Re: [avr-chat] Array access to ports

[David Brown]

Ron Kreymborg wrote:
> > -----Original Message-----
> > From: address@hidden 
> > [mailto:address@hidden On 
> > Behalf Of David Brown
> > Sent: Monday, 29 October 2007 7:25 PM
> > To: address@hidden
> > Subject: Re: [avr-chat] Array access to ports
> >
> >
> > 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;
> -----------------------------------------------------------------
> Thanks for the clue David. In fact it is even simpler than that:
>
> typedef struct {uint8* pPort; uint8 mask;} LedType;

pPort should be a pointer to a volatile uint8.  I like to build up types 
with several typedefs, as that makes it clearer and less error prone 
when using complicated types, but you want:

typedef struct {volatile uint8* pPort; uint8 mask;} LedType;


Your code will probably work in exactly the same way with and without 
the "volatile" - but under some circumstances, the compiler might 
optimise the code and give you different results (for example, turning 
an LED on then off in the same function might be omitted).  The general 
rule is that all hardware access is done through volatiles - don't break 
that rule just to save slightly on the typing.

> static LedType LedList[] = {{&PORTB, 0x20},{etc...}};
>
>    *LedList[index].pPort |= ledList[index].mask;
>
> This uses 13 instructions (OPT=s) - overhead I can live with.
>
> Thanks again
> Ron