Microcontroller
- 1. Wireless Embedded Systems Aaron Schulman CSE190 Winter 2020 Lecture 2 MCUs and IO
- 2. Reading for next week • Posted on Website – Please skim. – “ARM Cortex-M for Beginners”
- 4. Introduction to Microcontrollers • A microcontroller (MCU) is a small computer on a single integrated circuit consisting of a relatively simple central processing unit (CPU) combined with peripheral devices such as memories, I/O devices, and timers. – By some accounts, more than half of all CPUs sold worldwide are microcontrollers
- 5. Die shot of a microcontroller
- 6. Microcontroller VS Microprocessor • A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. • A microprocessor incorporates the functions of a computer’s central processing unit (CPU) on a single integrated circuit.
- 8. Types of Processors • In general-purpose computing, the variety of instruction set architectures today is limited, with the Intel x86 architecture overwhelmingly dominating all. • There is no such dominance in embedded computing. On the contrary, the variety of processors can be daunting to a system designer. • Things that matter – Peripherals, Concurrency & Timing, Clock Rates, Memory sizes (SRAM & flash), Package sizes
- 10. How to choose MCU for our project? • What metrics we need to consider? – Power consumption – Clock frequency – IO pins – Memory – Internal functions – Others
- 11. How to choose MCU for our project? • What metrics we need to consider? – Power consumption • We cannot afford mA MCU because the power budget of the system is 3.47mA. – Clock frequency (speed that instructions are executed) • kHz is too slow… • 100MHz is over kill... – IO pins • Lots of peripherals - Image sensor, UART debugger, SD card, DAC, ADC, microphone, LED
- 12. How to choose MCU for our project? • What metrics we need to consider? – Memory • We need to have sufficient memory to store: – Program (Non-volatile): Logic to read from sensors, communicate – Stack: Function calls are now expensive (no recursion) – Data: Constants (time periods), Sensor history, Communication state – Internal functions • Migrating data from the sensor to the radio (DMA)
- 13. How to choose MCU for our project? • Memory – Store accelerometer history data • 12bits each for X,Y,Z acceleration • sampled 2 thousand times a second (2 KHz) • = 12*3*2,000 bits per second (72kbits or 9 kBytes) • How many seconds can we hold if we have only 100 kBytes of storage – What types of memory are available on an MCU? • Internal memory: RAM, 0.5~128 kBytes • External memory: Flash, high power consumption, ~5mA for read and ~10mA for erase
- 14. How to choose MCU for our project? • Clock frequency – kHz is too slow • Smartphone camera frame rate is 60fps (1 KHz clock would leave only 60 clock cycles per frame) – 100MHz is too fast • Power consumption is high – Several MHz would be ideal
- 15. How to choose MCU for our project? • IO pins (interface for external peripherals) – Interfacing sensors, UART debugger, LEDs, Bluetooth – We need a large number of IO pins – We need various types of IO pins • Analog pins (input/output analog signals e.g., audio) • Digital pins (input/output digital signals e.g., busses, GPIOs)
- 16. The MCU used in our projects
- 17. Input and Output (I/O)
- 18. I/O Devices (sensors) • Keyboard, mouse, microphone, scanner, video/photo camera, etc. • Large diversity – Many widely differing device types – Devices within each type also differs • Speed – varying, often slow access & transfer compared to CPU – Some device-types require very fast access & transfer • Access – Sequential VS random – read, write, read & write
- 19. What operations does software need to perform on peripherals? 1. Get and set parameters 2. Receive and transmit data 3. Enable and disable functions
- 20. How can we imagine providing an interface to hardware from software? 1. Specialized CPU instructions (x86 in/out)
- 21. Port I/O • Devices registers mapped onto “ports”; a separate address space • Use special I/O instructions to read/write ports • Protected by making I/O instructions available only in kernel/supervisor mode • Used for example by IBM 360 and successors Port I /O • Devices registers mapped onto “ports”; a separate address space • Use special I/O instructions to read/write ports • Protected by making I/O instructions available only in kernel/supervisor mode • Used for example by IBM 360 and successors memory I/O ports
- 22. How can we imagine providing an interface to hardware from software? 1. Specialized CPU instructions (x86 in/out) 2. Treating devices like they are memory (MMIO)
- 23. Memory Mapped IO • Device registers mapped into regular address space • Use regular move (assignment) instructions to read/ write a device’s hardware “registers” • Use memory protection mechanism to protect device registers Memory Mapped I/ O • Device registers mapped into regular address space • Use regular move (assignment) instructions to read/ write registers • Use memory protection mechanism to protect device registers • Used for example by PDP-11 memory mapped I/O memory
- 24. MMIO is used for embedded systems • Why not Ports I/O? – special I/O instructions would be instruction set dependent (x86, ARM Thumb, MIPS) • Bad if there are many different instruction sets out there – Need special hardware to execute and protect instructions • Memory mapped I/O: – Can use all existing memory reference instructions for I/O • Can reuse code for reading and writing (e.g., memcpy) – Memory protection mechanism allows greater flexibility than protected instructions (protect specific registers) – Can reuse memory management / protection hardware to interface with hardware (saves space and power)
- 25. Reading and writing with MMIO is not like talking to RAM • MMIO reads and writes hardware device registers • Reads and write to registers can cause peripherals to begin or end an operation • By reading data, it may cause the hardware to do something!!! – E.g., Clear an interrupt flag, get the next byte on a bus • By writing data, it may cause the hardware to do something with it – E.g., Send this data over the UART bus
- 26. GPIOs are the general digital I/O device Each GPIO pin represents one bit in memory: if the pin is on it’s a 1, off it’s a 0 That bit can be an input or an output • GPIOs can be used to control lights (light on or off), but even more • Indicating that an event just happened – Interrupt the radio to tell it to transmit data – Interrupt the CPU to tell it a button was pressed – Read pin status to receive configuration messages • Debugging – Did this one part of my code actually execute? – Is the timer firing at the interval that I expect it to fire (connect GPIO to oscilloscope)? – Why using GPIO? • GPIO ops are lightweight
- 27. Topology of a GPIO pin
- 29. A fun extra feature: Drive Strength