Hello world.

1 files changed, 125 insertions, 0 deletions

diff --git a/src/uart.c b/src/uart.c
new file mode 100644
index 0000000..c5823e8
--- /dev/null
+++ b/src/uart.c
@@ -0,0 +1,125 @@
+#include <uart.h>
+
+#include <gpio.h>
+#include <mmio.h>
+
+enum
+{
+    // The base address for UART.
+    UART0_BASE = (GPIO_BASE + 0x1000), // for raspi4 0xFE201000, raspi2 & 3 0x3F201000, and 0x20201000 for raspi1
+    // The offsets for reach register for the UART.
+    UART0_DR     = (UART0_BASE + 0x00),
+    UART0_RSRECR = (UART0_BASE + 0x04),
+    UART0_FR     = (UART0_BASE + 0x18),
+    UART0_ILPR   = (UART0_BASE + 0x20),
+    UART0_IBRD   = (UART0_BASE + 0x24),
+    UART0_FBRD   = (UART0_BASE + 0x28),
+    UART0_LCRH   = (UART0_BASE + 0x2C),
+    UART0_CR     = (UART0_BASE + 0x30),
+    UART0_IFLS   = (UART0_BASE + 0x34),
+    UART0_IMSC   = (UART0_BASE + 0x38),
+    UART0_RIS    = (UART0_BASE + 0x3C),
+    UART0_MIS    = (UART0_BASE + 0x40),
+    UART0_ICR    = (UART0_BASE + 0x44),
+    UART0_DMACR  = (UART0_BASE + 0x48),
+    UART0_ITCR   = (UART0_BASE + 0x80),
+    UART0_ITIP   = (UART0_BASE + 0x84),
+    UART0_ITOP   = (UART0_BASE + 0x88),
+    UART0_TDR    = (UART0_BASE + 0x8C),
+    // The offsets for Mailbox registers.
+    MBOX_BASE    = 0xB880,
+    MBOX_READ    = (MBOX_BASE + 0x00),
+    MBOX_STATUS  = (MBOX_BASE + 0x18),
+    MBOX_WRITE   = (MBOX_BASE + 0x20)
+};
+
+// A mailbox message with set clock rate of PL011 to 3MHz tag.
+static volatile unsigned int  __attribute__((aligned(16))) mbox[9] = {
+  9*4, 0, 0x38002, 12, 8, 2, 3000000, 0, 0
+};
+
+// Loop <delay> times in a way that the compiler won't optimize away.
+static inline void delay(int32_t count) {
+  asm volatile("__delay_%=: subs %[count], %[count], #1; bne __delay_%=\n"
+    : "=r"(count): [count]"0"(count) : "cc");
+}
+
+void uart_init(int raspi) {
+  // Disable UART0.
+  mmio_write(UART0_CR, 0x00000000);
+  
+  // Setup the GPIO pin 14 && 15.
+
+  // Disable pull up/down for all GPIO pins & delay for 150 cycles.
+  mmio_write(GPPUD, 0x00000000);
+  delay(150);
+
+  // Disable pull up/down for pin 14,15 & delay for 150 cycles.
+  mmio_write(GPPUDCLK0, (1 << 14) | (1 << 15));
+  delay(150);
+
+  // Write 0 to GPPUDCLK0 to make it take effect.
+  mmio_write(GPPUDCLK0, 0x00000000);
+
+  // Clear pending interrupts.
+  mmio_write(UART0_ICR, 0x7FF);
+
+  // Set integer & fractional part of baud rate.
+  // Divider = UART_CLOCK/(16 * Baud)
+  // Fraction part register = (Fractional part * 64) + 0.5
+  // Baud = 115200.
+
+  // For Raspi3 and 4 the UART_CLOCK is system-clock dependent by default.
+  // Set it to 3Mhz so that we can consistently set the baud rate
+  if (raspi >= 3) {
+    // UART_CLOCK = 30000000;
+    unsigned int r = (unsigned int) (((uint64_t)(&mbox) & ~0xF) | 8);
+    // Wait until we can talk to the VC.
+    while (mmio_read(MBOX_STATUS) & 0x80000000);
+    // Send our message to property channel and wait for the response.
+    mmio_write(MBOX_WRITE, r);
+    while ((mmio_read(MBOX_STATUS) & 0x40000000) ||
+           (mmio_read(MBOX_READ) != r));
+  }
+
+  // Divider = 3000000 / (16 * 115200) = 1.627 = ~1.
+  mmio_write(UART0_IBRD, 1);
+  // Fractional part register = (.627 * 64) + 0.5 = 40.6 = ~40.
+  mmio_write(UART0_FBRD, 40);
+
+  // Enable FIFO & 8 bit data transmission (1 stop bit, no parity).
+  mmio_write(UART0_LCRH, (1 << 4) | (1 << 5) | (1 << 6));
+
+  // Mask all interrupts.
+  mmio_write(UART0_IMSC, (1 << 1) | (1 << 4) | (1 << 5) | (1 << 6) |
+                         (1 << 7) | (1 << 8) | (1 << 9) | (1 << 10));
+
+  // Enable UART0, receive & transfer part of UART.
+  mmio_write(UART0_CR, (1 << 0) | (1 << 8) | (1 << 9));
+}
+
+void uart_putc(unsigned char c) {
+  // Wait for UART to become ready to transmit.
+  while (mmio_read(UART0_FR) & (1 << 5));
+  mmio_write(UART0_DR, c);
+}
+
+unsigned char uart_getc() {
+  // Wait for UART to have received something.
+  while (mmio_read(UART0_FR) & (1 << 4));
+  return mmio_read(UART0_DR);
+}
+
+void uart_write(const void* buffer, size_t length) {
+  const uint8_t* bytes = buffer;
+  for (size_t i = 0; i < length; ++i) {
+    uart_putc(*bytes++);
+  }
+}
+
+void uart_print(const char* string) {
+  while (*string) {
+    uart_putc(*string++);
+  }
+}
+