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COSMOS Summer 2008 Peripheral Interfaces

COSMOS Summer 2008 Peripheral Interfaces. Rajesh K. Gupta Computer Science and Engineering University of California, San Diego. Roadmap. Topic: Computer interfaces: electricals, signaling, protocols This lecture

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COSMOS Summer 2008 Peripheral Interfaces

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  1. COSMOS Summer 2008Peripheral Interfaces Rajesh K. Gupta Computer Science and Engineering University of California, San Diego. 2008 R. Gupta, UCSD

  2. Roadmap • Topic: • Computer interfaces: electricals, signaling, protocols • This lecture • Concepts covered: Input/Output, Addressing, Synchronous, Asynchronous, Access Arbitration, Protocols, Packets, Messaging • Next lecture • The USB Interface • Reference • None Keywords: Bidirectional Address space Address decoding Multi-master Wired AND I2C, Master/Slave 2008 R. Gupta, UCSD

  3. A Processor’s Interfaces • Two basic types • Communicate ‘via’ memory • Communicate directly • How do we measure goodness of an interface? • Maximum data-rate of transfer? Bits or bytes per second? 2008 R. Gupta, UCSD

  4. Peripheral Interfaces • Two ‘low cost’ interfaces • SPI: Serial Peripheral Interface (4-wire) • I2C: Inter-Integrated Circuit (2-wire) • A microcontroller can control a network of devices with just two general-purpose IO pins and software. • Let us examine I2C • Used for: reading from DAC, ADC, changing monitor settings, sound volume control to intelligent speakers, controlling LCD displays, reading diagnostic sensors, reading real-time clocks, turning on/off power supply for components… • Many USB-to-I2C adapters to enable their use in PCs. 2008 R. Gupta, UCSD

  5. I2C • Connects multiple devices on a multi-drop bus • “Multi-master” • Bidirectional, low-speed, synchronous to a common clock • Devices can be attached or detached without affecting other devices • 7-bit address space, 16 reserved, 112 nodes maximum • Distances up to a few meters (400 picoFarad max) • Standard: 100 kbps (can go down to DC) • Low speed: 10 kbps, Fast: 400 kbps, FM+ 1 Mbps, HS 3.4 Mbps 2008 R. Gupta, UCSD

  6. I2C Wires • Two Wires, bidirectional wires • SDA: Serial Data • SCL: Serial Clock • ‘Open drain’: normally high when not in use • MASTER node issues the SCL and addresses SLAVES • SLAVE node receives the SCL and the address • “Wired AND” logical function. • Four modes for a given device: • Master TX, Master RX, Slave TX, Slave RX 2008 R. Gupta, UCSD

  7. So, how do we write or read? • Normally, both SDA and SCL are ‘high’ • “sense” before you drive a line • A device that wants to write pulls SDA low • Followed by SCL going low • So, everyone else knows that a transmission is starting 2008 R. Gupta, UCSD

  8. START, Data, Data,…,Data, STOP!

  9. Putting it together • Start Condition: With SCL low, SDA goes HL • Bits are ‘sampled’ on the rising edge of SCL • Stop Condition: With SCL high, SDA goes LH 2008 R. Gupta, UCSD

  10. “START”, “BIT” & “STOP” • Start Condition: With SCL low, SDA goes HL • Bits are ‘sampled’ on the rising edge of SCL • Stop Condition: With SCL high, SDA goes LH • 2008 R. Gupta, UCSD

  11. QUIZ Question 1: What is on SDA when it makes a transition with SCL low? Question 2: What is on SDA when it makes a transition with SCL high? Question 3: What happens if Slave is not ready to receive next BIT?

  12. “The Protocol” • Any number of bytes in an I2C packet • MSB first, each bytes transmitted must be acknowledged by the receiver • After each 8th bit, MASTER releases SDA and then generates an additional clock pulse on SCL • Receiver can then acknowledge by pulling SDA low • Receiver can always abort the transmission by holding SCL low • Can not go up by the MASTER, thus no bit sampling edge 2008 R. Gupta, UCSD

  13. Bi-directional Data Transfers

  14. “The Protocol” • Master sends START • followed by 7-bit address of the Slave • followed by single bit representing write to (0) or read from (1) the slave. • Slave responds with ACK bit for that address • Master then continues in either TX or RX mode • Communication transfer follows. • All other MASTERS monitor START and STOP bits. 2008 R. Gupta, UCSD

  15. Note the direction of signaling

  16. Multi-Master = Conflicts? • More than one Master may attempt to START at the same time • Signaling provides ability to prioritize and exclude 2008 R. Gupta, UCSD

  17. Conflicts and Tricks • When a node is transmitting ‘1’ and another transmits a ‘0’ • First node can sense something is wrong…since data node is not default high • Slave can do this on SCL to ‘stretch the clock’ • Or slow down the master • Master can do this on the SDA to ‘get control’ (arbitration) 2008 R. Gupta, UCSD

  18. More on Conflicts • What happens if two masters are sending a message to two different slaves? • Master sending to the lower slave address always ‘wins’ arbitration 2008 R. Gupta, UCSD

  19. Types of Messages on I2C • Single message where a Master writes to a Slave • Single message where a Master reads from a Slave • Combined messages where a Master issues at least two reads and/or writes to one or more slaves • Each RD or WR begins with START and the slave address • A Repeat START is not preceded by STOP • Slaves figure out that data following repeat START is part of the same message. 2008 R. Gupta, UCSD

  20. What do messages mean? • “Message Semantics” • Most commonly request/response control model • Bytes following a Write command are either a command or address 2008 R. Gupta, UCSD

  21. Role of Software in I2C • SDA and SCL lines can be controlled by simple subroutines. • Example: • I2Start: • check if SCL is high; • wait a few micro seconds; • Transition SDA H to L; • wait a few micro seconds; 2008 R. Gupta, UCSD

  22. EXAMPLE: Memory connection 2008 R. Gupta, UCSD

  23. What are the limitations of I2C? • Small address space • Speed? • I2C implemented in Software may not even support 100 kbps? • *All* devices must at least partially support the highest speed used or they may detect spurious addresses • Clock stretching affects everyone. 2008 R. Gupta, UCSD

  24. Exercise • Is I2C communication synchronous or asynchronous? • But START and STOP delimit data packets? • Is I2C duplex? Fully duplex? • Draw SDA and SCL waveforms in case of transmission of START followed by $A2 = (1010 0010) 2008 R. Gupta, UCSD

  25. 2008 R. Gupta, UCSD

  26. Recap • Computers interfaces with devices via memory or via direct ports • I2C defines a 2-pin interface that • Performs Master-Slave communications • The interface defines electrical behavior (signaling) • And the data transfer protocols (messaging) • What about the mechanicals? • Next: USB interface. 2008 R. Gupta, UCSD

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