More Detail
Contents
1. More Detail The Built in Assembler For the full gory details see http Awww nongnu org avr libc user manual assembler htm and the cookbook at http www nongnu org avr libc user manual inline_asm html It is very rare to need to use the assembler Normally the requirement is to do a single no op without changing anything in order to delay by 1 CPU cycle This can be achieved with asm and r0 r0 The Makefile A simple description of make and Makefiles is available here http Awww network theory co uk docs gccintro gccintro_16 html That covers everything in the sample Makefile except the first few lines CPPFLAGS I I 1ib instructs the compiler to add two I Include flags instructs the compiler to look for h header files in the current directory lib instructs the compiler to also look in the lib directory at lib MCU atmegal 68p is required by gcc avr to tell it which device is the target of the compilation The list of supported devices is here http www nongnu org avr libc user manual VPATH 1ib make will look for source files in the current directory then in directories listed after VPATH Libraries For the details see http Awww nongnu org avr libc user manual library html Common pitfalls Here is a list of common pitfalls in no particular order Interrupts One of the easiest traps to fall into is to spend so long in the interrupt routine that another interrupt has occurred befo2. re the first one is finished If timing is critcal then the extra latency before the second interrupt is serviced will probably cause a failure This suggests doing the minimum possible in the interrupt service routine ISR eg setting flags and the maximum outside the ISR A different strategy to the above is as follows Do everything in the interrupt routines and nothing in main Make sure that each interrupt handler sets up the conditions for the next interrupt to occur If only one interrupt is enabled at a time then there is no potential to clash Make sure that all interrupts are set up and conditions cleared before the Master Interrupt Enable bit is set otherwise an interrupt may occur immediately More Detail If you use global variables which are changed in more than one place declare such variables as volatile otherwise the compiler will optimise the code and not notice that the variable has changed V O It is very easy when modifying code to forget to define a new output Everything will compile correctly but the pin will not behave as you expect It is useful to copy a line such as PORTB 1 lt lt PB1 changing itto PORTB amp 1 lt lt PB1 However omitting the brackets in the second case will cause problems because of operator precedence Except while initialising it is very rare that you will need assignment such as PORTB 1 lt lt PB2 You almost always need or amp Analogue to Digital con
3. verter The most common pitfall with the A to D is forgetting to supply a reference volatge at AVcc Zero or very low readings are a symptom of this The next most common is having too high a clock rate If it is too fast then only the first few bits of the data will be correct Selecting the wrong analogue input is the next most common error Fuse errors If you can no longer program the fuses have you disabled the clock by mistake This is particularly easy to do when chaning from an internal to an external or crystal derived clock If you reprogram the fuses the device does not automatically restart You must power cycle it Changing the program does however force a restart
Download Pdf Manuals
Related Search