Silicon Laboratories EFM32 Installation and operating instructions

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EFM32 Interrupt Handling

AN0039 - Application Note

Introduction

This application note is an introduction to interrupts and wake-up handling in the

EFM32. It includes ways to optimize for low latency wake-up, interrupt prioritization

and energy saving operation.

This application note includes:

• This PDF document

• Source files (zip)

• Example C code

• Multiple IDE projects

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1 Interrupt Theory

Interrupts are a commonly used technique in microcontrollers allowing CPU-external systems to indicate

need for change in CPU execution. Instead of using polling loops to monitor these kinds of events and

wasting valuable processing time, interrupts do not require any action from the CPU unless they are

triggered.

When an Interrupt Request (IRQ) is received, the CPU will store its current state before it switches to

the Interrupt Service Routine (ISR) (Figure 1.1 (p. 2)). In older architectures there was only one

ISR and SW needed to determine which source triggered the IRQ. In modern architectures like the ARM

Cortex-M in the EFM32, each IRQ has its own ISR. The starting address for each ISR is stored in an

interrupt vector table.

When an interrupt is triggered, the CPU automatically jumps to the corresponding address in the vector

table which contains an address to the relevant ISR. The service routine then executes the tasks needed

to handle the event before the CPU returns to where it left off before the interrupt was received. A

commonwaytoacknowledgetheinterruptistocleartheinterruptsourceintheinterrupthandlerresulting

in the IRQ being de-asserted.

Figure 1.1. Basic Interrupt Operation

Main thread TIMER0 ISR

TIMER0 IRQ

TIMER0 ISR Address

UART1 ISR Address

Interrupt

Vector Table

UART0 ISR Address

TIMER1 ISR Address

As more than one interrupt can be triggered at the same time, interrupt priorities can be assigned to

the different IRQs. This allows lower latency for the most important interrupts for real-time control etc.

In interrupt controllers that support nesting, it is also possible for a high priority interrupt handler to start

immediately even if another lower priority handler is executing. The CPU will then continue where it left

off in the lower priority handler once it is done servicing the higher priority interrupt. Figure 1.2 (p. 3)

shows an example where a higher priority interrupt (2) is serviced in the middle of the lower priority

interrupt handler (1). The lookup in the vector table is done every time an IRQ is triggered, but left out

in the figure for simplicity.

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Figure 1.2. Nested Interrupts

Main thread

ISR 1

ISR 2

Interrupt 1

Interrupt 2 with higher

priority than interrupt 1

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2 Interrupts in the EFM32

The Nested Vector Interrupt Controller (NVIC) in the ARM Cortex-M processor in the EFM32 evaluates

the IRQ lines and switches the CPU execution to the triggered IRQs address in the vector table.

Figure 2.1 (p. 4) shows an overview of how interrupts are handled in the EFM32. Most of the

peripherals in the EFM32 can generate interrupts and control one or more interrupt lines (IRQ) each.

Figure 2.1. Interrupt overview

Cortex- M3 NVIC

Peripheral

IEN[n]

IF[n]

set clear

IFS[n] IFC[n]

Interrupt

condition IRQs

SETENA[n]/ CLRENA[n]

CPU

interrupt

SETPEND[n]/ CLRPEND[n]

set clear

Active interrupt

Software generated interrupt

2.1 Peripheral IRQ Generation

As shown in Figure 2.1 (p. 4) each IRQ line can be triggered by one or more interrupt flags (IF).

Normally these interrupt flags will be set by a hardware condition (e.g. timer overflow), but SW can also

set and clear these directly by writing to the IFS (Interrupt Flag Set register) or IFC (Interrupt Flag Clear

the IRQ. To acknowledge an interrupt the Interrupt Flag corresponding to the event should be cleared

(through the IFC register) in the ISR. The OR function between the interrupt flags ensures that the IRQ

line is asserted as long as there are unmasked interrupt flags that have not been acknowledged. As an

example, the TIMER0 peripheral contains 8 interrupt flags:

Example 2.1. TIMER0 interrupt flags/conditions

• OF - Overflow

• UF - Underflow

• CC0 - Compare Match/Input Capture on Channel 0

• CC1 - Compare Match/Input Capture on Channel 1

• CC2 - Compare Match/Input Capture on Channel 2

• ICBOF0 - Input Capture Buffer Overflow on Channel 0

• ICBOF1 - Input Capture Buffer Overflow on Channel 1

• ICBOF2 - Input Capture Buffer Overflow on Channel 2

These interrupt flags can be read through TIMER0_IF. The TIMER0_IFC and TIMER0_IFS registers

are used to clear and set the interrupt flags, while TIMER0_IEN masks the flags not contributing to the

IRQ. More detailed information on how to generate IRQs in the EFM32 peripherals are given in the

reference manual for the device, as well as in practical examples in application notes targeted for the

specific peripheral.

