Microcontroller: Timers, ADC

Amarjeet Singh

February 1, 2013

Logistics

2

Please share the JTAG and USB cables for your assignment Lecture tomorrow by Nipun

Revision from last class

3

When servicing an interrupt, how does the timing work? How are interrupt priorities decided? What are the primary blocks in a Timer/Counter? How is Buffer Value (BV) flag decided in both Capture and Compare

mode in xMega?

4

xMega: Timer/Counter Block Diagram

Registers: Counter (CNT), Period (PER), Compare Capture (CCx)

Size?

Counter compared with 0 or PER to generate BOTTOM/TOP

Compared with CCx to generate interrupt, events

Waveform generator mode uses CCx to set waveform period and pulse width

How to support 32 bit count?

5

Single Slope PWM Generation

Time period is controlled by PER register (counter counts from BOTTOM to TOP) while CCx register controls the duty cycle

New value of CCx Register loaded on UPDATE event WG output is set to 0 on compare match and set to 1 on TOP

6

Dual Slope PWM Generation

Time period is controlled by PER register (counter counts from BOTTOM to TOP and then TOP to BOTTOM) while CCx register controls the duty cycle

WG output is set to 1 on BOTTOM ; 0 on compare match during up count and 1 on compare match during down count

New value of CCx updated on UPDATE event

7

Timer/Counter 0/1: Associated Registers

8

Timer/Counter 0/1: Associated Registers

9

xMega: Timer/Counter2

Realized when Timer/Counter 0 is set in split mode A system of 2 8-bit Timer Counters (referred to as Low Byte and

High Byte Timer/Counter) with 4 compare channels each Provides 8 configurable PWM channels with individually

controlled duty cycle Only the low byte timer/counter can be used to generate compare

match interrupts, events Shared clock source and separate period and compare settings At a time can be only used as Timer/Counter 0 or 2

10

Timer/Counter2: Block Diagram

Counter operation same as Timer/Counter 0

Compare channel operation same as Timer/Counter 0

11

xMega Timers: Summary

TC-0/1: 16-bits, 32-bits support by cascading TC-2: 2 8-bits Multiple compare or capture (CC) channels for each timer

One compare match or input capture interrupt/event per channel

Used for count and direction control, capture Waveform generation:

Frequency generation Single slope PWM (Only this one available for TC-2) Dual slope PWM

Input capture: Frequency capture Pulse-width capture

12

Pic24F: Timers and Counters

Timer1: 16-bit timer/counter Timer2/3 and Timer4/5: 32-bit timers that can be independently

configured as 2 16-bit counters Separate module for Input Capture (on ICx pin)

Can capture rising/falling/both edges

13

Pic24F: Timers and Counters

Separate output compare module to generate output pulses

14

AVR32: Timers and Counters

15

xMega: Real Time Counter (RTC)

16-bit RTC runs continuously even in the low power sleep mode to keep track of time

Can wake up devices from sleep and/or generate interrupt at regular

time intervals

Reference clock is typically 1.024KHz output from high accuracy crystal of 32.768 KHz

Includes a 10-bit prescaler to scale down the reference clock before it

reaches the counter

What is the highest resolution with 32.768 KHz clock? With a resolution of 1 s what is the maximum timeout period?

16

xMega: Real Time Counter (RTC)

RTC can generate both interrupts and events Compare interrupt Overflow interrupt

17

xMega: RTC32

32-bit Real Time Counter Runs continuously even in low power sleep mode External 32.768 KHz clock must be used as clock reference fed into

the prescaler Reference clock: 1 Hz or 1.024 KHz

18

PIC24F: Real Time Clock and Calendar

Time (hours, minutes, seconds); Calendar (Weekday, date, month, year): Year range 2000 – 2099 (with leap year correction)

Requires external 32.768 KHz clock crystal Optimized for low power operation

19

xMega: Analog Comparators

Compares voltage levels on two inputs and gives a digital output based on the comparison

Input sources: Analog port pins, several internal signals, 64-level programmable voltage scales the input values on positive pin AIN0 and negative pin AIN1

20

Analog Comparator

When voltage on first input is greater than that on second input, Analog Comparator Output (ACOUT) is set

Output can be set to trigger Timer/Counter Capture operation or can also trigger interrupt specific to analog comparator

Interrupt triggering can be enabled on output rise, fall or toggle How can I use both AC0 and AC1 to check if input signal is within a

range?

