Datasheet

48
49 uart_reset(uart);
50 uart_unreset(uart);
51
52 // FIXME clk_peri enable mask needs to be set.
53
54 #if PICO_UART_ENABLE_CRLF_SUPPORT
55 uart_set_crlf(uart, PICO_UART_DEFAULT_CRLF);
56 #endif
57
58 // Any LCR writes need to take place before enabling the UART
59 uart_set_baudrate(uart, baudrate);
60 uart_set_format(uart, 8, 1, UART_PARITY_NONE);
61
62 // Enable the UART, both TX and RX
63 uart_hw(uart)->cr = UART_UARTCR_UARTEN_BITS | UART_UARTCR_TXE_BITS |
Ê UART_UARTCR_RXE_BITS;
64 // Enable FIFOs
65 hw_set_bits(&uart_hw(uart)->lcr_h, UART_UARTLCR_H_FEN_BITS);
66 // Always enable DREQ signals -- no harm in this if DMA is not listening
67 uart_hw(uart)->dmacr = UART_UARTDMACR_TXDMAE_BITS | UART_UARTDMACR_RXDMAE_BITS;
68 }
4.3.7.1. Baud Rate Calculation
The uart baud rate is derived from dividing clk_peri.
If the required baud rate is 115200 and UARTCLK = 125MHz then:
Baud Rate Divisor = (125 * 10^6)/(16 * 115200) ~= 67.817
Therefore, BRDI = 67 and BRDF = 0.817,
Therefore, fractional part, m = integer((0.817 * 64) + 0.5) = 52
Generated baud rate divider = 67 + 52/64 = 67.8125
Generated baud rate = (125 * 10^6)/(16 * 67.8125) ~= 115207
Error = (abs(115200 - 115207) / 115200) * 100 ~= 0.006%
Pico SDK: https://github.com/raspberrypi/pico-sdk/tree/pre_release/src/rp2_common/hardware_uart/uart.c Lines 77 - 97
77 uint uart_set_baudrate(uart_inst_t *uart, uint baudrate) {
78 invalid_params_if(UART, baudrate == 0);
79 uint32_t baud_rate_div = (8 * clock_get_hz(clk_peri) / baudrate);
80 uint32_t baud_ibrd = baud_rate_div >> 7;
81 uint32_t baud_fbrd = ((baud_rate_div & 0x7f) + 1) / 2;
82 invalid_params_if(UART, (baud_ibrd > 65535) || (baud_ibrd == 0));
83
84 // Load PL011's baud divisor registers
85 uart_hw(uart)->ibrd = baud_ibrd;
86 if (baud_ibrd == 65535) {
87 uart_hw(uart)->fbrd = 0;
88 } else {
89 uart_hw(uart)->fbrd = baud_fbrd;
90 }
91
92 // PL011 needs a (dummy) line control register write to latch in the
93 // divisors. We don't want to actually change LCR contents here.
94 hw_set_bits(&uart_hw(uart)->lcr_h, 0);
95 // todo what is supposed to be returned here?
96 return 0;
RP2040 Datasheet
4.3. UART 450