Timer Value IRQ IACK

Transcription

1 Real Time Clocks & s Programming with Real-time clocks Real-time clock is just another source of interrupts. Should have high priority in real-time systems Timing jitter must be accommodated or tolerated Software related use of s Single event scheduled for future time e.g. timeout for communications Regular scheduling of events e.g. time-slice multi-tasking 75

2 Devices Single shot operation Provides single interrupt after specied timeout Value 0 t IRQ IACK counter is loaded with the desired value and then set running. an interrupt occurs when the counter reaches zero. 76

3 Devices Continuous operation Provides regular interrupts at specied intervals Value 0 t IRQ IACK each time zero is reached, the counter is re-loaded with the interval value and countdown continues. 77

4 Devices System Clock May be consulted at any time, giving an absolute value for system time. Max Value 0 t IRQ IACK pre-load value is set to maximum. interrupt routine is responsible for keeping count of number of full sequences. 78

5 68230 Peripheral Interface / 24 bit counter provides extra timing information in single shot mode Enable $FFFFFF Value PRE-LOAD $ t IRQ IACK in fact this is really another continuous mode with a very long second cycle. 79

6 Sharing a Countdown While running as a system clock the timer may be accessed by many routines. sharing for single-shot or periodic interrupts is somewhat harder. Tick based timer queues Value 0 t IRQ the timer triggers an interrupt at regular intervals pending timer events are stored in a queue on each tick the interrupt routine checks to see if another timer event is due tick based queues offer limited resolution 80

7 Queues For accurate timing we must use a difference based timer queue: ptr to timeout queue funct ptr 1 funct ptr 2 funct ptr 3 timeout 1 timeout 2 timeout 3 absolute time of first event each queue entry contains a pointer to an event service routine timeout values are the time differences between adjacent events we need only know the absolute time for the rst event 81

8 Queues Value 0 t timeout 1 tim.2 timeout 3 IRQ IACK each time the counter reaches zero timeout n +1is automatically loaded into the counter interrupt routine loads timeout n +2into the pre-load registers interrupt routine adjusts absolute time register interrupt routine starts function n 82

9 Queues Queue manipulation Adding an event nd place in queue calculate difference timeout re-calculate difference timeout for following event Adding an event behind the rst queue element here we must modify the pre-load registers during counting Adding an event at the front of the queue we must re-start the timer with a new counter value 83

10 Queues Queue manipulation Periodic events we need an extra eld to indicate periodic events ptr to timeout queue funct ptr 1 funct ptr 2 funct ptr 3 timeout 1 timeout 2 timeout 3 S P period 2 S absolute time of first event periodic events must be added back into the queue when serviced 84

11 Hardware &s Timing accuracy is limited by unpredictable and variable response times timing jitter. For many hardware related problems, a greater accuracy is required. We can achieve this by connecting the hardware directly to the timer. input from hardware start/stop timer external clock input counts pulses enable timer output to hardware event strobes square wave generation 85

12 Hardware-Attached Timing Digital lter illustrating hardware attached timing S/H ADC DAC Analogue L.P.F. R.T. Processor INT Analogue L.P.F. provides separate regular strobe signals for different devices S/H ADC INT DAC 86

13 Hardware-Attached Timing Sharing a timer for hardware-attached applications: IRQ to controlled hardware devices 3 EN 8 line decoder Zero Detect PULSE OUT Pre-load Registers Counter Registers Device Select Register The PI/T provides a single congurable timer input line and a single timer output line 1. a device select register indicates which device receives next strobe the decoder activates the required strobe line when enabled 1 this doubles as the interrupt request 87

14 Hardware-Attached s the device select eld must be added to our timer queue information ptr to timeout queue absolute time of first event funct ptr 1 funct ptr 2 funct ptr 3 device sel 1 device sel 2 device sel 3 timeout 1 timeout 2 timeout 3 S P period 2 S the device select register is updated by the master service routine after the interrupt is cleared 88