Asynchronous data transfer

Computer Organization & Architecture - Asynchronous Data Transfer

1. Computer Organization & Architecture A synchronous Data Transfer Nithiyapriya Pasavaraj

2. Data transmission (i.e., from source to destination ) can be done either Parallel or Serial. In Parallel data transmission , the sequence of data can be transmitted parallel at a time . Nithiyapriya Pasavaraj Data Transmission

3. Nithiyapriya Pasavaraj

4. In a computer system , CPU & I/O interfaces are designed independently of each other. When the internal timing in each unit is independent from the other and the registers in Interface & registers of CPU uses its own private clock, Asynchronous data transfer can be used to coordinate the speed & its timing characteristics. Nithiyapriya Pasavaraj Asynchronous Data Transfer

5. The Asynchronous data transfer between two independent units requires that control signals transmitted between the communicating units , so that the time can be indicated at which they send data. The Asynchronous way of data transfer can be achieved by two methods. Strobe control Handshaking Nithiyapriya Pasavaraj

6. The strobe control method of asynchronous data transfer employs a single control line to time each transfer, which is known as strobe. It may be achieved either by source or destination. Nithiyapriya Pasavaraj Strobe Control

7. Nithiyapriya Pasavaraj Source Initiated Strobe for data transfer Source Unit Destination Unit Data Bus Strobe Block Diagram Timing Diagram Valid Data Data Strobe

8. The data bus carries the binary information from source unit to the destination unit. The strobe is a single line that informs the destination unit when a valid data word is available in the bus. The source unit first places the data on the bus. After a brief delay (to ensure that the data settle to a steady value), the source activates the strobe pulse. The information of the data bus and the strobe signal remain in the active state for sufficient time period to allow the destination unit to receive the data. The source removes the data from the bus for a brief period of time after it disables its strobe pulse. Nithiyapriya Pasavaraj

9. Nithiyapriya Pasavaraj Destination Initiated Strobe Valid Data Data Strobe Source Unit Destination Unit Data Bus Strobe Block Diagram Timing Diagram

10. The destination unit activates hr strobe pulse , informing the source to provide the data. The source unit responds by placing the required binary information on the unit to accept it. The data must be valid and remain in the bus long enough for destination unit to accept it. The falling edge of strobe pulse can be used again to trigger a destination register. The destination unit then disables the strobe. The source removes the data from the bus after a predetermined time interval. Nithiyapriya Pasavaraj

11. The source unit that initiates the transfer has no way of knowing whether the destination unit has actually received the data item that was placed in bus. The destination unit that initiates the transfer has no way knowing whether the source unit has actually placed the data on the bus. Nithiyapriya Pasavaraj Disadvantages of strobe method

12. The Handshaking method solves the problems raised in strobe method, by introducing a second control signal that provides a reply to the unit that initiates the transfer. The data transfer can be occur as Source initiated Transfer Destination initiated Transfer Nithiyapriya Pasavaraj Handshaking method

13. The Two Handshaking lines generated from the source are, Data Valid - Source to Destination to inform, whether there are valid data in the bus. Data Accept - Destination to source to inform, whether it can accept data. Nithiyapriya Pasavaraj Source Initiated Transfer using Handshaking

14. Nithiyapriya Pasavaraj Source Unit Destination Unit Data Bus Data Valid Timing Diagram Valid Data Data bus Data valid Block Diagram Data accepted Data Accepted

15. Nithiyapriya Pasavaraj Sequence of Events Source Destination

16. The source unit initiates the transfer by placing the data on the bus and enabling its data valid signal. The data accepted signal is activated by the destination unit after it accepts the data from the bus. The source unit then disable its data valid signal , which invalidates the data on the bus. The destination unit disable its data accepted signal and the system goes into its initial state. Nithiyapriya Pasavaraj The sequence of events shows the four possible states that the system can be at given time.

17. The two handshaking lines in this scheme are Data valid Ready for data The source unit in this case, does not place data on the bus until after it receives the ready for data signal from the destination unit. Nithiyapriya Pasavaraj The destination initiated transfer using Handshaking

18. Nithiyapriya Pasavaraj Source Unit Destination Unit Data Bus Data Valid Timing Diagram Valid Data Ready for data Data valid Block Diagram Ready for Data Data bus

19. Nithiyapriya Pasavaraj Sequence of Events Source Destination

20. The handshaking procedure follows the same pattern as in source initiated except the read for data signal has been converted from data accepted in case of source initiated. Nithiyapriya Pasavaraj