Interconnection Standards

Interconnection Standards

AU: Dec.-06, 07, 08, May-06, 11, 13

Universal Serial Bus (USB)

• USB gives fast and flexible interface for connecting all kinds of peripherals.

• USB is playing a key role in fast growing consumer areas like digital imaging, PC telephony, and multimedia games, etc.

• The presence of USB in most new PCs and its plug-n-play capability, means that PCs and peripherals (such as CD ROM drives, tape and floppy drives, scanners, printers, video devices, digital cameras, digital speakers, telephones, modems, key boards, mice, digital joysticks and others) will automatically configure and work together, with high degree of reliability, in this exciting new application areas. 

• USB opens the door to new levels of innovation and its use for input devices. There are also brand new opportunities of all types of peripherals from printers to scanners to high speed connection such as Ethernet, DSL, cable and satellite communications.

• USB has advantages that specifically benefit developers, including the hardware designers who select components and design the circuits, the PC programmers who write the software that communicates with USB peripherals, and peripherals programmers who write the code that resides inside USB peripherals.

USB Features

1. Simple connectivity

USB offers simple connectivity.

2. Simple cables

The USB cable connector are keyed so you cannot plug them in wrong.

3. One interface for many devices

USB is versatile enough to be usable with many kinds of peripherals with no need of having a different connector and protocols for each peripheral. USB supports all kinds of data, from slow mouse inputs to digitized audio and compresses video.

4. Automatic configuration

When a user connects a USB peripheral to a powered system, windows automatically detects the peripheral and loads the appropriate software driver for it. There is no need to locate and run a setup programme or restart the system before using the peripheral.

5. No user setting

USB peripherals do not have user selectable settings such as port address and interrupt request (IRQ) lines.

6. Frees hardware resources for other devices

Using USB for as many peripherals as possible frees up IRQ lines for the peripherals that do require them.

7. Hot pluggable (Plug-and play)

You can install or remove a peripheral regardless of the power state.

8. Data transfer rates

USB supports three data transfer rates, 480 Mb/s (high-speed), 12 Mb/s (full-speed) and 1.5 Mb/s (low-speed).

9. Coexistence with IEEE 1394

USB 2.0 and IEEE 1394 offer similar data rate primarily differ in terms of application focus.

10. Reliability

Reliability of USB results from both the hardware design and data transfer protocols.

11. Low cost

Even though USB is more complex than earlier interfaces, its components and cables are inexpensive. A device with a USB interface is likely to cost the same or less than its equivalent older interfaces.

12. Low power consumption

Power circuits and code automatically power down USB peripherals when not in use, yet keep them ready to respond when needed.

13. Flexibility

USB's four transfer types and three speed make it feasible for many types of peripherals.

14. Operating system support

Windows 98 was the first Windows operating system to reliably support USB, and its successors such as Windows 2000 support USB as well. Other computers and operating systems also have USB support. ON apple's iMac, the only peripherals connectors are USB. Other Macintoshes also support USB, and support is in progress for Linux, NetBSD, and FreeBSD.

USB System

• The Fig. 8.11.1 shows the basic components of USB system. It consists of USB host, USB device and USB cable. The USB host is a personal computer (PC) and devices are scanner, printer etc. There will be only one host in the USB system, however there can be 127 devices in the USB system.

USB Connector

• The Fig. 8.11.2 shows the pin-out of the USB connector.

• There are two types of USB connectors.

• In either case, there are four signals as indicated in Table 8.11.1 The 5.0 V and ground signals can be used to power the device connected on the PCI bus as long as the amount of current does not exceed 100 mA per device.

• The data signals are biphase signals. When + data represents 5.0 V the data represents 0 V and vice versa.

USB Data Encoding Format

• The USB uses NRZI (non-return-to zero, inverted) data encoding method for transmitting data packets. In this method, the signal level does not change for the transmission of logic 1. However, it is inverted for each change to a logic 0. This is illustrated in Fig. 8.11.3. In this method data bits are always transmitted beginning with the least significant bit first, followed by subsequent bits.

• To maintain the signal frequency in the specified range i.e. to achieve synchronization we have to insert a sync bit in the data stream, if a logic 1 is transmitted for more than six bits in a row. The process of inserting sync bit is known as bit stuffing.

