AVR Freaks

#define F_CPU 20000000UL

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

const uint32_t UART_BAUDRATE = 115200UL;
const uint8_t UART_ENABLE_RX = 0;
const uint8_t UART_ENABLE_TX = 1;

volatile uint8_t *UARTByteSource;
volatile uint8_t UARTBytesLeft;
volatile uint8_t _UARTTXBusy = 0;

//=========================================================
void setupClock() {

	// Unlock Configuration Change Protection
	CPU_CCP = CCP_IOREG_gc;
	// CLKOUT enabled (PB5/Pin 6), Clock source is OSC20M
	CLKCTRL_MCLKCTRLA = 0x80;
	// Unlock Configuration Change Protection
	CPU_CCP = CCP_IOREG_gc;
	// No prescaler on main clock
	CLKCTRL_MCLKCTRLB = 0x00;

}

//=========================================================
uint8_t UARTTXBusy() {
	return _UARTTXBusy;
}

//=========================================================
void setupUART_8N1(const uint32_t baudRate, const uint8_t enableRX, const uint8_t enableTX) {

	uint8_t sregBackup = CPU_SREG;

	// Recommended setup for full-duplex mode (p. 258)

	// For interrupt-driven operation, we disable global interrupts during initialization
	CPU_SREG &= ~CPU_I_bm;

	// Set TX pin to High, then set to Output
    PORTB_OUTSET = 0x04;
	PORTB_DIRSET = 0x04;

	// Set baudrate, Formula on p. 259
	USART0_BAUD = (64UL * F_CPU) / (16UL * baudRate);

	// Set operation mode: Asynchronous, N, 1, 8-bit (see p. 278)
	USART0_CTRLC = USART_CMODE_ASYNCHRONOUS_gc | USART_PMODE_DISABLED_gc |
	               USART_SBMODE_1BIT_gc | USART_CHSIZE_8BIT_gc;

	if (enableRX) {
		USART0_CTRLB |= USART_RXEN_bm;
		USART0_CTRLA |= USART_RXCIE_bm;
	}

	if (enableTX) {
		USART0_CTRLB |= USART_TXEN_bm;
		USART0_CTRLA |= USART_TXCIE_bm;
	}

	// Restore SREG
	CPU_SREG = sregBackup;

}

//=========================================================
// Non-blocking, except waiting for TX
void UARTSendBytes(uint8_t *p, const uint8_t n) {

  UARTByteSource = p;
  UARTBytesLeft = n;

  while (UARTTXBusy()) {
  }

  _UARTTXBusy = 1;
  // Trigger the interrupt immediately (the flag is set when there's NO activity)
  USART0_CTRLA |= USART_DREIE_bm;  

}

//=========================================================
void setup() {

	setupClock();
	setupUART_8N1(UART_BAUDRATE, UART_ENABLE_RX, UART_ENABLE_TX);

}

//=========================================================
int main(void) {

    uint8_t str[10] = "A";

    setup();

	// Enable interrupts
	CPU_SREG |= CPU_I_bm;

	while (1) {

		_delay_ms(10);
		UARTSendBytes(str, 1);

        }

}

//=========================================================
// USART receive completed
ISR (USART0_RXC_vect) {
	// TODO
}

//=========================================================
// USART transmit buffer ready for new data
ISR (USART0_DRE_vect) {

    USART0_TXDATAL = *UARTByteSource++;
    if (0 == --UARTBytesLeft) {
	     // Disable this interrupt
	     USART0_CTRLA &= ~USART_DREIE_bm;
    }

}

//=========================================================
// USART transmission completed
ISR (USART0_TXC_vect) {

	// Datasheet error? TXCIF has to be cleared manually?
	USART0_STATUS |= USART_TXCIF_bm;
	_UARTTXBusy = 0;

}

Not easy cleaning up this mess :-)

As it is above, with manually clearing of TXCIF in the ISR, everything works - I get the letter "A" output on the UART every 10ms. Actual output duration is ~80us.

If I comment out the manual clear, the output is only sent once every ~125ms, as far as I can tell it's because the TXC ISR is being called over and over again.

[Edit: P.S. yeah, initialized a uint8_t array as string... :-D ]

Is there direct bit access in TinyAVR 1-series using C/C++? How would you write this command?

Yes. This is not related to the issue at hand.

Cool, I wasn't aware of these... well, not enough to be working with them regularly But again, I don't think it's related to my issue with the TXC flag.

Hmmm... yes, the datasheet isn't too explicit about it (at least in the parts I read so far) but it seems right. Thanks!

The 4809 too? Good thing you caught it, I was about to start investigating it too

Thanks for confirming.