2.2 The Nested Vector Interrupt Controller (NVIC)

The Nested Vector Interrupt Controller (NVIC) is an integrated part of the ARM Cortex-M processor,

supporting both Cortex-internal interrupts (Hard fault, SysTick etc.) and up to 240 peripheral interrupt

requests (IRQs). In the EFM32, IRQs are generated by peripherals such as TIMERs and GPIOs as a

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response to events internal to, or acting upon the MCU. The NVIC's handling of the IRQs is controlled

by memory mapped registers (System Control Space). More information on the NVIC can be found in

the EFM32 Cortex-M3 Reference Manual.

As shown in Figure 2.1 (p. 4), each IRQ will set a Pending bit when asserted. This pending bit will

generate an interrupt request to the CPU if the corresponding enable bit (SETENA[n] in Figure 2.1 (p.

4) ) is also set. Note that the pending bit will be automatically cleared by hardware when the

corresponding ISR is entered.

Table 2.1 (p. 6) shows the interrupt vector table for the EFM32TG devices. The vector table is

common for all devices within the same device series (e.g. EFM32TG), but will vary between device

series (e.g EFM32LG devices have a different table than EFM32G devices). The interrupts generated

internally in the Cortex-M have negative IRQ numbers, while the peripheral IRQs start at 0.

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Table 2.1. EFM32 Interrupts

IRQ # Source

- Reset

-14 Non-maskable interrupt (NMI)

-13 Hard fault

-12 Memory management fault

-11 Bus fault

-10 Usage fault

-5 SVCall

-2 PendSV

-1 SysTick

0 DMA

1 GPIO_EVEN

2 TIMER0

3 USART0_RX

4 USART0_TX

5 ACMP0/ACMP1

6 ADC0

7 DAC0

8 I2C0

9 GPIO_ODD

10 TIMER1

11 USART1_RX

12 USART1_TX

13 LESENSE

14 LEUART0

15 LETIMER0

16 PCNT0

17 RTC

18 CMU

19 VCMP

20 LCD

21 MSC

22 AES

2.2.1 Interrupt Priority

Each IRQ in the EFM32 has 3 bits in the Priority Level Registers (IPRn) controlling interrupt priority. A

low value means a high priority. These bits are used to configure two types of priority:

• Preempt priority

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• Sub priority

The preempt priority level determines whether an interrupt can be executed when the processor is

already running another ISR. The sub priority is only evaluated if two interrupts have the same preempt

priority and are pending at the same time. The interrupt with the higher sub priority will then be handled

first. If two pending interrupts have the same preempt and sub priority, the interrupt with the lower IRQ

number will be handled first. By default, the priority of all interrupts is 0 (highest) out of reset. There

is no need to write special wrapper code to handle nested interrupts. You only need to configure the

appropriate priority for each IRQ.

The PRIGROUP bits in the AIRC Register controls how many of the priority bits are used to encode the

preempt priority and how many are used for sub priority as shown in Figure 2.2 (p. 7). By default

PRIGROUP is set to 0 and all 3-bits are therefore used for preempt priority.

Figure 2.2. Definition of Priority Fields in Priority Level Register

Bit 6Bit 7

Preempt priority

Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

Not implemented

Preempt priority Sub

priority

Preempt

priority Sub priority

Sub priority

Not implemented

PRIGROUP

0- 4

2.2.2 Interrupt Sequencing

Before an ISR can be entered the CPU registers must pushed to the stack (stacking), which takes 12

clockcycles (when flashisconfiguredto 0 waitstates).Returningfrom the ISRalsotakes12 clock cycles

as the CPU state must be restored (unstacking). For ISRs following immediately after (tail-chaining), or

nested inside another ISR, the ARM Cortex-M improves latency by not stacking and unstacking fully

between the ISRs. This reduces the latency between the handlers to only 6 clock cycles as shown in

Figure 2.3 (p. 7) .

Figure 2.3. Latency when entering interrupt handlers

2.3 Sleep Operation

If there is no other work for the CPU to do while waiting for an interrupt to trigger, energy can be saved

by putting the CPU to sleep until the interrupt triggered. In the EFM32 there are two ways of going to

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sleep, the Wait For Interrupt (WFI) and the Wait For Event (WFE) instructions. After executing one of

these instructions the CPU stays in sleep mode until one of the following conditions occur:

• An enabled interrupt request is asserted

• A Debug Entry request occurs, if Debug is enabled.

The SLEEPDEEP bit in the System Control Register (SCR) controls whether the processor uses Sleep

or Deep Sleep/Stop Mode as its low power mode according to Table 2.2 (p. 8). Energy Mode 4/

Shut Off Mode is entered by register commands to the Energy Management Unit (EMU) and will require

a reset to wake-up thus not allowing regular interrupt operation. For more information on the functional

and power consumption differences between the Energy Modes, please refer to the reference manual

for the relevant EFM32 device.

