ECE 353 Introduction to Microprocessor Systems

1. ECE 353Introduction to Microprocessor Systems Michael G. Morrow, P.E. Week 8

2. Topics • Logic Compatibility • Microprocessor peripherals • Timers • Periodic • Watchdog • Real-time • ADuC7026 timers • Pulse-width modulators (PWM) • Direct memory access (DMA)

3. FET Basics • Field effect transistors (FETs) are used as the switching elements in logic gates • When turned on, the FET presents a very low impedance to current, like a closed switch • When turned off, the FET presents a very high impedance to current, like an open switch • There are two ways to construct a FET • N-channel P-channel

4. Logic Outputs • Totem-pole • Three-state (tri-state) • Open-drain

5. Logic Family Characteristics • Voltage • VIHmin– minimum input voltage recognized as a logical 1 • VILmax – maximum input voltage recognized as a logical 0 • VOHmin – minimum voltage output for a logical 1 • VOLmax – maximum voltage output for a logical 0 • Current • Currents are defined positive in, negative out • IOHmax– maximum output current sourced for a logical 1 • IOLmax – maximum output current sunk for a logical 0 • IIHmax – maximum input current required at a logical 1 • IILmax – maximum input current required at a logical 0 • IOZH, IOZL – current drawn/sourced when tri-stated • Logic families

6. Simple Circuit Models • Drivers • Receivers

7. DC Noise Margins

8. Logic Compatibility • Static (DC) Compatibility • Voltage • Current • Overvoltage tolerant inputs • Dynamic compatibility • Capacitive loading, mutual induction, reflections, etc. • Exercises

9. Timer Peripherals • Timer/counter modules used to • Generate signals with specified frequency / duty cycle • Count external events, measure pulse width • Generate absolute delays, periodic interrupts • Building a timer peripheral • Basic free-running timer • Periodic timer enhancements • Clock selection and prescaling • Adding capture capability

10. A Basic Free-Running Timer

11. A Periodic Timer

12. Clock Selection and Prescaling

13. Capture Capability

14. Real-Time Clocks (RTCs) • RTCs provide microprocessor systems with absolute time information • Absolute time does not necessarily mean calendar/clock time • Typically operate from 32.768KHz crystal with battery or capacitor back-up power supply • Generate periodic interrupts • Often contain a small amount of RAM – historically this was where the PC stored its configuration (BIOS) settings since it was non-volatile. • Dallas Semiconductor DS1375

15. Watchdog Timers • Watchdog timers are used to guard a system against lock-up due to software errors or soft failures in hardware • Often included in microcontrollers and CPU supervisor circuits. • Retriggering is usually done in the main program loop • Watchdog output can be used to reset the CPU or as a nonmaskable interrupt (NMI) • Maxim MAX6323/MAX6324

16. ADuC7026 Timers • Timer0 • A basic periodic timer, intended to be used as the RTOS timer • 16-bit counter, free-running or period register • CPU core clock with prescaler • Generates interrupt and/or ADC conversion trigger • MMRs

17. ADuC7026 Timers • Timer1 • General-purpose timer • 32-bit counter • Multiple clock sources with prescaler • Capture register • Binary or H:M:S formats

18. ADuC7026 Timers • Timer2 • Wake-up timer • 32-bit counter • Can run on 32kHz clocks • Binary or H:M:S format

19. ADuC7026 Timers • Timer3 • Watchdog or general-purpose timer • 16-bit counter • 32kHz clock source • Watchdog timer is reset by writing to T3CLRI MMR • Requires pseudo-random sequence in secure clear mode

20. Timer3 Secure Clear Mode

21. PWM Peripherals • A basic pulse-width modulator peripheral creates a rectangular wave whose duty cycle can be controlled • PWM allows us to control the average power delivered to a load without changing the voltage supplied to it • The ADuC7026 contains a very capable 3-phase PWM that is intended to do motor control

22. DMA Controllers • Direct memory access (DMA) controllers are peripherals devices designed to offload data movement from the processor • A common use is in servicing peripherals by collecting a frame of data for the CPU to work on, or sending out a frame of data to a peripheral as it needs it • To use DMA, we need to • Program the DMA controller for the task • Processor does other things • The DMA controller interrupts the processor when it has completed. • DMA controllers usually have an auto-reload feature to do a repetitive task without the CPU having to reconfigure it every time.

23. DMA Controllers (cont) • Typical DMA controllers are programmed with the following information • Source address and destination address • Should address be modified at each transfer, and by how much? • Transfer size • How many bytes should it transfer each time? • Number of transfers • Trigger event • What causes the DMA controller to do transfers?

24. Wrapping Up • Homework #4 is due Wednesday, March 21 • Reading for next week • Textbook 11 • ADUC 9-10, 33-36, 43-47, 79-82

25. Logic Compatibility Exercises • For the following logic families, determine compatibility, noise margins, and fan-out. • 74ALS driving 74AC • 74AC driving 74ALS Note: For 74AC, top line is with CMOS load, bottom line is with TTL load.

26. TinyLogicTM and Little Logic

27. Timer0 MMRs • T0LD – load counter value • T0VAL – read counter value • T0CLRI – clear timer interrupt • T0CON - Configuration

28. DS1375RTC

29. MAX6323

30. PWM DAC • Use PWM digital output driver • LPF removes most of AC components