Add Baudrate 4800

[Markus1995]

Hello, Can you please implent also 4800 as Baudrate? Thank You

[thielj]

+1 - 4800 is commonly used for NMEA and GPS receivers

[SRGDamia1]

4800 is too slow to use without requiring the use of an additional timer. It could be supported, but it wouldn't meet the goal of having no requirements.

[thielj]

Hi @SRGDamia1, wouldn't this just be the following or am I missing something?

      case 4800:
        txBitWidth      = TICKS_PER_BIT_9600 << 1;
        bitsPerTick_Q10 = BITS_PER_TICK_38400_Q10 >> 3;

[SRGDamia1]

No, just doing that won't work. The timer that we need the ticks from is only 8-bit and with the "Arduino standard" pre-scaler set, each tick is 4µs. So the timer will roll over after 2564µs = 1.024ms. At 4800 baud you have (1+8+1)bits / 1character 1sec / 4800bits = 2.083ms. So you'd have no way of knowing how many ticks actually happened between bits because the timer might have rolled over (twice) in the middle.

You could get around this by summoning up another timer with a different pre-scaler (like Timer2, which the 8MHz boards already need). But I think one of @SlashDevin 's goals for this library is to have no extra requirements when being used on a "standard" board like an Uno.

[thielj]

Why not use micros() instead of accessing TCNT0 directly? It keeps track of timer overflows and should be safe to use at the beginning of the interrupt handler.

[SRGDamia1]

I tried using micros at 1200 baud, and it didn't work. I think the extra overhead of calling micros() is too much: https://github.com/SlashDevin/NeoSWSerial/issues/23

[thielj]

Even if you simplify micros and eliminate all costly math?

// to be called once at start of interrupt handler
inline uint16_t ticks() {
    uint8_t m = (uint8_t) timer0_overflow_count;
    uint8_t t = TCNT0;
    if ((TIFR0 & _BV(TOV0)) && (t < 255))
        ++m;
    return ((uint16_t)m << 8) | t;
}

[SRGDamia1]

I didn't try that.