Getting Started

Appen

define sbi(

dix 3: Debugging Tale

ubstitution yields:

sfr, bit) ( (*(volatile uint8_t *)( ((uint16_t) &(sfr)) )) |= _BV(bit))

t *)(((

impleme

n that m

ed reg

dress

mory

ed v

ed ea

bi:

high

RTB

RTB

RTB

S

And _BV is? In pgmspace.h it is:

#define _BV(bit) (1 << (bit))

More substitiuton yields:

#define sbi(sfr, bit) ((*(volatile uint8_ uint16_t) &(sfr)) )) |= (1 << (bit)))

ntation specific type qualifier that

ight, in our case, screw things up if

ters. W

By the by, what’s a volatile? It is an
tells a complier to suppress optimizatio
we are using pointers to memory mapp is e don’t want the compiler to
help us by using some other memory ad
e

since a register is hardwired into the
machine and though addressed like m , isn’t ordinary memory. Volatile also
tells the compiler that that the so modifi ariable can change unexpectedly (like
by an interrupt) so it needs to be check
s

ch time it is used and not just stored

omewhere like on the stack.

More substitution for an actual use of s

sbi(PORTB, 5); // set OC1A

yields:

(((volatile uint8_t )(((uint16_t) &(PO )) )) |= (1 << (5)))

But the complier doesn’t know what PO is. What is it?

From io169.h

#define PORTB _SFR_IO8(0x05)/ PO /

and _SFR_IO8 id defined in sfrdefs.h: