Embedded Systems: Guide to Timers, Counters, and Serial Transmission, Slides of Computer Science

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4-Integrating Peripherals in

Embedded Systems

Introduction

• Single-purpose processors

• Performs specific computation task
• Custom single-purpose processors
  • Designed by us for a unique task
• Standard single-purpose processors
  • “Off-the-shelf” -- pre-designed for a common task
  • a.k.a., peripherals
  • serial transmission
  • analog/digital conversions

Counters

• Counter: like a timer, but

counts pulses on a general

input signal rather than

clock

• e.g., count cars passing over a sensor
• Can often configure device as either a timer

or counter

16-bit up counter

Clk 16

Cnt_in

2x mux

Mode

Timer/counter

Top

Reset

Cnt

Other timer structures

• Interval timer
  • Indicates when desired time interval

has passed

• We set terminal count to desired

interval

• Number of clock cycles = Desired time interval / Clock period
• Cascaded counters
• Prescaler
• Divides clock
• Increases range, decreases

resolution

Top

Time with prescaler

16-bit up counter

Clk Prescaler

Mode

16-bit up counter

Clk 16

Terminal count

Top

Reset

Timer with a terminal count

Cnt

16-bit up counter

Clk

16-bit up counter

Cnt

Top

16/32-bit timer

Cnt

Watchdog timer

• Must reset timer every X

time unit, else timer

generates a signal

• Common use: detect failure,

self-reset

• Another use: timeouts
  • e.g., ATM machine
  • Scalereg: 11-bit timer, 1 μsec resolution.
  • Scalereg Top signal toggles every 2^11 *1μsec= 2. msec
  • Hence, Timereg: 16-bit timer, 2 msec. resolution
  • timereg value = maxCnt-time needed/resoloution
  • X = 65535-2min/2msec
  • X=65535-2*60/0.002 X=

scalereg

checkreg

timereg to system reset or interrupt

osc clk prescaler

Top overflow

12 MHz 1MHz^ 11 bit Up X counter

16 bit Up counter 0 disable 1 enable

/* main.c */

main(){ wait until card inserted call watchdog_reset_routine

while(transaction in progress){ if(button pressed){ perform corresponding action call watchdog_reset_routine }

/* if watchdog_reset_routine not called every < 2 minutes, interrupt_service_routine is called */ }

watchdog_reset_routine(){ /* checkreg is set so we can load value into timereg. Zero is loaded into scalereg and 11070 is loaded into timereg */

checkreg = 1 scalereg = 0 timereg = 5535 }

void interrupt_service_routine(){ eject card reset screen }

Serial Transmission Using UARTs

• UART: Universal Asynchronous

Receiver Transmitter

• Takes parallel data and transmits serially
• Receives serial data and converts to parallel
• Parity: extra bit for simple error

checking

• Start bit, stop bit
• Baud rate
  • signal changes per second
  • bit rate usually higher

embedded 1 device 0 0 1

Sending UART

Receiving UART

start bit data

end bit

1 0 0 1 1 0 1 1

Serial Transmission Using UARTs

• Clocks on sender and receiver are

never exactly in sync

• Requires synchronization of receiver
• High-low transition signals frame boundary

UART Tx Signals

• Example for two transmissions

Example UART /MAX 232 interface

MAX

• The MAX232 is a dual

driver/receiver that includes

a capacitive voltage

generator to supply

standard voltage levels for

TIA/EIA-232-F interface.

• Uses a single 5-V supply.

• Each receiver converts

TIA/EIA-232-F inputs to 5-V

TTL/CMOS levels.

• These receivers have a 14

MAX232: Receiver

MAX232: Driver