Electronic – How to detect Serial or TTL signal for a PIC

With TTL serial, there are two unidirectional data lines. Each is driven by the sender, both high and low. A 0 bit is represented by 0V a 1 bit by VCC.

The receiver's pin should be set to an input.

So, for a microcontroller to send a byte (8-N-1 no flow control) it could do something like this:

#define BAUDRATE 9600
#define DELAY (SYS_CLK/BAUDRATE)

#define UART_BITBANG_OFF     UART_BITBANG_PORT |= _BV(UART_BITBANG_PIN)
#define UART_BITBANG_ON      UART_BITBANG_PORT &= ~ _BV(UART_BITBANG_PIN)

#define UART_BITBANG_BIT(bit) {if (bit) UART_BITBANG_ON; else UART_BITBANG_OFF; _delay_us(DELAY);}

void uart_bitbang_init(void)
{
    UART_BITBANG_DDR &= ~ _BV(UART_BITBANG_PIN);        // TX output
}

void uart_bitbang_putc(uint8_t c)
{
    UART_BITBANG_BIT(1)
    UART_BITBANG_BIT((c & 0x1) == 0);
    UART_BITBANG_BIT((c & 0x2) == 0);
    UART_BITBANG_BIT((c & 0x4) == 0);
    UART_BITBANG_BIT((c & 0x8) == 0);
    UART_BITBANG_BIT((c & 0x10) == 0);
    UART_BITBANG_BIT((c & 0x20) == 0);
    UART_BITBANG_BIT((c & 0x40) == 0);
    UART_BITBANG_BIT((c & 0x80) == 0);
    UART_BITBANG_BIT(0);
}

(This code reads a bit backwards as it was originally meant for inverted TTL serial)

Of course, most MCUs have hardware UARTs which do all this for you.

Here's what you'd see on a scope:

https://www.pololu.com/docs/0J25/4.a

Here's a great video from ladyada explaining serial: http://www.adafruit.com/blog/2010/09/15/usb-serial-and-you-video-an-adafruit-after-school-special/

Things to check / try:

1. Make sure you have the right baud rate set for your clock speed. The calculation is made somewhat more tricky by the newer 16-bit baud rate generator option.

2. Make sure you are actually running at the clock speed you are expecting to be running at.

3. Check you have the clock polarity set right - try different settings:

bit 4 SCKP: Synchronous Clock Polarity Select bit

• Asynchronous mode:
• • 1 = Transmit inverted data to the TX/CK pin
• • 0 = Transmit non-inverted data to the TX/CK pin

There is a similar setting for the receive clock somewhere, but I can't find it at the moment.