CSE190 Winter 2019 Lecture 4 Serial Busses Wireless
CSE190 Winter 2019 Lecture 4 Serial Busses Wireless Embedded Systems Aaron Schulman Serial Buses in our project UART serial bus for sending debug messages to your development host I2C serial bus for communicating with sensors (e.g., the accelerometer) SPI serial bus for communicating with the Bluetooth Low Energy radio 2 Serial Interfaces traps & exceptions
ISA Software Hardware C Assembly Machine Code ldr (read) str (write) SVC# CPU
fault INT# System Buses AHB/APB interrupts Interrupts EM C DAC/ ADC AD
C DA C USART I2 C SP UA I RT In Ou put In tpu te t rru
pt Internal External Timers Co m p Ca are pt ur e GPIO/ INT
Internal & External Memory 3 Parallel Bus VS Serial Bus Simplistic View of Serial Port Operation Transmitter n 0 1 2 3 4 5 6 0 1 2 3 4 5 n+1
5 4 3 2 1 0 Interrupt raised when Transmitter (Tx) is empty a Byte has been transmitted and next byte ready for loading n n+1 n+2 n+3 n+4
n+5 n+6 n+7 n+8 7 6 5 4 3 2 1 0 7 6 5
4 3 2 1 7 6 5 4 3 2 7 6 5 4 3
7 6 7 5 6 7 4 5 6 7 Interrupt raised when Receiver (Rx) is full a Byte has been received and is ready for reading Serial Bus Interface Motivations Motivation Without using a lot of I/O lines I/O lines require I/O pads which cost $$$ and size I/O lines require PCB area which costs $$$ and size
Connect different systems together Two embedded systems A desktop and an embedded system Connect different chips together in the same embedded system MCU to peripheral MCU to MCU Often at relatively low data rates But sometimes at higher data rates So, what are our options? Universal Synchronous/Asynchronous Receiver Transmitter Also known as USART (pronounced: you-sart) 6 Serial Bus Design Space
Number of wires required? Asynchronous or synchronous? How fast can it transfer data? Can it support more than two endpoints? Can it support more than one master (i.e. txn initiator)? How do we support flow control? How does it handle errors/noise? How far can signals travel?
7 Serial Bus Examples S/A Type Duplex #Device s Speed (kbps) Distance
(ft) Wires RS232 A Peer Full 2 20 30
2+ RS422 A Multidrop Half 10 10000 4000
1+ RS485 A Multipoint Half 32 10000 4000 2
I2C S Multimaster Half ? 3400 <10 2
SPI S Multimaster Full ? >1000 <10 3+ Microwi
re S Master/ slave Full ? >625 <10 3+ 1-Wire
A Master/ slave half ? 16 1000 1+ UART Uses
PC serial port is a UART! Serializes data to be sent over serial cable De-serializes received data Slides from BYU CS 224 UART Uses Used to be commonly used for internet access Slides from BYU CS 224 UART Universal Asynchronous Receiver/Transmitter Hardware that translates between parallel and serial forms Commonly used in conjunction with communication standards such as EIA, RS-232,
RS-422 or RS-485 11 Protocol Each character is sent as a logic low start bit a configurable number of data bits (usually 7 or 8, sometimes 5) an optional parity bit one or more logic high stop bits with a particular bit timing (baud) 12 UART Example Send the ASCII letter W (1010111)
UART Hardware Connection UART Character Reception Slides from BYU CS 224 UART Character Reception Slides from BYU CS 224 UART Character Reception Slides from BYU CS 224 UART Character Reception Receiver also verifies that stop bit is 1
If not, reports framing error to host system New start bit can appear immediately after stop bit Receiver will resynchronize on each start bit Let us design a UART transmitter Slides from BYU CS 224 Transmitter/System Handshaking System asserts Send and holds it high when it wants to send a byte UART asserts Busy signal in response When UART has finished transfer, UART deasserts Busy signal System de-asserts Send signal
Slides from BYU CS 224 Transmitter Block Diagram Slides from BYU CS 224 Discussion Questions How fast can we run a UART? What are the limitations? Why do we need start/stop bits? How many data bits can be sent? 19200 baud rate, no parity, 8 data bits, 1 stop bit
22 Serial Peripheral Interconnect (SPI) Another kind of serial protocol in embedded systems (proposed by Motorola) Four-wire protocol SCLK Serial Clock MOSI/SIMO Master Output, Slave Input MISO/SOMI Master Input, Slave Output SS Slave Select Single master device and with one or more slave devices
Higher throughput than I2C and can do stream transfers No arbitration required But Requires more pins Has no hardware flow control No slave acknowledgment (master could be talking to thin air and not even know it) What is SPI? Serial Bus protocol Fast, Easy to use, Simple Everyone supports it SPI Basics A communication protocol using 4 wires Also known as a 4 wire bus Used to communicate across small distances