Table 2.2. Entering Energy Modes with WFI/WFE

SLEEPDEEP Low Frequency Oscillator Resulting Energy Mode when

running WFI/WFE

0 NA Energy Mode 1/Sleep Mode

1 Running Energy Mode 2/Deep Sleep Mode

1 Stopped Energy Mode 3/Stopped Mode

2.3.1 ISR Handling With Wait For Interrupt (WFI)

The Wait For Interrupt instruction (__WFI();), causes immediate entry to EM1/2/3. When an enabled

IRQ is asserted, the CPU wakes up and executes the associated ISR. When the ISR is done, the main

thread continues at the point it was before entering sleep mode.

2.3.2 Low Latency Wake-up With Wait For Event (WFE)

The Wait For Event instruction (__WFE();), causes immediate entry to EM1/2/3. The CPU will wake up

once it receives an event. In the EFM32 an event can be generated by a pending interrupt if the Send

Event on Pending Interrupt bit (SEVONPEND)in the System Control Register is set. Note that if the

interrupt is also enabled, the pending interrupt will also cause the ISR to be executed after waking up.

However, if the interrupt is not enabled, the CPU will skip the ISR and continue execution immediately

from the WFE instruction. You will then be able to react to the interrupt event faster than if using the

ISR as the storing of the CPU state (12 clock cycles) for the ISR is skipped. This allows wake-up in

only 1 clock cycle from EM1 and only 2µs from EM2/3. Note though that the interrupt pending bit is only

cleared automatically when entering the ISR, so when waking from an event without the ISR, you must

first clear the interrupt flag in the peripheral and then clear the pending bit manually before entering a

sleep mode again.

Whenever an event occurs, the event status register bit will be set. This is the bit that the WFE command

is waiting for. It is important to note that this bit is also set when regular interrupts are executed. If this

bit has been set by interrupts executed earlier in the application, and the WFE instruction is executed,

the device will immediately wake up again as the event status bit is already set. This bit is not readable

directlyby software. To make surethatthisbitiscleared, it is thereforerecommendedtorunthefollowing

sequence before the WFE used to go to sleep:

Example 2.2. Clearing event status bit:

__SEV(); /* Set event register bit */

__WFE(); /* Clear the event status bit */

/* __WFE() can now be used to go to sleep */

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Note that when using other interrupts at the same time as using WFE, the event status bit must be

cleared every time an interrupt executes.

2.3.3 Continuous Sleep With SLEEPONEXIT

NormallywhentheISRisdone,CPU executionreturnstowhereit leftoff.However,ifthe SLEEPONEXIT

bit in the System Control Register (SCR) is set to 1 the device enters sleep (depth set by DEEPSLEEP

bit) directly when finishing the ISR without returning to main. This feature is useful for applications where

the program would otherwise enter sleep repeatedly between interrupts. With SLEEPONEXIT set, only

oneWFIinstructionwouldneedtoberunandexecutionwouldnotreturntomainuntiltheSLEEPONEXIT

bit is cleared in the ISR after a desirable condition has been met. Note that if the SLEEPONEXIT bit is

changed as the last instruction in the ISR, a Data Synchronization Barrier (DSB assembly instruction)

must be run to make sure that the new value is registered before the ISR is exited.

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3 CMSIS

The ARM Cortex Microcontroller Software Interface Standard (CMSIS) is a common software standard

for all Cortex-M devices across all vendors. This means that features, such as the NVIC, which are

present in all Cortex microcontrollers, are handled in the same way. The example below shows an ISR

for TIMER0 written according to the CMSIS standard. This function is called by the interrupt vector table,

which can be found in the startup file for the device (e.g. startup_efm32tg.s).

Example 3.1. A TIMER0 ISR:

void TIMER0_IRQHandler(void)

{

/* Clear the interrupt flag in the beginning */

TIMER_IntClear(TIMER0, TIMER_IF_OF);

/* More code */

}

As the NVIC is a part of the ARM Cortex-M, CMSIS functions are also provided to handle things

like enabling IRQs, (done with the NVIC_EnableIRQ-function in the example below) and configuring

priorities.Theperipherals (like TIMER0) are not an integratedpartoftheCortexandcanvaryfrom device

to device. CMSIS does not dictate a set of functions for these, although a common naming standard is

specified. Energy Micro does however provide a full function library for all peripherals called emlib.

Clearing of interrupt flags (TIMER_IntClear-function above) and handling of interrupt enable bits

(TIMER_IntEnable-function below) are all handled by the emlib functions (functions for TIMER0 are

found in efm32_timer.h). Please note that the emlib and CMSIS functions themselves are shared

between all EFM32 devices and individual differences between the sub-series are handled internally

in the functions.

Example 3.2. Enabling TIMER0 Overflow Interrupt:

/* Enable TIMER0 interrupt in NVIC */

NVIC_EnableIRQ(TIMER0_IRQn);

/* Enable TIMER0 IRQ on Overflow */

TIMER_IntEnable(TIMER0, TIMER_IF_OF);

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