Design serial protocol

Through the SPI protocol, devices communicate with each other using master-slave architecture. Although multiple slave devices can be supported by SPI, the number of master devices is limited to one.

SPI is known as four-wire serial bus because it consists of four signals:. In full duplex mode, data rates over 1Mbps can be achieved - this is one of the main advantages of the SPI bus. Compared to I2C, SPI also supports using simple hardware interfacing and provides a higher throughput. However, the SPI protocol also has some drawbacks — the lack of error-checking mechanism and slave acknowledgment feature are some of the major disadvantages.

You can probably find a dozen USB cables and connectors lying around in your home. Originally developed in the s, it was intended to standardize the connection of a number of peripheral devices to a computer. When a device communicates with another device through USB protocol, data travels in the form of packets. All the data packets are composed of 8-bit bytes or multiples of 8-bit bytes, depending on the supported bitrate , where the LSB or Least Significant Bit is transmitted first.

If you are building an embedded system that involves USB, make sure you use a USB protocol analyzer to monitor the data on the bus. Total Phases offers products that support all of the above listed protocols. Click below if you would like to learn more, or have a personal demo designed to specifically address your needs.

Total Phase. So… the RGB format defines a 24 bit color. Our protocol needs to implement a minimum of 3 bytes 1 byte equals 8 bits to contain color data in RGB format. We also require a header to identify the protocol, and optional but extremely suggested is the use of a checksum byte as a validation method, this gives us a protocol with a fixed value of 5.

Validation gives us assurance that the data we receive is not corrupted sometimes by unknown causes we might have data corruption, for example: a bit value changes by electromagnetic effects in the transmission line, the color might change dramatically by this cause.

We are going to employ two Arduinos for testing. Reading a byte stream a buffer is tricky. We need to consider several factors.

So we have to design a flowchart for this endeavour:. By creating this protocol, we can now easily send and receive data that corresponds to the RGB Led Color. Some ideas to explore: Connecting these Arduinos wirelessly using two Xbees. Say for example you want to control the same RGB Led wirelessly, you have several options here: bluetooth, zigbee, wifi. Actually… many of the current Smart Led Bulbs in the market employ a similar protocol like the one we described here to manipulate its color, they map the RGB Color Components in bytes and send them in a defined sequence they send a little more data of course.

I have 4 analog signals to be transmitted and 8 buttons. Is there a way to send all the button values in one byte instead of 8 different bytes? It seems possible because i have 8 buttons so each button can be send as one bit. As such, for off-board communications, I 2 C is practically limited to under 10 feet for moderate speeds.

The data is clocked simultaneously into the slave and master based on SCK pulses the master supplies. It is important to ensure that these are compatible between master and slave.

This capability is, however, dependent on the phase selection of the SCK. But for small, low-pin-count microcontrollers, a multi-slave SPI interface might not be a viable solution. Microwire is a three-wire synchronous interface developed by National Semiconductor and present on their COP8 processor family.

A full-duplex bus, Microwire is capable of speeds of Kbps and faster capacitance permitting. Microwire devices from National come with different protocols, based on their data needs.

Microwire has the same advantages and disadvantages as SPI with respect to multiple slaves, which require multiple chip select lines. In some instances, an SPI device will work on a Microwire bus, as will a Microwire device work on an SPI bus, although this must be reviewed on a per-device basis. Both SPI and Microwire are generally limited to on-board communications and traces of no longer than 6 inches, although longer distances up to 10 feet can be achieved given proper capacitance and lower bit rates.

Like the I 2 C bus, 1-Wire is half-duplex, using an open-drain topology on a single wire for bidirectional data transfer. However, the 1-Wire bus also allows the data wire to transfer power to the slave devices, although this is somewhat limited.

Though limited to a maximum speed of 16Kbps, bus length can be upwards of 1, feet, given the proper pull-up resistor. For more detail on the 1-Wire bus, read H. Bit banging requires the software to be cognizant of the exact timing required for each bit, for it must toggle an output line for every bit change as well as monitor the receive pin for incoming data, if such interface is full-duplex.

Luckily for embedded developers, quite a few bit-banging routines are available on the Internet for every serial bus described here, and for use in almost every microcontroller architecture. In fact, several microcontroller manufacturers have developed and published their own such routines. Catching the right bus. As you can see, there is a multitude of serial communication buses to choose from. And we didn't even discuss wireless, networks, Firewire, and USB protocols.

Your choice in a serial bus should not only meet the needs of the product today, but also be available as well as viable for the life of the product. I hope this has helped you decide which serial interface is proper for your current embedded design.

When not spending time with his new family, John can be found in the garage working on one of his robots. He can be reached via e-mail at. Wilson, Michael R. You must Sign in or Register to post a comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. You must verify your email address before signing in. Check your email for your verification email, or enter your email address in the form below to resend the email.

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Why serial? Terminology Before we get into the individual interface details, we should define several terms: On an asynchronous bus, data is sent without a timing clock. A synchronous bus sends data with a timing clock. Full-duplex means data can be sent and received simultaneously.

The SPI may be configured either as master or as a slave. So it needs 6 wires to send and receive data from slave or master. Theoretically, the SPI can have unlimited number of slaves. The data communication is configured in SPI registers. The SPI can deliver up to 10Mbps of speed and is ideal for high speed data communication.

Both the lines must be connected to a positive supply using a pull up resistor. I2C can deliver speed up to Kbps and it uses 10 bit or 7 bit addressing system to target a specific device on the i2c bus so it can connect up to devices.

It has limited length communication and is ideal for onboard communication. I2C networks are easy to setup since it uses only two wires and new devices can simply be connected to the two common I2C bus lines.

A maximum of peripherals can be connected to a single USB host controller. USB acts as "plug and play" device. The USB are used in almost devices such as keyboards, printers, media devices, cameras, scanners and mouse. It is designed for easy installation, faster data rated, less cabling and hot swapping. It has replaced the bulkier and slower serial and parallel ports. USB uses differential signalling to reduce interference and allow high-speed transmission over a long distance.

A differential bus is built with two wire, one of represents the transmitted data and the other its complement. The idea is that the 'average' voltage on the wires does not carry any information, resulting in less interference. In USB, the devices are allowed to draw a certain amount of power without asking the host.

USB uses only two wires to for data transfer and are faster than the serial and parallel interface. USB versions supports different speeds such as 1. Length of individual USB cable can reach up to 5 meters without a hub and 40 meters with hub.

The CAN protocol is robust, low-cost and message based and covers in many applications - e. Each ECU contains a chip for receiving all transmitted messages, decide relevance and act accordingly - this allows easy modification and inclusion of additional nodes e. CAN bus data loggers. The CAN bus systems can provide speed up to 1Mbps. Data is shifted out on the falling edge of SK, and is valued on the rising edge. SI is shifted in on the rising edge of SK.

It supports speeds up to 3Mbps.