Re: [avr-chat] Array access to ports

[David Brown]

Ron Kreymborg wrote:
> I am addressing a dozen or so LEDs on a number of different ports, turning
> them on and off. I would like to address them through an array structure
> that stores the relevant port and mask. For example:
>
> typedef enum
> {
>     T_LED_OFF,
>     T_LED_ON
> } LED_STATE_TYPE;
>
> typedef struct
> {
>     MYPORT    port;
>     uint8     mask;
> } LED_TYPE;
>
> I could designate the respective LEDs with names. For example:
>
> typedef enum
> {
>    LEDOUT,
>    etc
> } LED;
>
> I had hoped to be able to preload the array something like:
>
> static LED_TYPE LedList[] = {
>     {PORTB, 0x20},                // LEDOUT
>     {etc}
> };
>
> Where the 0x20 is the mask for pin5 of PORTB for the LEDOUT led. I could
> then use a simple function to address all leds. For example, turning the
> LEDOUT led on (high) would look like:
>
>    ChangeLedState(LEDOUT, T_LED_ON);
>
> And the partial function:
>
> void ChangeLedState(LED index, LED_STATE_TYPE state)
> {
>    if (state == T_LED_ON)
>       LedList[index].port |= LedList[index].mask;
>    .
>    .
>
> The snag of course is that PORTB is not a constant but an SFR. Can anyone
> suggest whether assigning a port like this is possible, and what would be
> the definition of MYPORT?
>
> Cheers
> Ron

You can't store an array of SFR's like this - but you can store an array 
of pointers to them:

typedef volatile uint8_t vuint8_t;
typedef vuint8_t pvuint8_t;

typedef struct { pvuint8_t pPort; uint8_t mask; } ledType;

static ledType ledList[] = { { &PORTB, 0x20 }, { &PORTC, 0x10 } };

Then you can write:
        *(ledList[index].pPort) |= ledList[index].mask;


Of course, the run-time overhead for this sort of thing is quite 
significant, especially if your code almost always uses compile-time 
constants (i.e., your "ChangeLedState" function is always called with 
fixed values).  You can avoid that by making your ChangeLedState 
function inline (possibly with the "always_inline" attribute), so that 
the compiler handles the lookups at compile-time, and gives you a single 
sbi or cbi instruction.

mvh.,

David