Serial buses free I/O pins

More times than designers might like to admit, they wish for just one more I/O pin on a microcontroller. Three chip-to-chip serial buses can help overcome I/O pin limits. The following information provides a quick overview and points you to sources of more information.



I SPY SPI

The Serial Peripheral Interface (SPI) provides basic communication between a master and one or more slave devices through synchronous-serial communications. The master initiates all communications with slaves and provides the clock signal that governs the bit rate. The SPI offers no high-level protocol, so communications between a master, say a microcontroller, and a peripheral, such as an ADC or a codec, do not require time for housekeeping tasks. The lack of a protocol means SPI-chip vendors come up with their own bit and byte arrangements. The simplest SPI interface requires three signal lines (clock, master-in slave-out and master-out slave-in) and ground.



SPI communications occur a byte at a time but lack an automatic acknowledgement of the receipt of data by a slave device. Also, communications cannot address a specific SPI chip, so each SPI slave must provide an active-low chip-select (/CS) input pin. The master selects a slave device by driving its individual /CS pin to a logic 0. If you have only one SPI slave, you could hardwire the chip-select pin to ground (and save an I/O pin); although an ADC, for example, might use the leading edge of the /CS signal to start a conversion.



SPI devices can quickly use up I/O pins needed to produce /CS signals. If you have more than three SPI devices, consider using a 3-line to 8-line decoder/demultiplexer to generate a /CS signal for as many as eight SPI devices from three or four I/O pins on the master. You also can use an SPI chip such as the Maxim MAX7317 to add 10 general-purpose I/O pins to a circuit.

SERIAL BUS GOES SQUARE

The Inter IC (I²C) communication link, developed by Philips Semiconductors in the 1980s, requires two bidirectional signal lines, one for a clock and another for data, plus ground. Unlike and SPI link, an I²C connection can include more than one controlling device in a multimaster configuration. Peripheral devices include real-time clocks, memories, LCD drivers and analog converters.



All I²C devices have preset unique addresses, so they require no chip-select lines. Manufacturers of I²C I/O devices may include programming pins that allow several of the same devices to operate on an I²C bus. The Analog Devices AD7991, a 2-channel 12-bit ADC, lets developers use as many as five of these chips on an I²C bus. To start, a committee assigned chip manufacturers unique 7-bit device addresses, which quickly ran out. The I²C specification now accommodates 10-bit addresses and engineers can mix chips with a 7- or a 10-bit address on an I²C bus.



Communications occur at one of three speeds, 100Kbps (standard), 400Kbps (fast), or up to 3.4Mbps (high speed). A maximum wire capacitance of 400pF limits the length of I²C communications, although companies offer bus extenders and repeaters. NXP Semiconductor, for example, offers 20 such ICs.



Unlike SPI slaves, the I²C protocol includes an acknowledgment. The wired-AND nature of the signal lines lets a receiving device pull the data line low for one clock period to acknowledge receipt of each byte. The I²C specification does not include error correction, although software can implement that capability.



1-WIRE BUS TAKES TWO

The proprietary 1-Wire technology from Dallas Semiconductor uses one wire for communications and power. (You need a ground wire, too.) Communications involve a master and one or more slave devices, such as sensors, EPROMs, and battery monitors. Asynchronous communications use carefully timed pulse-widths to represent data. A logic-1 condition pulls the data line low for 15µs or less and a logic 0 pulls the line low for at least 60µs (Peripherals can pull small amounts of power from the communication line when it is idle.)



Each 1-Wire device comes with a unique 64-bit ID number that supplies an 8-bit device-type identifier, a 48-bit address and an 8-bit CRC value. The 48-bit address ensures no conflict along devices on a 1-Wire line. Software in the master can discover the device types and their serial numbers. Dallas Semiconductor – part of Maxim Integrated Products – produces many 1-Wire devices. But as far as I know, no other manufacturer offers 1-Wire chips.



For further reading

“Overview of 1-Wire Technology and Its User,” Applcation Note 1796. Maxim Integrated Products. www.maxim-ic.com/appnotes.cfm/appnote_number/1796



“SPI Block Guide (V03.6),” Document Number S12SPIV3/D www.freesacale.com/files/microcontrollers/doc/ref_manual/SP12SPIV3.pdf



“The I2C Bus Specification,” Version 2.1, January 2000. NXP Semiconductor www.nxp.com/acrobat_download/literature/9398/39340011.pdf





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Figure 1