• The bit stuffing isillustrated in Fig. 8.11.4.Bit stuffing ensures that Digital data

the receiver can maintain synchronization for long strings of logic 1s.

• The Fig. 8.11.5 shows the flowchart to generate USB data from the raw digital serial data. (SeeFig. 8.11.5 on next page.)

8.11.1.5 USB Commands

• USB data is transmitted to particular receptor with the use of USB commands. •The communication begins with the transmission of the sync byte (80H).

• It is followed by the packet identification byte (PID). The PID contains eight bits, but only the rightmost four bits contain the type of packet. The leftmost four bits of the PID are the complement form of four rightmost bits.

• For example, if a command is 1001, the actual PID byte is 0110 1001. The Table 8.11.2 shows the available 4-bit PIDs and their 8-bit codes. The PIDs are also used as token indicators, as data indicators, and for handshaking.

USB Packet Format

• The Fig. 8.11.6 shows the formats of data, token, handshaking and start-of frame packets used on the USB.

• The data packet begins with PID, then data byte and ends with CRC (cyclic redundancy check) code.

• Packets use two types of CRC codes : one is a 5-bit CRC and the other (used for data packets) is a 16-bit CRC.

• The 5-bit CRC is generated with the X⁵ + X² +1 polynomial.

• The 16-bit CRC is generated with the X¹⁶ + X¹⁵ + X² + 1 polynomial.

• The USB uses the ACK (acknowledge) and NAK (Not acknowledge) tokens to co-ordinate the transfer of data packets between the host system (host is a PC or other computer that contain two components: controller and a root hub). (A hub is device that contains one or more connecters or internal connections to USB devices along with the hardware to enable communicating with each device and the USB device.)

• Once a data packet is transferred from the host to the USB device, the USB device either transmits and ACK or a NAK token back to the host.

• If the data and CRC are received without error, the ACK is sent; otherwise, the NAK is sent.

• If the host receives a NAK token, it retransmits the data packet until the receiver receives it without error.

• This method of data transfer is known as stop and wait flow control. In this method, the host has to wait for the client to send an ACK or NAK before transferring additional data packets.

Data Transfer Types

• The USB is designed to handle many types of peripherals with varying requirements for transfer rate, response time, and error correcting. There are four types of data transfers each handle different needs and a peripheral can support the transfer types that are best suited for its purpose.

1. Control transfer

• Control transfers are the only type with functions defined by the USB specification. These transfers enable the host to read and select configurations and other settings on the devices being enumerated. Control transfers may also send custom requests that send and receive blocks of data for any purpose. All USB devices must support control transfers.

• This data transfer exchanges configuration, setup, and command information between the device and host. CRCS check the data and initiate retransmissions when needed to guarantee the correctness of these packets.

• Control transfers use message pipes. In a message pipe, each transfer begins with a Setup transaction containing a request. To complete the transfer, the host and device may exchange data and status information, or the device may just send status information. There is always at least one transaction that sends information in each direction.

• If the request is one that the device supports, it takes the requested action. A device may also respond with a code that indicates that it does not support the request.

2. Bulk transfer

• Bulk transfers are intended for situations where the rate of transfer isn't critical, such as sending a file to a printer or receiving data from a scanner. In these cases quick transfers are nice, but the data can wait if necessary. If the bus is very busy with other transfers that have guaranteed transfer rates, bulk transfers must wait, but if the bus is idle, bulk transfers are very fast. Only full-speed devices can do bulk transfers. Devices are not required to support bulk transfers, but a specific I device class might require it.

• This data transfer moves large amounts of data when timely delivery is not critical. Typical applications include printers and scanners. Bulk transfers are fillers, claiming unuse USB bandwidth when nothing more important is going on. CRCs protect these packets.

3. Interrupt transfer

• Interrupt transfers are for devices that must receive the host's or device's attention quickly. Other than control transfers, interrupt transfers are the only way that low speed devices can transfer data. A keyboard or mouse can use interrupt transfers to send keypress or mouse movement data. Both full and low speed devices can do interrupt transfers. Devices aren't required to support interrupt transfers, but a specific device class might require it.

• This data transfers, though not interrupt in the CPU diverting sense, poll devices to see if they need service. Peripherals exchanging small amounts of data that need immediate attention (such as mice and keyboards) use interrupt transfers. Error checking validates the data.

