Standard Single Purpose Processors: Peripherals Flashcards
What can a Timer on a Microprocessor be used for?
Mesure time intervals, to generate timed output or to mesure input events.
Hoe do you calculate the CLK resolution?
Clk resolution = 1/Clk freq
What is the Clk range?
Clk range = time until overflow
What is TOP?
-indicates top count reached, wrap-around
- is connect to pin of processor or directly mapped to
interrupt flag
Draw a Basic timer!
See Chapter 5 Slide 4
Draw a basic Timer/Counter, or explain the difference to a basic timer
See Chapter 5 Slide 5
Counter: like a timer, but counts
pulses on a general input signal
rather than clock
Clk/Cnt Mode, TCNT, Reset, Top
What is a Timer with Terminal Count
▪ Timer indicates when desired time interval has passed (up- or down-counter) ▪ Up-counter: Set terminal count to desired interval but setting the value to which to count to
Explain a Cascaded counter
Multiple counters connected in a row
2x 16 bit Can be configured for 32 bits
Frist counter reaches top, 2. counter is incremented by 1
What is a Prescaler, what is the purpose?
• Divides clock • Prescaler output signal has reduced frequency (1/2, 1/4, …, determined by mode) • Increases range, decreases resolution
What is a Real-Time Clock (RTC)? What is the purpose?
▪ RTC: Clock keeping track of current time in human units
RTCs often
• have an alternate source of power (lithium battery, supercapacitor)
• use crystal oscillator with frequency 32768 Hz, i.e. resolution 30.52 µs
(overflow of 15 Bit counter after 1 second)
Purpose:
• Low power consumption (important when running from batteries)
• Frees main system for time-critical tasks
Explain Conventional Counters!
Count number of input cycle trigger events per second (called gate time)
Gate time not synchronized with input signal
• uncertainty of measurement is ±1 input
• cycle count, i.e. resolution is 1 Hz with
a 1 s gate time for all input signal frequencies
–> Counts for a fixed time persiode the numbers of signals
How do you Increase resolution for a Conventional Counters!?
Increase resolution with longer gate-times
• Gate time of 10 s increases resolution tenfold, adds one digit to read-out
• Resolution increases with frequency
–>Conventional counters
are bad for low-medium
frequencies and adequate
for high frequencies only
Explain Reciprocal Counting!
Input signal trigger controls start of gate-time
▪ Two counting registers: one counts number of input cycles and other counts the clock pulses
–> counts X numbers of signals and mesures time with clock
What is the advantage of Reciprocal Counting?
Resolution is independent of frequency
The ±1 input cycle error is avoided. Truncation errors are now in the time count; i.e. ±1 clock pulse
What is a Watchdog Timer (WDT)
Computer hard- or software timer that
triggers a system reset or other corrective action
if main program hangs (due to fault condition)
▪ WDTs are last line of defense against product failure
• If all else fails then let watchdog reset system
What is “feed the dog”?
Must reset timer every x time units (feed the dog), else WDT generates interrupt which resets program or jumps to save part
Code to feed the dog must be
• small and amount of system resources used, especially CP U cycles, must
be reasonable
• carefully integrated into software
Most common usage of WDT?
Common usage is to detect:
• infinite loops
• deadlocks (if some lower priority tasks are not getting to run because higher priority tasks are hogging the CPU)
• general failures (e.g. in firmware) and timeouts
Difference between the terms UART and RS -232
UART is peripheral on microcontroller which can send and receive serial
data asynchronously
• RS-232 is a signaling standard which mandates the logic levels and
control signals
• While AVR’s normal logic levels are about 3-5V, RS-232 communication
uses a low of +3V to +25V for a digital ’0’, and -3V to -25V for a digital ’1
What are Standards for voltage signaling?
RS-232, RS-422 and RS-485
Name the three minmal required wires for RS - 232.
when are only these used?
- transmit data (TX)
- receive data (RX)
- ground (GND)
Used when full facilities of RS-232 are not required
When can a 2-wire connection be used? What are the wires?
2-wire connection (data and ground) can be used if data flow is one way (half duplex)
What is a Asynchronous interface?
Transmitter and receiver clocks are independent, and a resynchronization is performed for each byte at the start bit
Typical usage:
Connect a PC for debugging
What is a Synchronous interface?
Include separate clock signal line
• Simplifies transmitter & receiver, but is more susceptible to noise when used over long distances
• Examples:
Serial peripheral interface (SP I) and I²C
• Typical usage:
connect external EEPROM
Explain UART and name one advantage?
UART: Universal Asynchronous Receiver
Transmitter
• Takes parallel data and transmits serially
• Receives serial data and converts to parallel
Adv:
• More cost effective than parallel transmission
What dose UART usually consists of`?
a clock generator, usually a multiple of bit rate
to allow sampling in the middle of a bit period
• input & output shift registers
• transmit/receive control
• read/write interface
What is required for a UART TX and RX?
Transmitting & receiving UARTs must have same values for bit speed (baud rate), number of data bits (word length), parity, and stop bits for correct operation
Error types of USART?
Overrun error (DORn) • indicates that a new data character was received before previous character was read. Data loss occurred
Parity error (UPEn) • set when parity of an incoming data character does not match expected value
Frame error (FEn)
• indicates a missing stop bit. This error can only happen when
• communication settings of transmitter/receiver (e.g. baud-rate) don’t match
• when connection was lost, or
• transmitter is faulty