Multiple Slaves, Single Master Synchronized 25 SPI Capabilities Always Full Duplex Communicating in two directions at the same time Transmission need not be meaningful Multiple Mbps transmission speed Transfers data in 4 to 16 bit characters Multiple slaves Daisy-chaining possible 26 SPI Protocol Wires:
Master Out Slave In (MOSI) Master In Slave Out (MISO) System Clock (SCLK) Slave Select 1N Master Set Slave Select low Master Generates Clock Shift registers shift in and out data 27 SPI Wires in Detail MOSI Carries data out of Master to Slave MISO Carries data from Slave to Master Both signals happen for every transmission SS_BAR Unique line to select a slave SCLK Master produced clock to synchronize data
transfer 28 SPI uses a shift register model of communications Master shifts out data to Slave, and shifts in data from Slave http://upload.wikimedia.org/wikipedia/commons/thumb/b/bb/SPI_8-bit_circular_transfer.svg/400px-SPI_8-bit_circular_transfer.svg.png 29 SPI Communication 30 SPI clocking: there is no standard way Four clocking modes
Two phases Two polarities Master and selected slave must be in the same mode During transfers with slaves A and B, Master must Configure clock to Slave As clock mode Select Slave A Do transfer Deselect Slave A Configure clock to Slave Bs clock mode Select Slave B Do transfer Deselect Slave B Master reconfigures clock mode on-the-fly! 31
SPI timing diagram Timing Diagram Showing Clock polarities and phases http://www.maxim-ic.com.cn/images/appnotes/3078/3078Fig02.gif 32 SPI Pros and Cons Pros: Fast and easy Fast for point-to-point connections Easily allows streaming/Constant data inflow No addressing/Simple to implement Everyone supports it Cons:
SS makes multiple slaves very complicated No acknowledgement ability No inherent arbitration No flow control 33 I2C bus in our projects Communication with the accelerometer Read from the accelerometer Pros Simple wire connection Two wires bus that can connect multiple peripherals with the MCU Cons Complexity is significantly higher
How to operate the camera? Accel I2C MCU I2C register 1 register 2 . Springs https://www.youtube.com/watch?v=eqZgxR6eRjo I2C Details
Two lines Serial data line (SDA) Serial clock line (SCL) Only two wires for connecting multiple devices I2C Details Each I2C device recognized by a unique address Each I2C device can be either a transmitter or receiver I2C devices can be masters or slaves for a data transfer Master (usually a microcontroller): Initiates a data transfer on the bus, generates the clock signals to permit that transfer, and terminates the transfer Slave: Any device addressed by the master at that time Bit Transfer on the I2C Bus
In normal data transfer, the data line only changes state when the clock is low SDA SCL Data line stable; Data valid Change of data allowed 38 of 40 Start and Stop Conditions
A transition of the data line while the clock line is high is defined as either a start or a stop condition. Both start and stop conditions are generated by the bus master The bus is considered busy after a start condition, until a stop condition occurs SDA SDA SCL SCL
Start Condition Stop Condition 39 of 40 I C Addressing 2 Each node has a unique 7 (or 10) bit address Peripherals often have fixed and programmable address portions Addresses starting with 0000 or 1111 have special functions:
0000000 Is a General Call Address 0000001 Is a Null (CBUS) Address 1111XXX Address Extension 1111111 Address Extension Next Bytes are the Actual Address 40 of 40 I2C-Connected System Example I2C-connected system with two microcontrollers (Source: I2C Specification, Philips) Master-Slave Relationships Who is the master? master-transmitters master-receivers
Suppose microcontroller A wants to send information to microcontroller B A (master) addresses B (slave) A (master-transmitter), sends data to B (slave-receiver) A terminates the transfer. If microcontroller A wants to receive information from microcontroller B A (master) addresses microcontroller B (slave) A (master-receiver) receives data from B (slave-transmitter) A terminates the transfer In both cases, the master (microcontroller A) generates the timing and terminates the transfer Exercise: How fast can I2C run?
How fast can you run it? Assumptions 0s are driven 1s are pulled up Some working figures Rp = 10 k Ccap = 100 pF
VDD = 5 V Vin_high = 3.5 V Recall for RC circuit Vcap(t) = VDD(1-e-t/) Where = RC 43
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