ECE 425 Timer
Timer Functions • Two main modes: • Input capture • Used for timing external events • Match (Output compare) • Allows use as a simple function generator • Runs off PCLK or a scaled down version. • PCLK is CCLK * 1, ½ or ¼ • 32-bit prescale register allows long periods
Timer Overview • Two independent timers • 32 bit counters • 4 channels per counter • Can generate interrupts on timeout/match • Programmable for edge sensitivity • Rising edge, falling edge, any edge
Timer in a Nutshell • Set up time base. • Derivative of system clock • Input capture: See how many timer cycles elapse between events or just count the number of events. • Match (output compare): every N cycles toggle, set or clear an output. • Can change N every time to make eccentric waveforms of arbitrary complexity.
Timer Registers • 17 per timer • Control registers: enables, set frequency, set mode • Check & clear interrupts • Capture values for timing measurements • Counters: prescale & time base • Lots of choices, lots of complexity
Timer Control Register • Only two bits used. • Bit 0: time base and prescale counters enable. Set to 1 to enable. Off (Logic 0) is default. • Both are enabled/disabled with this bit. No individual control. • Bit 1: reset. Set to 1 to synchronously clear both counters. • Addresses: • Timer 0: 0xE000_4004 • Timer 1: 0xE000_8004
Count Control Register • Selects Timer or Counter mode • Timer mode: increment time base counter every PCLK or scaled PCLK rising edge. • Counter mode: timer counter is incremented on the specified edge of a Capture input pin. • When in counter mode, selects which input pin will trigger the action. • Addresses: • Timer 0: 0xE000_4070 • Timer 1: 0xE000_8070
Counter Control Register Input edge? That’s next.
Counter Control Registers CCR bits 7:4 are reserved and currently unused. Should always be left at 0.
CAP Triggers and Their Pins • CAP0.0 (3 pins) : P0.2, P0.22 and P0.30 • CAP0.1 (1 pin) : P0.4 • CAP0.2 (3 pin) : P0.6, P0.16 and P0.28 • CAP0.3 (1 pin) : P0.29 • CAP1.0 (1 pin) : P0.10 • CAP1.1 (1 pin) : P0.11 • CAP1.2 (2 pins) : P0.17 and P0.19 • CAP1.3 (2 pins) : P0.18 and P0.21
CAP0-3 • Each Timer has four possible input sources, CAP0-3. • Each of the sources is associated with one or more pins. • Pins have multiple duties. Being able to select one of several pins for Capture triggers will simplify board/system design. • Selection is done via Pin Connect Block. • When more than one pin is selected for a Capture input on a single channel, the pin with the lowest Port number wins. • Example: if pins 30 (P0.6) and 46 (P0.16) are selected for CAP0.2, only pin 30 will be used by TIMER0 to perform CAP0.2 function.
Timer Counter • The heart of the timer system: a 32-bit counter. • Incremented according to the value set in the prescale register. Increments every (Prescale + 1) PCLK cycles. • It that is zero, increments every PCLK rising edge • Rolls over at terminal count (0xFFFF_FFFF). • Roll over does not generate an interrupt or have any particular significance.
