There are many ways of generating delays in AVR or any other microcontroler. One way is to use software counter like delays withing loops. This is not recommended way and should be avoided because this utilizes processor and is hardly predictable due to compiler optimizations.
But to get high precision you definitely need to use timer counter. This way you are free from compiler optimizations. Single timer can deal with rather short delays. If you need longer delays generated by hardware only, why not to put another timer to work. ATMEL app note suggests using two timers for generating long delays. The method is pretty obvious from illustration – first timer generates timer1 compare signal on dedicated pin (OC1A), This signal then ticks second timer0. Now imagine if first timer can generate maximum 10s. If timer0 is 8-bit timer, then whole configuration can give 2560s. The only drawback of this method is that first timer output has to be connected to second input. In existing boards this may be tricky, but why not think of this on new design where application is dependent on long delays.
Volatile is probably least documented keyword in most tutorials and books. Probably this is main cause of most misuses and bugs related to it. If you already are programming microcontrollers, you probably know that volatile is always used on global variables that are accessed from interrupt service routines. Otherwise code won’t work.
After few requests I decided to drop few lines about volatile keyword. This keyword is commonly used to tag memory type. We hear “volatile memory”, “non-volatile memory” when talking about computer hardware. As quick reminder – “non-volatile memory” is type of memory that stores its contents even when power is off. Such type of memory is EEPROM, Flash, FRAM. This is easy from hardware perspective. But what volatile keyword means in C or C++ code? This is an indicator (called qualifier) to compiler that tells that this variable may be changed during program flow even if it doesn’t look like to be. This means that compiler must treat this value seriously and keep optimizer away from it. Continue reading
GPS modem is a device which receives signals from satellite and provides information about latitude, longitude, altitude, time etc. The GPS navigator is more famous in mobiles to track the road maps. The GPS modem has an antenna which receives the satellite signals and transfers them to the modem. The modem in turn converts the data into useful information and sends the output in serial RS232 logic level format. The information about latitude, longitude etc. is sent continuously and accompanied by an identifier string.
The connection of GPS modem with AVR microcontrollers shown in the circuit diagram. The ground pin of max 232 and serial o/p of GPS modem is made common. Pin2 of MAX232 is connected to pin 3 of GPS modem and pin 3 of max 232 is connected to pin 2 of modem. This type of connection is called a serial cross cable. Continue reading
Connecting a button as an input to a micro-controller is a relatively easy task, but there are some problems. The main problem is that buttons bounce, i.e. when you press (or release) a button it will often change level a couple of times before it settles at the new level. So if you, for example, connect the button to a pin with an external interrupt enabled, you will get several interrupts when you press the button once. This behavior is normally not wanted. Even if the button’s didn’t bounce (with filtering hardware for example) we still want to capture the event of a pushed button and take some action one time for every button press, so we need to keep track of the state of the button as well.
One technique, used in this tutorial, to handle this is to check (poll) the button(s) periodically and only decide that a button is pressed if it have been in the pressed state for a couple of subsequent polls. Continue reading