attached to a computer that drives everything), this can be a problem when two systems with slightly different clocks try to communicate with each other. To work around this problem, asynchronous serial connections add extra start and stop bits to each byte help the receiver sync up to data as it arrives.
Both sides must also agree on the transmission speed (such as 9600 bits per second) in advance. Slight differences in the transmission rate aren’t a problem because the receiver re-syncs at the start of each byte. (By the way, if you noticed that “11001010” does not equal 0x53 in the above diagram, kudos to your attention to detail. Serial protocols will often send the least significant bits first, so the smallest bit is on the far left.
The lower nybble is actually 0011 = 0x3, and the upper nybble is 0101 = 0x5.) Asynchronous serial works just fine, but has a lot of overhead in both the extra start and stop bits sent with every byte, and the complex hardware required to send and receive data. And as you’ve probably noticed in your own projects, if both sides aren’t set to the same speed, the received data will be garbage.
This is because the receiver is sampling the bits at very specific times (the arrows in the above diagram). If the receiver is looking at the wrong times, it will see the wrong bits. A Synchronous Solution SPI works in a slightly different manner. It’s a “synchronous” data bus, which means that it uses separate lines for data and a “clock” that keeps both sides in perfect sync. The clock is an oscillating signal that tells the receiver exactly when to sample the bits on the data line. This could be the rising (low to high) or falling (high to low) edge of the clock signal; the datasheet will specify which one to use.
When the receiver detects that edge, it will immediately look at the data line to read the next bit (see the arrows in the below diagram). Because the clock is sent along with the data, specifying the speed isn’t important, although devices will have a top speed at which they can operate (We’ll discuss choosing the proper clock edge and speed in a bit). One reason that SPI is so popular is that the receiving hardware can be a simple. This is a much simpler (and cheaper!) piece of hardware than the full-up UART (Universal Asynchronous Receiver / Transmitter) that asynchronous serial requires.
Receiving Data You might be thinking to yourself, self, that sounds great for one-way communications, but how do you send data back in the opposite direction? Here’s where things get slightly more complicated. In SPI, only one side generates the clock signal (usually called CLK or SCK for Serial ClocK).
The side that generates the clock is called the “master”, and the other side is called the “slave”. There is always only one master (which is almost always your microcontroller), but there can be multiple slaves (more on this in a bit). When data is sent from the master to a slave, it’s sent on a data line called MOSI, for “Master Out / Slave In”.
If the slave needs to send a response back to the master, the master will continue to generate a prearranged number of clock cycles, and the slave will put the data onto a third data line called MISO, for “Master In / Slave Out”. Notice we said “prearranged” in the above description. Because the master always generates the clock signal, it must know in advance when a slave needs to return data and how much data will be returned. This is very different than asynchronous serial, where random amounts of data can be sent in either direction at any time. In practice this isn’t a problem, as SPI is generally used to talk to sensors that have a very specific command structure. For example, if you send the command for “read data” to a device, you know that the device will always send you, for example, two bytes in return.
(In cases where you might want to return a variable amount of data, you could always return one or two bytes specifying the length of the data and then have the master retrieve the full amount.) Note that SPI is “full duplex” (has separate send and receive lines), and, thus, in certain situations, you can transmit and receive data at the same time (for example, requesting a new sensor reading while retrieving the data from the previous one). Your device’s datasheet will tell you if this is possible. Slave Select (SS) There’s one last line you should be aware of, called SS for Slave Select. This tells the slave that it should wake up and receive / send data and is also used when multiple slaves are present to select the one you’d like to talk to. The SS line is normally held high, which disconnects the slave from the SPI bus. (This type of logic is known as “active low,” and you’ll often see used it for enable and reset lines.) Just before data is sent to the slave, the line is brought low, which activates the slave.
When you’re done using the slave, the line is made high again. In a, this corresponds to the “latch” input, which transfers the received data to the output lines.
Multiple slaves There are two ways of connecting multiple slaves to an SPI bus:. Do you have to file a will in florida. In general, each slave will need a separate SS line.
