STC8G1K08A tiny 349 byte UART hello world demo

#include "fw_uart.h"

/* Keep the baud rate in one place so the Timer1 reload formula stays obvious. */
#define UART1_BAUD_RATE 115200UL

/* Reproduce the library's SYSCLK calculation without calling any runtime code. */
#define UART1_SYSCLK_HZ (__CONF_FOSC / ((__CONF_CLKDIV == 0) ? 1UL : (unsigned long)__CONF_CLKDIV))

/* UART1 mode 1 uses Timer1 overflow divided by four as its baud-rate clock. */
#define UART1_TIMER1_RELOAD (65536UL - (UART1_SYSCLK_HZ / (4UL * UART1_BAUD_RATE)))

/* Keep the entire message in one memory area so the library can stream it out. */
static unsigned char uart_message[] = "Hello, world from UART1!\r\n";

static void UART1_SendString(unsigned char *str)
{
    while (*str)
    {
        SBUF = *str++;
        TI = 0;
        while (!TI)
        {
        }
    }
}

#define TICKS_MS (__CONF_FOSC / ((__CONF_CLKDIV == 0) ? 1UL : (unsigned long)__CONF_CLKDIV) / 9000UL)

static void delay_ms(unsigned int ms)
{
    unsigned int i;

    do
    {
        i = TICKS_MS;
        while (--i)
        {
        }
    } while (--ms);
}

static void UART1_Init(void)
{
    /*
     * Open the extended SFR bank so we can write CLKDIV before bringing UART1 up.
     * B7 = 1: enable access to the extended SFR region.
     * B6..B0 = 0: leave the other bank-switching and peripheral-routing controls unchanged.
     */
    P_SW2 = 0x80;

    /* Apply the configured clock divider with a direct whole-register write. */
    CLKDIV = __CONF_CLKDIV;

    /*
     * Return to the normal SFR bank now that the clock divider is programmed.
     * B7 = 0: disable extended SFR access again.
     * B6..B0 = 0: keep the remaining P_SW2 controls inactive in this minimal demo.
     */
    P_SW2 = 0x00;

    /*
     * Select the high-speed internal RC oscillator band that the trim values were tuned for.
     * 0x03 keeps only IRCBAND[1:0], which are the band-select bits.
     * Any upper bits in the configuration value are discarded so we only touch the documented band field.
     */
    IRCBAND = (__CONF_IRCBAND & 0x03);

    /* Load the matching voltage trim byte for that RC band. */
    VRTRIM = __CONF_VRTRIM;

    /* Load the high-speed RC fine-trim byte. */
    IRTRIM = __CONF_IRTRIM;

    /*
     * Load the low-speed RC trim field even though this demo runs from the fast internal clock.
     * 0x03 keeps only LIRTRIM[1:0], which are the valid low-speed trim bits.
     * Any upper bits are forced low so we do not write undefined values into that register.
     */
    LIRTRIM = (__CONF_LIRTRIM & 0x03);

    /*
     * Put UART1 into the standard 8-bit asynchronous mode and allow the receiver to run.
     * 0x50 = 0101 0000b.
     * SM0 = 0, SM1 = 1: select UART mode 1, the usual 8-bit UART with variable baud rate.
     * SM2 = 0: disable multiprocessor address filtering, so every received byte is accepted.
     * REN = 1: enable the UART receiver.
     * TB8 = 0, RB8 = 0: ninth-bit transmit/receive state starts cleared because mode 1 does not use it.
     * TI = 0, RI = 0: clear both transmit and receive status flags before the first character.
     */
    SCON = 0x50;

    /*
     * Configure Timer1 as the baud-rate generator that UART1 will count from.
     * 0x40 = 0100 0000b.
     * B6 = 1: run Timer1 in 1T mode so it increments every system clock instead of every 12 clocks.
     * B0 = 0: keep UART1's baud source on Timer1 rather than switching it to Timer2.
     * B5..B1 = 0: leave the unrelated AUXR features disabled in this demo.
     */
    AUXR = 0x40;

    /*
     * Put Timer1 into the reload mode expected by the FwLib UART code path.
     * 0x00 = 0000 0000b.
     * GATE1 = 0: Timer1 starts and stops only from TR1, not from the external INT1 pin.
     * C/T1 = 0: Timer1 counts clock ticks, not external events.
     * M1_1 = 0, M0_1 = 0: select the Timer1 mode that this STC8 library uses for UART baud generation.
     * The Timer0 half of TMOD is also cleared because this demo does not use Timer0.
     */
    TMOD = 0x00;

    /* Load the high byte of the compile-time Timer1 reload value. */
    TH1 = (unsigned char)(UART1_TIMER1_RELOAD >> 8);

    /* Load the low byte so the very first overflow already uses the right baud rate. */
    TL1 = (unsigned char)(UART1_TIMER1_RELOAD & 0xFF);

    /* Start Timer1 so UART1 can derive its bit timing. */
    TR1 = 1;
}

void main(void)
{
    /* A zero-initialized unsigned loop variable gives a compact full-range delay. */
    unsigned int settle = 0;

    UART1_Init();
    
    /* Hand the whole NUL-terminated message buffer to the local string transmit helper. */
    while (1)
    {
        UART1_SendString(uart_message);
        delay_ms(1000);
    }
}

picocom -b 115200 /dev/ttyUSB0

===== Memory usage summary for uart_minimal_size_demo =====
Internal RAM layout:
      0 1 2 3 4 5 6 7 8 9 A B C D E F
0x00:|0|0|0|0|0|0|0|0|S|S|S|S|S|S|S|S|
0x10:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x20:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x30:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x40:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x50:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x60:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x70:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x80:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x90:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0xa0:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0xb0:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0xc0:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0xd0:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0xe0:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0xf0:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
0-3:Reg Banks, T:Bit regs, a-z:Data, B:Bits, Q:Overlay, I:iData, S:Stack, A:Absolute

Stack starts at: 0x08 (sp set to 0x07) with 248 bytes available.
No spare internal RAM space left.

Other memory:
   Name             Start    End      Size     Max     
   ---------------- -------- -------- -------- --------
   PAGED EXT. RAM                         0      256   
   EXTERNAL RAM     0x0001   0x0020      32     1024   
   ROM/EPROM/FLASH  0x0000   0x015c     349     8192   
==========================================================