Timer Counter • Suppose you want to know how many times the counter has rolled over? • Necessary for measuring events with long periods • Just set one of the match registers to generate an interrupt every time it does go to 0. • Timer Counter Addresses: • Timer 0: 0xE000_4008 • Timer 1: 0xE000_8008
Prescale Register • Set the maximum value for the Prescale Counter. • 32-bit register, range is 0 – 0xFFFF_FFFF • When set to 0, Timer will run off PCLK with no scaling. • Anything else, timer will be scaled. • Addresses: • Timer 0: 0xE000_400C • Timer 1: 0xE000_800C
Prescale Counter • 32-bit counter. • Runs off PCLK. • Increments every PCLK cycle up to the point where it matches what was set in the Prescale Register. • When it hits, it returns to 0 on the next cycle • Addresses: • Timer 0: 0xE000_4010 • Timer 1: 0xE000_8010
Comparators Bit-by-bit XOR followed by NOR reduction. Output is 1 iff all bits are equal. A0 B0 A1 B1 A2 B2 A3 B3
Match Registers • 4 32-bit match registers per timer. • When one matches counter value, something can be triggered. • Options are • Generate an interrupt • Reset the timer • Stop the timer • Some combination of the above
Match Register Addresses Timer 0 Timer 1 MR0: 0xE000_8018 MR1: 0xE000_801C MR2: 0xE000_8020 MR3: 0xE000_8024 • MR0: 0xE000_4018 • MR1: 0xE000_401C • MR2: 0xE000_4020 • MR3: 0xE000_4024 Procedure: Write a value to one of the registers. Enable one or more of the match functions via the match control registers. When the timer matches a match register, the enabled function will trigger.
Match Control Registers • Three bits per Match register • Enable/disable • Interrupt on match • Stop on match • Return to zero on match • Addresses: • Timer 0: 0xE000_4014 • Timer 1: 0xE000_8014
Match Control Bits For all functions, Logic 1 is enabled, Logic 0 is disabled. Only bits 11:0 are defined. All the rest are reserved and should always be left at 0.
Capture Registers • Captures timer value when the specified event occurs. • Event is programmed into Capture Control Registers • Events are edges on specified pins. • Elapsed time is calculated in program by subtracting 1st from 2nd. • Implied: must store first somewhere else. • Must check for rollover to get valid difference.
Capture Register Addresses Timer 0 Timer 1 CAP0: 0xE000_802C CAP1: 0xE000_8030 CAP2: 0xE000_8034 CAP3: 0xE000_8038 • CAP0: 0xE000_402C • CAP1: 0xE000_4030 • CAP2: 0xE000_4034 • CAP3: 0xE000_4038 Procedure: set one or more control registers to trigger on a specified edge. When the event occurs, store the timer value in a CAP register. Save the captured value in memory or another register. Wait for the next event. When that happens, subtract (adjusting for rollover, if it occurred) the second from the first, yielding the elapsed time in PCLK cycles.
Capture Control Registers • Three bits per Capture Register • Options are: • Capture on rising edge • Capture on falling edge • Enable interrupt • Any combination of the above • Addresses: • Timer 0: 0xE000_4028 • Timer 1: 0xE000_8028
Capture Control Bits For all bits, logic 1 means enabled and logic 0, disabled. Only bits 11:0 are defined. All the rest are reserved and should always be left at 0.
External Match • We’ve already seen how match works. • But that is internal to the silicon. • To use it as a signal generator, need to get those transitions to the outside world. • That’s done through External Match. • Four per timer, some can be connected to more than one pin.
External Match Functions • Each match (4 per timer) can cause an associated output pin to: • Do nothing • Go to logic 0 (no effect if already 0) • Go to logic 1 (no effect if already 1) • Toggle • Pins must previously have been set to output (all default to input) and associated with match via Pin Control Block.
External Match Register • Four bits per timer reflecting match output as set in control bits. Will show pin output whether or not pins have been associated with the match. • It’s a R/W register but these four bits should be considered read only • Eight bits setting action • Two bits per channel • Ignore, set, clear, toggle • These eight bits are for you to set.
External Match Register Only 12 bits are defined. The rest should always be left at zeros. Addresses: Timer 0: 0xE000_4014, Timer 1: 0xE000_8014
Interrupt Register • Four bits for match interrupts • Four bits for capture interrupts • After one is set, it can be cleared by writing a one to the same bit. • Toggle logic • Writing zeros has no effect • Addresses: • Timer 0: 0xE000_4000 • Timer 1: 0xE000_8000
Timer Interrupt • VIC only has one interrupt per timer. • Timer 0: bit 4 of VICSoftInt register, 0xFFFF_F018 • Timer 1: bit 5 of same register • If more than one timer interrupt is possible, the timer ISR must read the timer interrupt register to determine which one fired. • Clear interrupt in VIC (toggle logic again) as well as timer interrupt register. • ISR must do both to re-enable interrupt.
