The right "}" is considered the "return" command - in fact "return " passed after "state = !state " is ignored and won't produce an error. So in this case of attachInterrupt(0, toggle, FALLING) zero corresponds to interrupt 0 on DP2, toggle() is the ISR routine at the bottom of the program, and RISING means when an electrical signal goes from 0V to 5V the toggle() ISR performs its function - what started as state = LOW is now state = HIGH and vise-versa. This is done internally by the compiler and hidden from the user. In setup() we come to the function attachInterrupt(interrupt, function, mode) where "interrupt" is the interrupt number 0 to 5 "function" is known as the interrupt service routine or ISR a function address pointed to by the interrupt vector location "mode" configures the hardware electrical characteristics for an interrupt. The variable is set to 0 - in reality we are using only one of the eight bits, bit 0. If being used as an interrupt routine variable be it byte, float, etc. This differs from the usual byte, integer, or float in how the system and compiler use it. The second line defines the variable "state" as both a byte (8-bit variable) and "volatile". So digitalWrite(LED, HIGH) is the same as digitalWrite(9, HIGH) which is the same as digitalWrite(LED, 1), etc. The label "LED" to the compiler is defined as the number 9. Our First ProgramĪbove is our first sample program. Same is true of FALSE and TRUE in comparison operations. A "0" or "1" to the compiler is understood as LOW or HIGH respectively. When closed the pin goes HIGH or electrical 5V. When SW0 or SW1 is open a LOW or 0V is on the corresponding Arduino pin. In all of the programming examples SW0 control an LED on DP9 and SW1 controls an LED on DP10. (INTR1) The switches are "debounced" to eliminate electrical contact noise that cause erratic operation. SW0 through the "debounce" circuit formed by a SN7414 and a 1uF cap controls DP2 (INTR0) while SW1 corresponds to DP3. Here we are interested in the INTR0 and INTR1.įigure 1 shows the rest setup we will be using. The Arduino Mega has an additional four INTR2-INTR5. Most Arduino boards have two external interrupts: INTR0 (on DP2) and INTR1 (DP3). There are four available functions for controlling interrupts with Arduino: attachInterrupt(), detachInterrupt(), interrupts(), and noInterrupts(). In the case of Arduino this is set by both the boot loader and the compiled program - access by the user is limited unless one wants to get into assembly programming. This is programmed by setting or clearing bits in internal registers. Typical microcontrollers such as Arduino or PIC have a number of interrupt sources most of them tied into internal hardware modules such as timers and comparators, while some are tied into external hardware pins. The advantage of hardware interrupts is the CPU doesn't waste most of its time "polling" or constantly checking the status of an IO pin. The CPU will "pop" the original memory address from the "stack", load it into program counter, and continue execution of original program from where the interrupt occurred. The CPU loads that memory address and executes the function or subroutine that should end with a "return" instruction.Ħ. The program jump to an specific memory location (interrupt vector) which is an indirect pointer to a subroutine or function.ĥ. The CPU will "push" the program counter onto a "stack" or memory storage location for later retrieval.Ĥ. Finish execution of current instruction.ģ. This is an electrical signal change on a microcontroller pin that causes the CPU to do the following largely in this order:ġ. Here we will explore hardware interrupts on the Arduino microcontroller.
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