To talk to a particular slave, you’ll make that slave’s SS line low and keep the rest of them high (you don’t want two slaves activated at the same time, or they may both try to talk on the same MISO line resulting in garbled data). Lots of slaves will require lots of SS lines; if you’re running low on outputs, there are that can multiply your SS outputs. On the other hand, some parts prefer to be daisy-chained together, with the MISO (output) of one going to the MOSI (input) of the next. In this case, a single SS line goes to all the slaves. Once all the data is sent, the SS line is raised, which causes all the chips to be activated simultaneously. This is often used for daisy-chained shift registers and.
Note that, for this layout, data overflows from one slave to the next, so to send data to any one slave, you’ll need to transmit enough data to reach all of them. Also, keep in mind that the first piece of data you transmit will end up in the last slave. This type of layout is typically used in output-only situations, such as driving LEDs where you don’t need to receive any data back. In these cases you can leave the master’s MISO line disconnected. However, if data does need to be returned to the master, you can do this by closing the daisy-chain loop (blue wire in the above diagram). Note that if you do this, the return data from slave 1 will need to pass through all the slaves before getting back to the master, so be sure to send enough receive commands to get the data you need.
Programming for SPI Many microcontrollers have built-in SPI peripherals that handle all the details of sending and receiving data, and can do so at very high speeds. The SPI protocol is also simple enough that you (yes, you!) can write your own routines to manipulate the I/O lines in the proper sequence to transfer data. (A good example is on the.) If you’re using an Arduino, there are two ways you can communicate with SPI devices:. You can use the and commands. These are software-based commands that will work on any group of pins, but will be somewhat slow.
Or you can use the, which takes advantage of the SPI hardware built into the microcontroller. This is vastly faster than the above commands, but it will only work on certain pins. You will need to select some options when setting up your interface. These options must match those of the device you’re talking to; check the device’s datasheet to see what it requires. The interface can send data with the most-significant bit (MSB) first, or least-significant bit (LSB) first.
In the Arduino SPI library, this is controlled by the function. The slave will read the data on either the rising edge or the falling edge of the clock pulse. Additionally, the clock can be considered “idle” when it is high or low.
In the Arduino SPI library, both of these options are controlled by the function. SPI can operate at extremely high speeds (millions of bytes per second), which may be too fast for some devices. To accommodate such devices, you can adjust the data rate. In the Arduino SPI library, the speed is set by the function, which divides the master clock (16MHz on most Arduinos) down to a frequency between 8MHz (/2) and 125kHz (/128). If you’re using the SPI Library, you must use the provided SCK, MOSI and MISO pins, as the hardware is hardwired to those pins. There is also a dedicated SS pin that you can use (which must, at least, be set to an output in order for the SPI hardware to function), but note that you can use any other available output pin(s) for SS to your slave device(s) as well.
On older Arduinos, you’ll need to control the SS pin(s) yourself, making one of them low before your data transfer and high afterward. Newer Arduinos such as the Due can control each SS pin automatically as part of the data transfer; see the for more information. Resources and Going Further Tips and Tricks.
Because of the high speed signals, SPI should only be used to send data over short distances (up to a few feet). If you need to send data further than that, and consider using. If things aren’t working the way you think they should, a logic analyzer is a very helpful tool. Smart analyzers like the can even decode the data bytes for a display or logging. Advantages of SPI:. It’s faster than asynchronous serial.
The receive hardware can be a simple shift register. It supports multiple slaves Disadvantages of SPI:. It requires more signal lines (wires) than other communications methods. The communications must be well-defined in advance (you can’t send random amounts of data whenever you want). The master must control all communications (slaves can’t talk directly to each other).
It usually requires separate SS lines to each slave, which can be problematic if numerous slaves are needed. Further Reading Check out the, which includes lots of good information on SPI and other synchronous interfaces. Presents a more correct way to set up an SPI network amongst your embedded devices, particularly for use with an microcontroller. A number of SparkFun products have SPI interfaces. For example, the has an easy-to-use SPI interface that you can use to turn any of 30 LEDs on or off. Other communication options:.
Now that you’re a pro on SPI, here are some other tutorials to practice your new skills:. In 2003, CU student Nate Seidle blew a power supply in his dorm room and, in lieu of a way to order easy replacements, decided to start his own company. Since then, SparkFun has been committed to sustainably helping our world achieve electronics literacy from our headquarters in Boulder, Colorado.
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