How did you get baud rate of 19200 for 10ns system clk and 80ns uart clk?

[srvedulageth]

Hi
May i know how you get 19200 baud for these parameters?

I am seeing in the waveform clk = 100MHz which is 10ns and inside uart module this multiplied by 8 which gives 80ns.
80ns = 12.5MHz.

I think the prescalar is not standard 16.
What is the prescalar value?

[srvedulageth]

Is it 41?

[stnolting]

The clock divider is automatically calculated from the CPU clock rate (via the top's CLOCK_FREQUENCY generic) and the baud rate.

neorv32/sw/lib/source/neorv32_uart.c

Lines 47 to 100 in b29cc30

	void neorv32_uart_setup(neorv32_uart_t *UARTx, uint32_t baudrate, uint32_t irq_mask) {
	uint32_t prsc_sel = 0;
	uint32_t baud_div = 0;
	// reset
	UARTx->CTRL = 0;
	// raw clock prescaler
	uint32_t clock = neorv32_sysinfo_get_clk(); // system clock in Hz
	#ifndef MAKE_BOOTLOADER // use div instructions / library functions
	baud_div = clock / (2*baudrate);
	#else // division via repeated subtraction (minimal size, only for bootloader)
	while (clock >= 2*baudrate) {
	clock -= 2*baudrate;
	baud_div++;
	}
	#endif
	// find baud prescaler (10-bit wide))
	while (baud_div >= 0x3ffU) {
	if ((prsc_sel == 2) || (prsc_sel == 4))
	baud_div >>= 3;
	else
	baud_div >>= 1;
	prsc_sel++;
	}
	uint32_t tmp = 0;
	tmp |= (uint32_t)(1 & 1U) << UART_CTRL_EN;
	tmp |= (uint32_t)(prsc_sel & 3U) << UART_CTRL_PRSC0;
	tmp |= (uint32_t)((baud_div - 1) & 0x3ffU) << UART_CTRL_BAUD0;
	tmp |= (uint32_t)(irq_mask & (0x1fU << UART_CTRL_IRQ_RX_NEMPTY));
	#ifdef UART0_SIM_MODE
	#warning UART0_SIM_MODE (primary UART) enabled! \
	Sending all UART0.TX data to text.io simulation output instead of real UART0 transmitter. \
	Use this for simulation only!
	if (((uint32_t)UARTx) == NEORV32_UART0_BASE) {
	tmp |= 1U << UART_CTRL_SIM_MODE;
	}
	#endif
	#ifdef UART1_SIM_MODE
	#warning UART1_SIM_MODE (secondary UART) enabled! \
	Sending all UART1.TX data to text.io simulation output instead of real UART1 transmitter. \
	Use this for simulation only!
	if (((uint32_t)UARTx) == NEORV32_UART1_BASE) {
	tmp |= 1U << UART_CTRL_SIM_MODE;
	}
	#endif
	UARTx->CTRL = tmp;
	}

---

The target baud rate is 19200, which corresponds to a bit time of 52.1µs = 5210ns. To do this, the above function calculates a clock divider and a prescaler.

For this particular case, the function calculates a clock divider of 650 and selects the prescaler "2" which divides the clock (according to the table) by 8 again.

Finall we have: 100_000_000 Hz / [8 * ( 650 + 1 )] = 5208 ns