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UART3 returns a stream of garbage

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Lee Lup Yuen 2023-04-20 18:16:24 +08:00
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@ -1107,14 +1107,52 @@ We register UART3 as `/dev/ttyS1` in [`arm64_serialinit`](https://github.com/lup
[(Here's the log)](https://github.com/lupyuen2/wip-pinephone-nuttx-apps/blob/7e9706c3dccc26dd33ddf71fd0ea64f9a17de4a1/examples/hello/hello_main.c#L76-L340)
Be careful when logging to the UART Port! If the UART Port is busy, we won't be able to set the Baud Rate...
Inside the UART Driver, be careful when logging to the UART Port! If the UART Port is busy doing logging, it won't allow us to the Baud Rate...
> "This register may only be accessed when the DLAB bit (UART_LCR[7]) is set and the UART is not busy (UART_USR[0] is zero)"
That's why we always wait for UART Not Busy before setting the Baud Rate. [(Like this)](https://github.com/lupyuen2/wip-pinephone-nuttx/blob/83566af2e58f92ae42fd434ce97f919404245986/arch/arm64/src/a64/a64_serial.c#L486-L500)
_What happens if UART Is Busy when we set the Baud Rate?_
```text
up_setup: Enter DLAB=1
up_serialout: addr=0x1c2800c, before=0x3, after=0x83
up_setup: Set the BAUD divisor
up_serialout: addr=0x1c28004, before=0x0, after=0x0
up_serialout: addr=0x1c28000, before=0x0, after=0xd
up_setup: Clear DLAB
up_serialout: addr=0x1c2800c, before=0x3, after=0x3
```
[(Source)](https://github.com/lupyuen2/wip-pinephone-nuttx/blob/83566af2e58f92ae42fd434ce97f919404245986/arch/arm64/src/a64/a64_serial.c#L1306-L1313)
From above, we see that the Baud Rate is not read correctly...
```text
up_serialout: addr=0x1c28000, before=0x0, after=0xd
// For UART0: Before should be 0xd
```
And DLAB doesn't get cleared correctly...
```text
up_setup: Enter DLAB=1
up_serialout: addr=0x1c2800c, before=0x3, after=0x83
...
up_setup: Clear DLAB
up_serialout: addr=0x1c2800c, before=0x3, after=0x3
// Before should be 0x83, not 0x3
```
We fix this by disabling the logging and waiting for UART Not Busy before setting the Baud Rate. [(Like this)](https://github.com/lupyuen2/wip-pinephone-nuttx/blob/83566af2e58f92ae42fd434ce97f919404245986/arch/arm64/src/a64/a64_serial.c#L486-L500)
Let's test the PinePhone LTE Modem on UART3...
## Test UART3 Port
In the previous section we have configured UART3 for PinePhone's [4G LTE Modem](https://lupyuen.github.io/articles/lte).
This is how we read and write the UART3 port via `/dev/ttyS1`: [hello_main.c](https://github.com/lupyuen2/wip-pinephone-nuttx-apps/blob/7e9706c3dccc26dd33ddf71fd0ea64f9a17de4a1/examples/hello/hello_main.c#L42-L69)
```c
@ -1196,6 +1234,8 @@ TODO: Check the Allwinner A64 UART Register Addresses
TODO: Why the spurious UART interrupts?
TODO: Inside UART Driver, log the received chars
# Boot NuttX on PinePhone
Read the article...