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Real Time Embedded Systems
www.atomicrhubarb.com/embeddedLecture 1 – January 17, 2012
Topic
Section Topic
• Where in the books– Catsoulis chapter/page– Simon chapter/page– Zilog UM197 (ZNEO Z16F Series Flash Microcontroller Contest Kit User Manual)
– Zilog UM171 (ZiLOG Developer Studio II—ZNEO User Manual)
– Zilog PS220 (ZNEO Z16F Series Product Specification)
– Zilog UM188 (ZNEO CPU Core User Manual)
– Assorted datasheets
Signals
Analog
DigitalReferencevoltage
Digital-Analog Connecting a sensor to digital circuitry is
simple if the sensor provides digital output.
We have seen that switches, and logic encoders are easily interfaced with gate circuits due to the nature of their signals.
When analog sensors are involved the interfacing becomes more complex.
We need a way to electronically translate analog signals into digital quantities, and visa-versa.
Analog Signals
How do we input and output analog signals?
Signal Converters
Analog to Digital signal converter on the inputs
Digital to Analog signal converter on the outputs
Digital -> Analog
A Digital to Analog converter is an electronic circuit which accepts a digital number at its input and produces a corresponding analog signal (usually a voltage) at the output
Digital -> Analog
DAC Several Types
PWM output + low pass filter Binary Weighted DAC R2R Ladder Kelvin Divider
DAC
Kelvin Divider
DAC
R-2R ladder network is a simple Digital-Analog converter.
Only two resistance values are used anywhere in the entire circuit.
Modern DACs use the ‘R-2R ladder’
But ...
We don't have a Digital to Analog converter on the ZNEO (or any of the Zilog microcontrollers).
Fortunately we know how to add that capability if we need
it Add a DAC chip I2C, SPI, 1-wire, … Or parallel digital input 1000's to choose from (really!)
Like this one!
Analog -> Digital
An Analog to Digital converter is an electronic circuit which accepts an analog input signal (usually a voltage) and produces a corresponding digital number at the output
ADC
Analog-to-Digital Conversion The workhorse of the test and
measurement world. Providing a means for a micro-
controller to measure analog signals.
ADC Several Types (ways to convert analog
to digital) Digital Ramp Successive Approximation Subranging Tracking Slope Integration Flash Sigma Delta
Flash ADC
Start with a voltage comparator Determines of a signal is greater than a
reference signal (a 1-bit analog to digital converter).
Vref
Vin
VoutVref
Vin
Vout
+
‐
Flash ADC
n-bit resolutionrequires2n-1 comparators
8-bit means255 comparators
10-bit means1023 comparators
Digital Ramp A binary counter counts up
from 0. The output of the counter to feed into a digital to analog converter.
The generated analog signal is compared to the input signal.
When the generated signal is greater than the input signal the counter stops and the binary value is the digital value for this signal
Sigma Delta ADC
High resolution and Low cost The Z16 uses Sigma-Delta ADC One of the more advanced ADC
technologies
Why so many different ways?
Resolution vs Speed
ADC Signal Sampling
Sampling can cause errors (aliasing)
Nyquist rate = 2x highest frequency of interest
Practically: always sample at least 5x, or higher
What kinds of things can you measure with an ADC?
Sensors
Sensors with analog output Temperature Pressure Light intensity Position Velocity, RPM ...
Analog Input
Don't like the ADC on your microcontroller?
Add an external one. Many to choose from Parallel data out I2C, SPI, 1-wire, ....
MAX19588
100 Million samples per second 16 bit
MAX1072 1.8 Million samples per second 10-bit data 1 micro amp in shutdown
Why so many different ADC?
Why
Speed (samples/second) Low Power for battery/portable
applications Resolution (8,10,12,14,18 bits) Number of channels Interface Range of analog input (3v, 12v, +/- 12v,
...)
Z16 ADC
One ADC 12 Switched inputs 10-bit resolution Internal voltage reference generator Ability to supply external reference
voltage.
Z16 ADC
The Pins
ANA0..11 AVDD, AVSS VREF
ADC
10 bit conversion (needs 2 bytes for result)
12 analog input sources are multiplexed with general-purpose I/O ports
Interrupt upon conversion complete Internal voltage reference generator
ADC
• Each ADC measurement consists of three phases:
1. Input sampling (programmable, minimum of 1.0 μs).
2. Sample-and-hold amplifier settling (programmable, minimum of 0.5 μs).
3. Conversion is 12 ADCLK cycles.
Z16 ADC Registers
ADC0CTL – Control Register ADC0DH, ADC0DL – ADC data ADCSST – Sample Settling Time ADCST – Sample Time ADCCP – Clock Prescaler ADC0MAX – Max Chanel Register ADCTCAP_H,L – Timer0 Capture
Register
Z16 ADC Registers
Don't forget GPIO alternate functionand interrupt control registers
ADC0CTL
ADC0D_H,L
Interrupt?
Why do I want to generate an interrupt on conversion?
Single ShotADC
Single ADC
1. Enable the alternate function of the GPIO pins that you want to use for analog inputs (PORT B & H)2. Write to ADC0CTL to configure which input source (one
source can be active at a time)3. Set ADCCTL START0 to enable conversion.4. Watch STAT0 bit until conversion is complete. 5. Read value from ADC0D_H, ADC0D_L
Example Example
Analog_Photodiode
Photodiode
Like a diode (allows current to flow in one direction and not the reverse)
BUT, light changes that and allows current to flow in the reverse direction also.
Dark = little current flow More light = more current flow
5v_| Photo| diode LED 100+---|<----+--->|--------/\/\/\----+
| || v GND+--> ANA0
LED for visualindicator
Photodiodeis blocked, LED not on
Photodiodeis blocked, LED not on
Photodiodeis illuminated, LED is on
Photodiodeis illuminated, LED is on
Why would we want an External Reference Voltage?
External Reference
This circuit gives us access to supply our own reference voltage (on J7) or
Use a MAX6160 voltage reference (and potentiometer) to provide a variable VREF
VREF
Internal VREF=3.3v 3FF 10 bits 3.3v 0FF 8 bits 0.82v 00F 4 bits 0.05v 001 1 bit
0.003v
With external VREF=1v 3FF 10 bits 1.0v 0FF 8 bits 0.25v 00F 4 bits 0.0156v 001 1 bit 0.001v
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