Interrupts and timers
הוצג| English | משמעות |
|---|---|
| interrupt/ˈɪntərʌpt/ | הפרעה |
| interrupt service routine/ˈɪntərʌpt ˈsɜːvɪs ruːˈtiːn/ | תוכנית שירות הפרעות |
| timer | טיימר |
| prescaler | מחלקת תדרים |
A week or a year
- The sensor node must run for a year on one battery. If the CPU polls all day, the battery is empty in about a week.
- Interrupts let the CPU sleep until something needs it. This lesson shows how they work and what they cost.
The interrupt
- An interrupt 中断 stops the main program where it is. The hardware saves the return state and jumps to an interrupt service routine 中断服务程序 (ISR).
- The NVIC (nested vectored interrupt controller) picks the ISR and ranks priorities. The ISR runs, clears its flag and returns.
Put the handling of one interrupt in order.
רמת קושי 3/5 · 200 מטבעות + בונוס קטן בהשלמה הראשונה היום
Save, jump, serve, clear, return. Forget the clear and the last step never comes.
The timer
- A timer 定时器 is a counter clocked through a divider, the prescaler 分频器. It raises an interrupt each time it reaches its reload value.
- Two divisions turn a fast clock into a slow, exact tick.
The timer equation
- The interrupt rate is $f_{\text{int}} = \dfrac{f_{\text{clock}}}{\text{prescaler} \times \text{reload}}$.
- Both factors live in 16-bit registers, so each must be at most 65 535. A design is valid only when both fit and the product equals the clock divided by the rate.
A timer runs from 84 MHz with prescaler 84 and reload 1000. What is the interrupt rate in Hz?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
$84 \times 10^{6} / (84 \times 1000) = 1000$ Hz: a 1 ms tick, with both factors inside 16 bits.
Worked example: a 1 Hz heartbeat
- From 84 MHz, prescaler 8400 gives 10 kHz. Reload 10 000 then gives exactly 1 Hz.
- Check: 8400 and 10 000 both fit 16 bits, and $8400 \times 10\,000 = 84 \times 10^{6}$. A prescaler of 84 000 would not fit.
From an 84 MHz clock, prescaler 4200 and reload 20 000 give an interrupt at
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
$4200 \times 20\,000 = 84 \times 10^{6}$, the clock, so exactly 1 Hz; and 20 000 ≤ 65 535 fits a 16-bit reload.
Prescaler 1 and reload 84 000 is a valid 1 kHz design on 16-bit timer registers.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
The product is right, but 84 000 is more than 65 535, so the reload does not fit. Prescaler 84 with reload 1000 works.
Worked example: interrupt or poll?
- An ISR with 12 cycles of entry, 30 instructions and 10 cycles of return costs 52 cycles, or 0.62 µs at 84 MHz.
- That cost is paid on every event: 0.06% of the CPU at 1 kHz of events, but 62% at 1 MHz. At very high rates, polling wins.
A 52-cycle ISR at 84 MHz, serving 1 MHz of events, uses about
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
$52 \times 10^{6}$ cycles a second out of $84 \times 10^{6}$ is 62%. The same ISR at 1 kHz uses only 0.06%: the event rate decides.
The same 52-cycle ISR serves 10 kHz of events at 84 MHz. What percentage of the CPU does it use? Give 1 decimal place.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
$52 \times 10\,000 = 520\,000$ cycles a second; $520\,000 / 84 \times 10^{6} = 0.62\%$.
Three common slips
- Forgetting to clear the flag. The CPU returns and enters the ISR again, for ever.
- Long ISRs. Do the minimum, set a flag, and let the main loop do the work.
- Reading a 16-bit timer in two halves. A carry between the reads corrupts the value; read the high byte twice or use a latch.
An ISR that forgets to clear its interrupt flag causes
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
The flag still shows pending when the ISR returns, so the CPU jumps straight back in. Clear the flag and keep the ISR short.
Recap
- Interrupt: vector, short ISR, clear the flag.
- Timer: $f_{\text{clock}}$ divided by prescaler × reload, with each factor inside its register.
- Interrupts win at low event rates; polling wins at very high ones.