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#include <pic16f84a.h>
typedef unsigned char uint8_t;
__sfr __at(0x70) UART_TX;
__sfr __at(0x72) UART_STATUS;
/*
* OPTION_REG, T0IF, T0IE, GIE are all provided by pic16f84a.h.
* OPTION_REG 0x07: T0CS=0 (Fosc/4), PSA=0, PS=111 (1:256)
* At 50 MHz: overflow every 256*256 / (50M/4) = 5.24 ms → ~191 ovf/sec
*/
#define OVF_PER_SEC 191
static volatile uint8_t ovf_count = 0;
void isr(void) __interrupt(0) {
if (INTCON & 0x04) { /* T0IF = bit 2 */
ovf_count++;
INTCON &= ~(uint8_t)0x04;
}
}
static void uart_send(uint8_t b) {
while (UART_STATUS & 0x01);
UART_TX = b;
}
static void uart_puts(const char *s) {
while (*s) uart_send((uint8_t)*s++);
}
static void print_uint8(uint8_t v) {
uint8_t started = 0;
if (v >= 200) { uart_send('2'); v -= 200; started = 1; }
else if (v >= 100) { uart_send('1'); v -= 100; started = 1; }
{
uint8_t t = '0';
while (v >= 10) { v -= 10; t++; }
if (started || t != '0') uart_send(t);
}
uart_send(v + '0');
}
void main(void) {
uint8_t seconds = 0;
/* T0CS=0 (Fosc/4), PSA=0, PS=111 (1:256 prescaler) */
OPTION_REG = 0x07;
TMR0 = 0;
INTCON = 0xa0; /* T0IE | GIE */
uart_puts("\x1B[2J\x1B[H");
uart_puts("Timer0 test\r\n");
uart_puts("-----------\r\n");
while (1) {
ovf_count = 0;
while (ovf_count < OVF_PER_SEC);
uart_puts("t = ");
print_uint8(seconds);
uart_puts(" s\r\n");
PORTA ^= 0x01;
seconds++;
}
}
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