printf("int8_t size %u\n", (uint32_t)sizeof(int8_t));
printf("uint32_t size %u\n", (uint32_t)sizeof(uint32_t));

uint32_t a,b;
char c;
scanf("%u%c%u", &a, &c, &b);

uint8_t A,B;

while(1)
  {
  A = -1, B = -1;
  scanf  ("%hhu%hhu",&A,&B); 
  printf ("A='%u', B='%u'\n", A,B);
  }

uint8_t A,B;

while(1)
  {
  A = -1, B = -1;
  scanf ("%" SCNu8 "%" SCNu8 , &A,&B);
  printf ("A='%" PRIu8 "', B='%" PRIu8 "'\n", A,B);
  }

hhu - спецификатор для unsigned char

scanf ("%" SCNu8 "%" SCNu8 , &A,&B);
printf ("A='%" PRIu8 "', B='%" PRIu8 "'\n", A,B);

The length modifiers and their meanings are:

hh Specifies that a following d,i,o,u,x,or X conversion specifier
applies to a signed char or unsigned char argument  (the  
argument  will  have been  promoted  according  to  the  integer  
promotions,  but  its  value  shall  be converted to signed char
or unsigned char before printing); or that a following n 
conversion specifier applies to a pointer to a signed char
argument.

Macros for format specifiers

Each of the following object-like macros expands to a character
string literal containing a conversion  specifier, possibly  
modified  by  a  length  modifier, suitable  for  use  within  
the format argument of a formatted input/output function when 
converting the corresponding integer type.  These macro names 
have the general form of PRI (character string literals for  the
fprintf and fwprintf family)  or SCN (character  string  literals  
for  the fscanf and fwscanf family), followed by the conversion 
specifier, followed by a name  corresponding  to  a  similar  
type  name  in  7.20.1.  In  these  names, N represents  the
width  of  the  type  as  described  in  7.20.1.  For example,
PRIdFAST32 can  be  used  in  a format string to print the value 
of an integer of type int_fast32_t.