02-05-2012, 05:23 PM
SERIAL COMMUNICATION OF DEVICES
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Synchronous communication: when a byte/character or frame of data is received or transmitted at constant time intervals with uniform phase differences, the communication is called synchronous. Bits of a data frame are sent in a fixed maximum time interval. When the maximum time interval varies, it is a special case known as iso-synchronous.
Asynchronous communication: when a byte/character or frame of data is received or transmitted at variable time intervals, the communication is called asynchronous. Voice data on line is sent on asynchronous mode. Telephone line communication is asynchronous.
Serial Data Communication using the ports
Microcontrollers have internal devices for SPI, SCI or SI. Each device has separate registers for control, status, serially received data bits and transmitting serial bits. Each device is programmable. These devices can be used in programmed IO modes or interrupt driven reception or transmission.
Synchronous peripheral interface port (SPI) : it has full duplex feature for synchronous communication. There are signals SCLK for serial clock, MOSI and MISO output from and input to master. An SPI feature is programmable rate for clock bits. It is also programmable for defining the occurrence of negative and positive edges with in an interval of bits at serial data out or in. It is also programmable for device selection for master or slave.
Serial connect interface port (SCI) : it is a UART asynchronous mode port. Communication is in the full duplex mode for the SCI transmission and receiver. SCI baud rates are fixed. The SCI receiver has a wake up feature and is programmable by RWU (Receiver Wakeup Unavailable) bit. It is enabled if RWU bit is set. If RWU is set, then the receiver of a slave is not interrupted by the succeeding bytes. RWU feature helps in inter-processor communication. SCI has 2 control register bits TB8 and RB8.
Serial interface port (SI) : it is a USART port. Its features include a programmable rates register. SI operates in half duplex synchronous mode of operation called mode0 and full duplex asynchronous mode of operation called mode1, mode2 or mode3.
Secure Digital Input Output (SDIO)
SDIO is an SD card with programmable IO functionalities such that it
(a) can be used up to eight logical functions
(b) can provide additional memory storage in SD format
© can provide IOs using protocols in systems.
It supports SPI and SD formats. Both SPI and SD formats specify that there should be interrupt handling for the IOs and the CRC checks on transferred data. SDIO is an SPI based 9 pin connector card. Six pins are used for communication.
Parallel Device Ports
The parallel port of devices transfers a number of bits over the wires in parallel. Parallel wires capacitive effect reduces length up to which parallel communication can be done. Parallel ports inputs I0 to I7 may be to a keypad controller. Parallel ports outputs O0 to O7 may be output bits to LCD display output controller. BRi and BRo are the input and output data buffers at bi-directional IO port. A device port connects to the address bus signals, Ai and Aj through a port address decoder. IORD and IOWR are additional control signals for the port device read and write respectively in case of IO mapped IOs. The memory read and write signals, RD and WR are used in the processor with memory mapped IOs.
• A device port may have multi byte data input buffers and data output buffers.
• A port may have a data direction register (DDR).
• There may be multiple functionality or alternate functionality in the port pins.
• A port may have provision for multiplexed output.
• A port may have provision for de-multiplexed inputs.