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# Tutorial 05 - UART0, PL011
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Finally, we can set up `UART0` thanks to the mailbox interface. This tutorial
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produces the same output as tutorial 04, but it prints the serial number on
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`UART0`.
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## uart.rs
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In the init function, we use the mailbox to set a base clock for the UART:
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```rust
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mbox.buffer[0] = 9 * 4;
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mbox.buffer[1] = mbox::REQUEST;
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mbox.buffer[2] = mbox::tag::SETCLKRATE;
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mbox.buffer[3] = 12;
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mbox.buffer[4] = 8;
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mbox.buffer[5] = mbox::clock::UART; // UART clock
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mbox.buffer[6] = 4_000_000; // 4Mhz
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mbox.buffer[7] = 0; // skip turbo setting
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mbox.buffer[8] = mbox::tag::LAST;
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// Insert a compiler fence that ensures that all stores to the
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// mbox buffer are finished before the GPU is signaled (which
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// is done by a store operation as well).
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compiler_fence(Ordering::Release);
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if mbox.call(mbox::channel::PROP).is_err() {
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return Err(UartError::MailboxError); // Abort if UART clocks couldn't be set
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};
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```
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Afterwards, we can program the rate divisors:
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```rust
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self.IBRD.write(IBRD::IBRD.val(2)); // Results in 115200 baud
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self.FBRD.write(FBRD::FBRD.val(0xB));
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```
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Baud rate calculation won't be covered in detail here. Please see [this
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reference from ARM](http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0183g/I49493.html)
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for details.
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The API for using the UART is identical to the `UART1` API.
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## main.rs
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We query the board's serial number and display it on the serial console.
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