src/drivers/uart/util.c

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2
   3 #include <console/uart.h>
   4 #include <types.h>
   5 #include <timer.h>
   6
   7 /* Calculate divisor. Do not floor but round to nearest integer. */
   8 unsigned int uart_baudrate_divisor(unsigned int baudrate,
   9         unsigned int refclk, unsigned int oversample)
  10 {
  11         return (1 + (2 * refclk) / (baudrate * oversample)) / 2;
  12 }
  13
  14 #if !CONFIG(UART_OVERRIDE_INPUT_CLOCK_DIVIDER)
  15 unsigned int uart_input_clock_divider(void)
  16 {
  17         /* Specify the default oversample rate for the UART.
  18          *
  19          * UARTs oversample the receive data.  The UART's input clock first
  20          * enters the baud-rate divider to generate the oversample clock.  Then
  21          * the UART typically divides the result by 16.  The asynchronous
  22          * receive data is synchronized with the oversample clock and when a
  23          * start bit is detected the UART delays half a bit time using the
  24          * oversample clock.  Samples are then taken to verify the start bit and
  25          * if present, samples are taken for the rest of the frame.
  26          */
  27         return 16;
  28 }
  29 #endif
  30
  31 #if !CONFIG(UART_OVERRIDE_REFCLK)
  32 unsigned int uart_platform_refclk(void)
  33 {
  34         /* Specify the default input clock frequency for the UART.
  35          *
  36          * The older UART's used an input clock frequency of 1.8432 MHz which
  37          * with the 16x oversampling provided the maximum baud-rate of 115200.
  38          * Specify this as maximum baud-rate multiplied by oversample so that
  39          * it is obvious that the maximum baud rate is 115200 when divided by
  40          * oversample clock.  Also note that crystal on the board does not
  41          * change when software selects another input clock divider.
  42          */
  43         return 115200 * 16;
  44 }
  45 #endif
  46
  47 /* Helper function to allow bitbanging an 8n1 UART. */
  48 void uart_bitbang_tx_byte(unsigned char data, void (*set_tx)(int line_state))
  49 {
  50         const int baud_rate = get_uart_baudrate();
  51         int i;
  52         struct stopwatch sw;
  53         stopwatch_init(&sw);
  54
  55         /* Send start bit */
  56         set_tx(0);
  57         while (stopwatch_duration_usecs(&sw) < MHz / baud_rate)
  58                 stopwatch_tick(&sw);
  59
  60         /* 'i' counts the total bits sent at the end of the loop */
  61         for (i = 2; i < 10; i++) {
  62                 set_tx(data & 1);
  63                 data >>= 1;
  64                 while (stopwatch_duration_usecs(&sw) < i * MHz / baud_rate)
  65                         stopwatch_tick(&sw);
  66         }
  67
  68         /* Send stop bit */
  69         set_tx(1);
  70         while (stopwatch_duration_usecs(&sw) < i * MHz / baud_rate)
  71                 stopwatch_tick(&sw);
  72 }