Microcontroller: 1-wire Interface, SPI Interface Amarjeet Singh February 5, 2012
Logistics Website updated with lecture slides from last 2 lectures Assignment-3 Any problem
Assignment-4 Final assignment – and probably requires maximum effort Make sure to build upon your previous assignments Link for Hex file for boot loader given on the course website
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Students Presentations – After Mid Sem Student presentations on specialization topics in Cyber Physical Systems
Will share the list of topics with spreadsheet with everyone Mandatory for CSE-537 students Select one of the topics that you want to present by this weekend and assign it to yourself in the spreadsheet If CSE-337 students also present, their summary evaluation will also carry 5 marks only
Presentations will start after mid semester
I will post the schedule after you all have selected the papers All need to submit 1 page summary on the same paper before the class (through email) Each group need to submit one summary only
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Revision from last two classes For the 16-bit RTC in xMega: What is the highest resolution obtained with 32.768 KHz clock? With a resolution of 1 second, what is the maximum timeout period? How can you use the two Analog Comparator modules in xMega to check if a signal is within a range? What is the maximum ADC resolution in xMega? How many maximum single ended signals can be supported using ADC block in xMega What is the reference voltage given to the negative channel for: Single ended signal unsigned mode Single ended signal signed mode Differential signal (signed) 4
Revision from last two classes What limits the sampling rate of ADC?
How is propagation delay given for ADC block in xMega? What are things to consider when selecting a sensor to interface with your microcontroller? What is the function of voltage regulators? What are some common types of voltage regulators? Where and why are decoupling capacitors used? Where and why are pull up/down resistors used?
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Revision from last two classes Why is it important to take a look at DC characteristics while interconnecting two ICs I2C protocol How many lines? What is the packet format? Why is it not suitable for peer to peer communication?
DS18S20: 1-Wire Interface Temperature Sensor from Maxim 9-bit temperature value 0.5 degree celsius accuracy from -10 to +85 Can derive power directly from the data line Unique 64-bit code that allows multiple temperature sensors to be connected on the same line – e.g. temperature monitoring in HVAC
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DS18S20: Function Commands DS18S20 Transaction: Initialization ROM Command (followed by any required data exchange) DS18S20 Function Command (followed by any required data exchange)
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DS18S20: Operation Example 1 Multiple DS18S20 on the same line (in parasitic mode) Master initiates a temperature conversion in a specific sensor Master reads the scratchpad to recalculate the CRC and verify the data
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DS18S20: Operation Example 2 Single DS18S20 on the bus (in parasitic mode) Master writes to TH and TL registers in scratchpad and then reads the scratchpad to verify the values Master then copies scratchpad into EEPROM
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DS2482: I2C to 1-wire DS2482-100: I2C to 1-wire bridge device Interfaces with standard or fast I2C masters to perform bidirectional conversion between I2C master and downstream 1-Wire slave devices Relative to 1-Wire slave devices, it acts as 1-Wire master
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SPI Signals Four main signals: Master Out Slave In (MOSI) Master In Slave Out (MISO) Serial Clock (SCK) Chip Select (CS)
Since there is a separate CS for each device on board, SPI does not scale up well in comparison with I2C
MOSI: Generated by master, received by slave Also labeled as Serial Input (SI) or Serial Data In (SDI)
MISO: Generated by slave, under control by the master Also labeled as Serial Output (SO) or Serial Data Out (SDO)
Chip Select (Slave Select) generated using spare I/O of master 13
SPI Communication Master configures SCK with frequency less than or equal to maximum frequency supported by slave device (1-70 MHz) Master pulls the CS low Full duplex data transfer:
Both master and slave contain shift registers: Master starts the transfer by writing to its SPI shift register As master transfers the byte to the slave on MOSI, slave transfers the contents of its shift register back to the master on MISO After 5 bits are transferred what is the state of TX Buffer?
Both the write and the read performed simultaneously
For write only operation, read byte is ignored For read only operation, a dummy byte is written to initiate slave transmission 14
SPI: Modes of Operation Four modes of operation depending on clock polarity and clock phase Low clock polarity (CPOL = 0): SCK is low when idle and toggles high during transfer High clock polarity (CPOL = 1): Reverse of low clock polarity
Clock phase zero (CPHA = 0): MOSI and MISO are valid on rising/falling edge of SCK for low/high clock polarity respectively
Clock phase zero and low clock polarity How does the timing diagram change for CPOL = 1? 15
SPI: Modes of Operation Clock phase one (CPHA = 1): MOSI and MISO are valid on falling/rising edge of SCK for low/high clock polarity respectively
SPI: Interfacing External Flash Atmel AT25DF641: 64 Mbit Serial Flash Memory Supports SPI mode 0 and 3 Works at 75 MHz Low power dissipation: 5 mA (typical) active read current 100,000 Program/Erase cycles
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SPI: Interfacing External Flash Atmel AT25DF641: 64 Mbit Serial Flash Memory Supports SPI mode 0 and 3 Works at 75 MHz Low power dissipation: 5 mA (typical) active read current 100,000 Program/Erase cycles
Which SPI mode does this waveform represent?
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SPI: Key Observations 4 lines required: MOSI, MISO, SCK, CS Allows simultaneous read and write making it more efficient Higher data rate: Up to 4 Mbps or more Multiple devices can be connected simultaneously but with only a single master Device selection based on Chip Select: Implies an extra pin for each device on the master chip More efficient in point to point connection: Why? No acknowledgement mechanism to confirm data receipt
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SPI vs I2C SPI more suited for applications that are thought as data streams (as opposed to reading/writing address locations in slave devices) Gains efficiency in applications that take advantage of its duplex capability e.g. codec (that requires sending samples in and out) Due to lack of built-in device addressing, SPI requires more hardware resources when more than one slave device is attached But SPI more efficient and simpler in point-to-point links Cost and complexity of I2C does not scale with number of devices
With its collision detection and acknowledgement scheme, I2C is a true multi-master bus