That’s All • 17 x 2 registers. • Need to initialize them to use timer subsystem. • Defaults are all off.
Timer Applications • What’s the timer good for? • Timing things. • Like a race. • How about a marathon?
Marathon Timer Requirements • Accuracy: say 1/100 of a second. • Time: track checkpoints every ¼ mile. • Make it long enough for the 400 lb. Sumo wrestler. • 10 hours or so without aliasing.
Where Do We Start? • It will be an ARM core, same peripheral units as we’ve been discussing. • What functions will we need? • Is it a Match function? • How about input capture? • What registers need initialization?
Processor Clock • Could choose anything, but let’s stick with the lab machines: 60 MHz internal clock. • How many clock ticks does 1/100 of a second take? • 600,000, which is 927C0h • And 10 hours? • 36,000 seconds, 6x107 cycles per second 2.16x1012
Clock Choice • Only need to measure to 1/100 of a second. 600,000 cycles per measurement seems excessive. • 2x1012 is a pretty big number even for a 32 bit machine. • FFFF_FFFF is about 4.3 x 10 9, so there would be lots of roll overs. • Nothing wrong with that, we can handle it, but it’s an unnecessary complication. • Maybe we can do something to make the timer better match the task.
Clock Scaling • We can scale the PCLK by up to 4, then scale the timer clock by up to 232. • What would be a good number? • Only need two clock ticks per 1/100 of a second. • Get max. power savings by dividing CCLK by 4. • As it happens, that’s the default: 00 to VPBDIV register • Then divide 15 MHz PCLK by something to count at around 200 Hz. • 75,000: something like that can be stored in the timer’s Prescale Register.
Timer Setup • Counter Control Register Mode (bits 1:0) • Default is timer mode, no need to do anything • Counter Control Register source (bits 3:2) • May as well use the default here too, if we are only going to time one runner. • How about the Match registers? What do we need to do with them? • Nothing. They are used for setting outputs and have nothing to do when timing external events.
More Things To Set Up • Capture control registers: need to set something there. All are off by default. • Assume we’ll just use one channel. • They’re all the same. Pick one arbitrarily. • Pick an edge. Rising or falling, matters not, but usually rising is used. • Both would be a bad idea. • Enable interrupt. Could theoretically make it work by polling, but this is what interrupts are for.
Timer Register Write Summary • Assumption: using Timer 0, Channel 0 • 00 VPBDIV (0xE01F_C100) • 75,000 Timer Prescale Register (0xE000_400C) • 0000 Timer Controller Register (0xE000_4070) • Timer mode, channel 0, increment on rising edges • 4 Capture Control Register (0xE000_4028) • Enable channel 0 interrupt
Timer ISR • After enabling the timer, all the action will be in the ISR. • What does it need to do? • Homework X: write a task list and then organize it into a flow chart for the marathon timer ISR. Include any necessary initialization, which may need to be done before the interrupt is enabled.
Output Compare Application • Two tone siren. • Set fast (1200 Hz) wave on an output pin. • Wait half a second. • Set slow (300 Hz) wave on the pin. • Wait half a second. • Repeat.
Values for 1200 Hz • Want a 1200 Hz signal square wave for the siren. • Suppose PCLK is 15 MHz. • 15,000,000/1200/2= 6250, easily fits into a 32 bit register. No need to scale PCLK. • Read Timer Counter, add 6250 & store, wait for match. • Alternate program: reset counter, only compare to 6250. • Advantage: no repeated adds in ISR • Disadvantage: no freerunning counter, can’t be used for other functions simultaneously • Upon match, toggle output and repeat.