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cpu/sam0_common: UART: implement arithmetic BAUD mode

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SAMD20 does not implement a Fractional Baud mode, so we have to
implement Asynchronous Arithmetic mode.
This commit is contained in:
Benjamin Valentin 2020-06-21 23:01:45 +02:00
parent ecab75b529
commit 28c1c502bc
1 changed files with 45 additions and 8 deletions
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53
cpu/sam0_common/periph/uart.c
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@ -19,6 +19,7 @@
* @author Troels Hoffmeyer <troels.d.hoffmeyer@gmail.com>
* @author Hauke Petersen <hauke.petersen@fu-berlin.de>
* @author Dylan Laduranty <dylanladuranty@gmail.com>
* @author Benjamin Valentin <benjamin.valentin@ml-pa.com>
*
* @}
*/
@ -35,6 +36,11 @@
#define UART_HAS_TX_ISR
#endif
/* default to fractional baud rate calculation */
#if !defined(CONFIG_SAM0_UART_BAUD_FRAC) && defined(SERCOM_USART_BAUD_FRAC_BAUD)
#define CONFIG_SAM0_UART_BAUD_FRAC 1
#endif
/**
* @brief Allocate memory to store the callback functions & buffers
*/
@ -57,6 +63,35 @@ static inline SercomUsart *dev(uart_t dev)
return uart_config[dev].dev;
}
static void _set_baud(uart_t uart, uint32_t baudrate)
{
const uint32_t f_src = sam0_gclk_freq(uart_config[uart].gclk_src);
#if IS_ACTIVE(CONFIG_SAM0_UART_BAUD_FRAC)
/* Asynchronous Fractional */
uint32_t baud = (((f_src * 8) / baudrate) / 16);
dev(uart)->BAUD.FRAC.FP = (baud % 8);
dev(uart)->BAUD.FRAC.BAUD = (baud / 8);
#else
/* Asynchronous Arithmetic */
/* BAUD = 2^16 * (2^0 - 2^4 * f_baud / f_src) */
/* = 2^(16-n) * (2^n - 2^(n+4) * f_baud / f_src) */
/* = 2^(20-n) * (2^(n-4) - 2^n * f_baud / f_src) */
/* 2^n * f_baud < 2^32 -> find the next power of 2 */
uint8_t pow = __builtin_clz(baudrate);
/* 2^n * f_baud */
baudrate <<= pow;
/* (2^(n-4) - 2^n * f_baud / f_src) */
uint32_t tmp = (1 << (pow - 4)) - baudrate / f_src;
uint32_t rem = baudrate % f_src;
uint8_t scale = 20 - pow;
dev(uart)->BAUD.reg = (tmp << scale) - (rem << scale) / f_src;
#endif
}
static void _configure_pins(uart_t uart)
{
/* configure RX pin */
@ -116,11 +151,15 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
/* set asynchronous mode w/o parity, LSB first, TX and RX pad as specified
* by the board in the periph_conf.h, x16 sampling and use internal clock */
dev(uart)->CTRLA.reg = (SERCOM_USART_CTRLA_DORD |
SERCOM_USART_CTRLA_SAMPR(0x1) |
SERCOM_USART_CTRLA_TXPO(uart_config[uart].tx_pad) |
SERCOM_USART_CTRLA_RXPO(uart_config[uart].rx_pad) |
SERCOM_USART_CTRLA_MODE(0x1));
dev(uart)->CTRLA.reg = SERCOM_USART_CTRLA_DORD
#if IS_ACTIVE(CONFIG_SAM0_UART_BAUD_FRAC)
/* enable Asynchronous Fractional mode */
| SERCOM_USART_CTRLA_SAMPR(0x1)
#endif
| SERCOM_USART_CTRLA_TXPO(uart_config[uart].tx_pad)
| SERCOM_USART_CTRLA_RXPO(uart_config[uart].rx_pad)
| SERCOM_USART_CTRLA_MODE(0x1);
/* Set run in standby mode if enabled */
if (uart_config[uart].flags & UART_FLAG_RUN_STANDBY) {
dev(uart)->CTRLA.reg |= SERCOM_USART_CTRLA_RUNSTDBY;
@ -139,9 +178,7 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
#endif
/* calculate and set baudrate */
uint32_t baud = (((sam0_gclk_freq(uart_config[uart].gclk_src) * 8) / baudrate) / 16);
dev(uart)->BAUD.FRAC.FP = (baud % 8);
dev(uart)->BAUD.FRAC.BAUD = (baud / 8);
_set_baud(uart, baudrate);
/* enable transmitter, and configure 8N1 mode */
if (uart_config[uart].tx_pin != GPIO_UNDEF) {