{"id":10012,"date":"2015-12-05T22:23:03","date_gmt":"2015-12-06T03:23:03","guid":{"rendered":"http:\/\/embedded-lab.com\/blog\/?p=10012"},"modified":"2016-12-18T19:43:18","modified_gmt":"2016-12-19T00:43:18","slug":"xmega-external-interrupt","status":"publish","type":"post","link":"https:\/\/embedded-lab.com\/blog\/xmega-external-interrupt\/","title":{"rendered":"XMega External Interrupt"},"content":{"rendered":"

External interrupts are a must have feature in any microcontroller. Interrupts solve a lot of problem that would have otherwise been dependent on polling methods. For instance when we press the volume up key of a TV tuner\u2019s remote controller, the remote controller quickly responds by transmitting the volume up command to the TV set and in turn the TV\u2019s volume increases. This fast response is due to external interrupt issued by the remote\u2019s button to the microcontroller it is connected to. If, however, all the keys of the remote were regularly and frequently scanned and then responded up on a press, the process would have been both slow and energy consuming because its microcontroller would then have never went to sleep or low power states and continuously kept scanning. In other words, the micro would have always ran despite no mandatory necessity and during standby conditions. This would have quickly drained the batteries. Since interrupt is typically used in such cases the remote controller will respond to a button press fast, wake up from sleep\/idle\/low power mode, transmit command data and then go back to sleep\/idle\/low power state. Thus the overall energy consumption is reduced while achieving fastest possible reaction. This is how real world applications work applying external interrupts.<\/p>\n

\"XMega<\/a><\/p>\n

 <\/p>\n

<\/p>\n

XMega devices just like any micro are equipped with interrupts for both external and internal peripherals but it is more than you can even guess. Any I\/O pin or group of I\/O pins can be used for external interrupt. Now that\u2019s a cool feature and a great relief as well because most traditional 8-bit micros like the ATMega328P, which is by the way the most widely used by the Arduino community, has only two external interrupts and fixed to one I\/O port only. Other larger Mega AVRs have up to eight external interrupts. However XMega\u2019s interrupt system is much more complex than those traditional AVRs. XMega devices support both maskable and non-maskable interrupts (NMI). Currently, however, there is only one NMI and it occurs due to crystal oscillator failure event. NMI is independent of global interrupt enable status. Unlike most traditional micros, interrupt priority can be set apart from edge selection and other stuffs. Priority setting has the effect of determining which interrupt can or will further interrupt another interrupt and so forth.<\/p>\n

\"Interrupt<\/a><\/p>\n

 <\/p>\n

The PMIC and Interrupt Priority<\/h4>\n

The Programmable Multilevel Interrupt Controller (PMIC)<\/i><\/b> is responsible for handling and prioritizing interrupt requests. As with any microcontroller the global interrupt flag (SREG_I_bit<\/i><\/b>) should be set before using the interrupt system. When a given interrupt condition is enabled and present, the PMIC receives interrupt request and it either acknowledges it or keeps it waiting based on interrupt priority and ongoing interrupts (if any). After executing an interrupt request, the PMIC returns to the proper interrupt level or to a state before the occurrence of the interrupt.<\/p>\n

 <\/p>\n

XMega reference manual states that there are three possible interrupt priority levels:<\/p>\n

\"Interrupt<\/a><\/p>\n

The PMIC prioritizes interrupts according to the following order:\"Interrupt<\/a><\/p>\n

This shows that a higher order interrupt will interrupt a lower order interrupt if both are occurring at the same instance. After processing the higher order interrupt the lower order interrupt is processed. Within a given order, interrupts with lower interrupt vector addresses have higher priorities than those with higher addresses. Typically reset has the highest priority and the lowest interrupt vector address. The same is true for XMega micros too. NMI is the next highest priority interrupt request.<\/p>\n

\"ISR<\/a><\/p>\n

 <\/p>\n

In the XMega, there are two types of interrupt and these are static and dynamic. Static interrupts are those which have fixed vector addresses while low priority interrupts are dynamic and this dynamic behaviour is achieved by Round-robin Scheduling (RRS)<\/i><\/b> technique. The RRS ensures that the low level interrupts are not skipped. If this technique is applied then the last invoked interrupt will have the lowest priority for one or more interrupts. Thus within the low level interrupt there is a mechanism that makes sure that all of them are served within a certain time frame.<\/p>\n

\"Round
\n<\/a><\/p>\n

<\/h4>\n

Interrupt Service Routine in XMega Devices<\/h4>\n

All maskable interrupt peripherals have interrupt flags and settings to set interrupt priority as well as enabling them. Irrespective of its settings an interrupt flag will be set when it occurs. The interrupt flags in most cases are automatically cleared up on executing them or by setting them in software.<\/p>\n

Some key points regarding interrupt handling:
\n<\/i><\/b><\/p>\n