Microprocessor-based Systems

1. Microprocessor-based Systems Course 9 Design of the input/output interfaces (continue)

2. Transfer through direct memory access (DMA- Direct memory access) • Why is it used? • To increase the transfer speed • In order to release the main processor from the transfer task • Implementation mode: • Specialized circuit for handling the transfer – the DMA controller • The transfer is programmed by the processor and than it is performed by the DMA controller: the transfer is made between the memory and an I/O interface without the direct implication of the main processor • The transfer phases: • Initialization: - the main processor programs the parameters of the transfer (the DMA controller is behaving as a slave) • The address of the memory zone • The direction of the transfer • The number of transferred data • The transfer phase: the controller performs the transfer and solve the synchronization with the peripheral device (the DMA controller is a master) • End of the transfer: the processor test the correctness of the transfer by reading the status of the DMA controller (the DMA controller is a slave)

3. The scheme of connecting a DMA controller into a microprocessor system

4. The sequence for handling a DMA request • An interface requires a transfer by activating the DRQi signal to the DMA controller • The DMA controller tests if the request is enabled and if there are no other higher priority transfers in progress • If the request is enabled and there is no other request than it requests the control of the bus by generating a HOLD signal to the processor • The processor finishes the last cycle on the bus and than disables its bus amplifiers (leaving the control of the bus to the DMA controller) and grants the HOLD request with the signal HOLDA (Hold Acknowledge) to the controller • The DMA controller take over the control of the bus by activating its bus amplifiers; it generates an address for the memory and DACKi (DMA Acknowledge) signal to the interface • The DMA controller generates a memory read signal (MRDC/) or an interface read signal (IORC/) • The addressed unit (memory or interface) generates the required data • The DMA controller generates an interface write signal (IOWC/) or a memory write signal (MWTC/); as result the data is transferred from the memory to the interface or vice-versa • The interface disable the DRQi signal and as a result the DMA controller disables its bus amplifiers • The DMA controller disables the HOLD signal, giving up the control of the bus in favor of the processor • The processor disables the HLDA signal, it activates its bus amplifiers and continues with the execution of instructions

5. Time diagram for a DMA transfer

6. Internal structure of the I8237 DMA controller

7. Features of the I8237 DMA controller • can handle up to 4 peripheral devices (it has 4 independent DMA channels) • transfer speed 1,6Mocteţi/s • more controllers may be connected in a cascading mode in order to increase the handled channels • the maximum dimension of a transferred block: 64KB • it can handle memory to memory transfers • More transfer modes: • Single Transfer • Block Transfer (burst) • Auto-initialized Transfer • Memory to memory transfer

8. Transfer through I/O processor • When is it used: • a higher speed is required • the peripheral device is complex (it has a complex behavior) • we want to discharge the main processor form I/O tasks • Implementation: • Input/Output processor + • Transfer program • we use DMA-like mechanisms and interrupts to synchronize the main and the I/O processors • Types of processors used for this purpose: • Specialized I/O processors • General purpose processors integrated into an interface (ex: Z80) • Microcontrollers and signal processors (ex: 8048, 8032, etc.) • Processors specialized for a specific peripheral device (e.g. graphical processor)

9. Advantages and drawbacks of the I/O processor-based transfer • Advantages: • Higher speed • Can be adapted to the behavior of a complex peripheral device (through a program) • Drawbacks: • Hard to implement and test • Higher costs • Concurrency problems on the system bus and memory inconsistency

10. Comparison between transfer modes

11. The serial interface • Interface types: • Serial – reduced number of signals (2-3) and a bit-by-bit transfer • Parallel – a number of parallel signals (usually 8 for data and some for control); transfer of data is made in parallel • Serial transmission: • A limited number of signals used for the transmission (usually 2 or 3 for a transmission direction) • the transmission is made sequentially bit-by-bit • used for long distance transfers • used to interconnect a computer with a peripheral device or two or mode computers • Serial transmission types: • Based on the synchronization mode: • synchronous serial transmission – the transfer is controlled with a clock signal • Asynchronous serial transmission – no clock signal is used • based on the interconnection mode: • point-to-point – connection between two equipments • Multi-point – more equipments connected on the same link • based on the direction of the transfer: • Unidirectional transfer – only one direction • Bidirectional transfer: • Full duplex – 2 independent directions • Half duplex – 2 directions, but alternatively

12. Synchronous serial transfer • Issue: synchronization between the transmitter and the receiver; when is the optimal time for reading the next bit • Synchronous transfer = the speed of the transfer is controlled with a clock signal • Examples: • I2C – serial bus for microcontrollers • the keyboard interface of PCs • Advantages: easy implementation, reliable on short distances • Drawbacks: extra wire for clock signal, limited transmission distances

13. Asynchronous serial transfer • no clock signal is used; synchronization is made through the specific structure of the transmitted data • Types: • Character-based asynchronous transmission • Message-based asynchronous transmission • The communication rules established through a standard protocol • The protocol specifies: • Codification of the binary information (e.g. voltage levels, current levels, light impulses, etc.) • The communication environment (twisted or coaxial wires, optical fibers, radio waves, etc.); topology • The structure of the transmitted data • The method used for synchronization • The method used of error detection • The method used for the flow control

14. The RS232 (V24) standard • RS232 the best known and most used serial asynchronous transfer standard • Information coding: voltage levels • “0” – (+3..+15V) ; usually 12V • “1” – (-3..-15V); usually -12V • Communication environment: electric wires (including phone wires) • The structure of the transmitted data: • synchronization: through the first bit, stop bits and frequencies: 300,600, 1200, 2400,… 19200 Bauds • Error detection: parity bit

15. The RS232 (V24) standard • The data-flow control: • Hardware – through pairs of signals • DSR-DTR – Dataset ready, Data Terminal Ready • RTS-CTS – Request to Send – Clear To Send • Software: through special ASCII codes • XON-XOFF – Start/stop transmission • Transmission distance: max. 100 m

16. calculator DTE DCE DCE Phone network RS232 RS232 MODEM Modulated analog signals The RS232 (V24) standard • Long range transmission through MODEM and phone networks • Signals: • TXD – transmission • RXD – reception • GND – ground • DSR – Data Set Ready • DTR – Data Terminal ready • RTS – Request To send • CTS – Clear To sent • RI - ring

17. Emiter Receiver TXD TXD RXD RXD GND GND DSR DSR DTR DTR CD CD RTS RTS CTS CTS - Bidirectional transfer, protocol DTR-DSR The RS232 (V24) standard • Transmission modes: • One direction, • 2 wires/signals • Bidirectional with software control of data flow (XON –XOFF) • 3 wires/signals • Bidirectional with hardware control of data flow (DTR-DSR or CTS-RST) • 5-7 wires/signals

18. Universal Serial Asynchronous Controller (USART) I8251

19. Other serial transmission standards: • RS485: • Features: • digital information coding through differential voltages (not through voltage levels) • twisted pair of wires for bidirectional transmission • more devices connected on the same cable – multi-point transmission • used as low level protocol for industrial networks (ex: Profibus, CAN) • Advantages: • longer transmission range (till 1 km) • higher noise immunity (because of twisted wires and differential coding) • used mainly in industrial environments

20. The RS485 standard

21. Preambul Adrress Control Useful Date CRC Other serial transmission standards: • HDLC/SDLC • Serial asynchronous transmission on messages • used for network transmission • More efficient than the character-based transmission (useful data/total no. of bits ration is better) • Very good error detection mechanism (CRC) • Synchronization through PLL (Phase Lock Loop) • The structure of the transmitted data: message