Window mode supported internally by the device

21

Analog to Digital Converter (ADC)

12-bit resolution (Can also work in faster 8-bit resolution) Up to 2 million samples per second (minimum 3.5 uS conversion time) Flexible input selection: Both single ended and differential

Up to 16 single-ended input; 4 channels of differential (with/without gain)

Built in differential gain: 1/2x – 64x Multiple input sources – Internal temperature sensor, DAC output etc.

22

Analog to Digital Converter (ADC)

Pipelined architecture with several stages: Allows high sample rate at low system clock frequency

New input can be sampled while older ADC conversions still going on

4 conversion channels with individual input selection: Allow 4 parallel operations

23

ADC Voltage Conversion

Several sources for reference voltage selection

Result is written to corresponding Channel result register (RES) Global setting for signed/unsigned conversion

Unsigned for single ended or internal signals Signed for differential signals generating both positive and

negative results What will be the TOP value for signed/unsigned conversion? What will be the ADC transfer function in terms of TOP?

24

ADC Voltage Conversion: Signed mode

Single ended measurement in signed mode: Negative connected to ground

ADC transfer function in signed mode

25

ADC Voltage Conversion: Signed mode

Signed differential input

ADC transfer function in signed mode

26

ADC Voltage Conversion: Unsigned mode

Single ended measurement in unsigned mode: Negative connected to half the reference voltage minus a fixed offset (nominal value 0.05*VREF)

Positive will vary from 0 to VREF

Offset to allow calibration when internal ground is higher than external ground

ADC transfer function in unsigned mode

27

xMega: ADC

Compare function: ADC has in-built 12-bit compare function ADC value can be compared with the ADC Compare Register to

generate an interrupt/event if the value is above/below the threshold All 4 ADC channels share the same Compare Register

Starting a conversion: Select the input source for one or more ADC channels ADC conversion started by application software writing to start

conversion bit of the Channel or from any events in the Event System

28

xMega ADC Block Diagram

29

ADC Clock and Conversion Timing

Clocked from Peripheral Clock that can be prescaled to provide ADC Clock

Maximum ADC sampling rate given by ADC clock frequency Propagation delay of ADC measurement given by

RES: Resolution (8- or 12-bit) Greater than one clock cycle but pipelined architecture implies

sampling rate is not limited by propagation delay but by clock frequency

30

ADC Clock and Conversion Timing

MSB of the result is converted first Rest of the bits are converted in the next 3 (for 8-bit resolution) or 5

(12-bit resolution) clock cycles Converting 1 bit takes half a clock cycle During the last clock cycle, result is prepared before the interrupt

flag is set

Single conversion without Gain

31

ADC Clock and Conversion Timing

Single Conversion with Gain

32

ADC Clock and Conversion Timing

Single Conversion on 2 ADC Channels (both without Gain) Both conversion requests may be placed at the same time Due to pipelined design, second conversion can start in the next

clock cycle after the first conversion has started

33

ADC Clock and Conversion Timing

Single Conversion on 2 ADC Channels (Channel0 with Gain and Channel1 without Gain)

34

ADC Clock and Conversion Timing

Single Conversion on 2 ADC Channels (Channel0 without Gain and Channel1 with Gain)

35

ADC Clock and Conversion Timing

Free Running mode (All 4 channels – Channel 0 and 1 without gain, Channel 2 and 3 with Gain)

What did you learn today?

36

On a small sheet of paper, give me a short (2-3 sentences) description of what you learned today

You can use it to give any broad comments on the class as well