I have 4 digits and need to supply a maximum of 15mA per segment, where each segment (RED) needs 2.1V. For example, the refresh rate is 220Hz, hence 55Hz/digit. How can I calculate the value of each of the 8 resistors that will allow me to pass 15mA to each segment during the time the segment is lit?
I am asking this because I have a lot of 82R resistors and I wanted to use them, what is the safe refresh rate that will yield 15mA/segment consumption. Vcc is 5V, of course.
Necessary part of the code is shown for reference
//common anode or common cathode
//this will save 2 bytes, no need to use
//bitwise NOT operation later on
//1 --> common anode
//0 --> common cathode
#define CA 1
volatile unsigned int val = 0;
#if CA
unsigned char digits[] = {
~(SEG_a|SEG_b|SEG_c|SEG_d|SEG_e|SEG_f), // 0
~(SEG_b|SEG_c), // 1
~(SEG_a|SEG_b|SEG_d|SEG_e|SEG_g), // 2
~(SEG_a|SEG_b|SEG_c|SEG_d|SEG_g), // 3
~(SEG_b|SEG_c|SEG_c|SEG_f|SEG_g), // 4
~(SEG_a|SEG_c|SEG_d|SEG_f|SEG_g), // 5
~(SEG_a|SEG_c|SEG_d|SEG_e|SEG_f|SEG_g), // 6
~(SEG_a|SEG_b|SEG_c), // 7
~(SEG_a|SEG_b|SEG_c|SEG_d|SEG_e|SEG_f|SEG_g), // 8
~(SEG_a|SEG_b|SEG_c|SEG_d|SEG_f|SEG_g), // 9
};
#else
unsigned char digits[] = {
(SEG_a|SEG_b|SEG_c|SEG_d|SEG_e|SEG_f), // 0
(SEG_b|SEG_c), // 1
(SEG_a|SEG_b|SEG_d|SEG_e|SEG_g), // 2
(SEG_a|SEG_b|SEG_c|SEG_d|SEG_g), // 3
(SEG_b|SEG_c|SEG_c|SEG_f|SEG_g), // 4
(SEG_a|SEG_c|SEG_d|SEG_f|SEG_g), // 5
(SEG_a|SEG_c|SEG_d|SEG_e|SEG_f|SEG_g), // 6
(SEG_a|SEG_b|SEG_c), // 7
(SEG_a|SEG_b|SEG_c|SEG_d|SEG_e|SEG_f|SEG_g), // 8
(SEG_a|SEG_b|SEG_c|SEG_d|SEG_f|SEG_g), // 9
};
#endif
ISR(TIMER0_COMP_vect)
{
//turn off all digits
CA_PORT &= 0xF0;
unsigned char cnt = 0;
if ( val == 0) cnt = 1;
static unsigned char active_digit = 0;
unsigned char digit_val[4] = {0, 0, 0, 0}; // NUMBER OF DIGITS
unsigned int tmp;
//extract each digit's value
tmp = val; // necessary to prevent modifying val variable
while (tmp > 0)
{
digit_val[cnt++] = tmp % 10;
tmp /= 10;
}
/*
Leading zeros suppresion
cnt from above calculation can tell us how many
digits should be lit without using if statement
*/
active_digit = ++active_digit % cnt;
SEGMENT_PORT = digits[digit_val[ active_digit]];
//turn corresponding digit on
CA_PORT |= (0x01 << active_digit);
};