4. Isochronous transfer (USB for audio devices)

• Isochronous transfers are for devices that must transfer data at a constant rate, such as audio files to be played in real time, or other data that needs a guaranteed delivery rate or time. This is the only transfer type that doesn't support automatic re-transmitting of data received with errors, so occasional errors must be acceptable. Only full speed devices can do isochronous transfers. Devices are not

required to support isochronous transfers, but a specific device class might require

it.

• This data transfer handling trimming data like an audio or video device. It is time sensitive information so within limitation it has guaranteed access to a USB bus. No error checking occurs so the system must tolerate occasional scrambled bytes. Above three transfers namely Bulk transfer, Interrupt transfer, and Isochronous transfer use Stream Pipes.

• In a stream pipe, the data has no format defined by the USB specification. The receiving device just accepts whatever arrives. The device firmware or host software can then process the data in whatever way is appropriate for the application. Of course, even with stream data, the sending and receiving devices will need to agree on some type of format.

Review Questions

1. Write short note on advantages of USB over older I/O bus architectures.  AU: Dec.-06, Marks 8

2. Write notes on USB.  AU: Dec.-07, Marks 8

3. Explain the protocols of USB.  AU May-11, Marks 8

4. Discuss the salient features of the USB operation.  AU May-06, Marks 6

5. What are the advantages of USB interface?  AU: Dec.-08, May-13, Marks 2

SATA

• A serial advanced technology attachment (serial ATA, SATA or S-ATA) is a computer bus interface that connects host bus adapters with mass storage devices like optical drives and hard drives.

• As its name implies, SATA is based on serial signaling technology, where data is transferred as a sequence of individual bits

• This interface is commonly used to connect hard disk drives to a host system such as a computer motherboard.

• The first version of SATA communicated at 150 megabytes per second (MBps). The standard was soon upgraded to 300 MBps in 2004 and 600 MBps in 2009-estimated to be sufficient to accommodate 10 years of advances in device throughput.

Features

• Low Voltage Requirement: SATA operates on 500 mV (0.5 V) peak-to-peak signaling. This help in promoting a much low interference and crosstalk between conductors.

• Hot Plugging: This feature helps users to change or remove storage devices even when the computer is running.

• Staggered Spin-Up: Allows sequential hard disk drive startup, which helps even out power load distribution during system booting.

• Native Command Queuing (NCQ): Usually, the commands reach a disk for reading or writing from different locations on the disk. When the commands are carried out based on the order in which they appear, a substantial amount of mechanical overhead is generated because of the constant repositioning of the read/write head. SATA II drives use an algorithm to identify the most effective order to carry out commands. This helps to reduce mechanical overhead and improve performance.

• Port Multipliers: Allows the connection of up to 15 drives to a SATA controller. This facilitates the building of disk enclosures.

• Port Selectors: Facilitates redundancy for two hosts connected to a single drive, allowing the second host to take over in the event of a primary host failure. 

• Simplified construction: PATA cables had 40-pin/80-wire ribbon cable. This was complex in structure. In comparison, SATA had a single 7 pin data cable and a 15 pin power cable. This cable resulted in a higher signaling rate, which translates to faster throughput of data.

• Differential Signaling: SATA uses differential signaling. Differential signaling is a technology which uses two adjacent wires to simultaneously the in-phase and out-of-phase signals. Thus, it is possible to transfer high-speed data with low operating voltage and low power consumption by detecting the phase difference between the two signals at the receiver's end.

• High data transfer rate: SATA has a high data transfer rate of 150/300/600 MBS/second. This capability of SATA allows for faster program loading, better picture loading and fast document loading.

• Large Cable Length: SATA cable can be of length up to 1 meter, whereas PATA cable can only have a length of maximum 18 inches.

Operating Modes

• SATA operates on two modes :

• IDE mode: IDE stands for Integrated Drive Electronics. This mode is used to provide backward compatibility with older hardware, which runs on PATA, at low performance.

• AHCI mode: AHCI is an abbreviation for Advanced Host Controller Interface. AHCI is a high-performance mode that also provides support for hot-swapping. •The Serial ATA [SATA] bus is defined over two separate connectors, one connector for the data lines and one for the power lines.

SATA Data pinout

SATA Power pinout

Review Questions

1. Write a short note on SATA.

2. List the features of SATA.