.. title: UART on Galaxy S3 and Note 2
.. slug: uart-on-galaxy-s3
.. date: 2017-11-01 23:00:00 UTC+11:00
.. tags: linux, exynos, midas
.. category: mainline
.. link: 
.. description: 
.. type: text

Over the next few months (years?) I plan on working on porting several Android-based Exynos 4412-based devices - namely, the
[Galaxy S3](https://en.wikipedia.org/wiki/Galaxy_S_III>) (of which there are three Exynos4412-based variants),
and the [Galaxy Note 2](https://en.wikipedia.org/wiki/Samsung_Galaxy_Note_II) (of which there are two variants).
The first step, I figured, was probably to get UART working on the vendor-provided Linux 3.0 kernel: Fortunately, I was not the first person to attempt this,
and there are [a few guides](https://forum.xda-developers.com/galaxy-s3/general/info-i9300-uart-guide-how-to-talk-to-t2928854)
on constructing a cable suitable for the process.

<!-- TEASER_END -->

Note, the UART output is multiplexed over the MicroUSB port by a MAX77693 MUIC, not an FSA9485 as the XDA guide suggests.

So, equipped with the knowledge I needed, I went shopping (yay!). I bought a [CP1202 USB-to-UART adapter](
https://www.ebay.com.au/itm/USB-to-UART-5-Pin-CP2102-Module-Serial-Converter-for-Electrical-Engineering/142295732271>),
and a [microUSB cable with a 523kΏ resistor](https://www.ebay.com.au/itm/UART-Cable-with-RJ45-and-USB-Connectors-Based-on-523k-Resistor/122062611479>).
After a lot of waiting, both pieces finally arrived, and I could get hacking. I ~~tore apart~~ carefully cut the cable - only the microUSB end is of any use.

## Making it work

Before soldering, I thought it might be good idea to use some jumper wires (conveniently packaged alongside the UART adapter) to check that my wiring was connect.

This was not a good idea.

It turns out, UART is pretty reliant on having a stable ground connection (who would've thought?). Jumper cables don't make providing such a connection easy. After much fighting with the cables,
I managed to get a couple of lines of semi-readable output on the UART console - the trick was to solder the USB cable connectors to the relevant ports on the UART, and to swap the TX and RX cables around.

So, I end up with this:

<table class="table table-bordered">
<tr><th>USB</th><th>UART</th></tr>
<tr><td>White</td><td>GND</td></tr>
<tr><td>Black</td><td>RXD</td></tr>
<tr><td>Green</td><td>TXD</td></tr>
</table>

Or, in photographic form (I cannibalised some jumpers to ease the soldering process):

[![uart cable](/images/i9300uart.thumbnail.jpg "Click for full size")](/images/i9300uart.jpg)

## Success!

After successfully soldering the cable, I plugged my hand-crafted UART cable into the computer and launched a serial console (`sudo screen /dev/ttyUSB0 115200`). Next, I powered off the phone,
and plugged it in - success! The phone starts to boot, and I get this on the UART console:

```
PMIC rev = PASS2(4)
BUCK1OUT(vdd_mif) = 0x06
BUCK3DVS1(vdd_int) = 0x28
[MMC] there are pending interrupts 0x00010000
cardtype: 0x00000007
SB_MMC_HS_52MHZ_1_8V_3V_IO
mmc->card_caps: 0x00000311
mmc->host_caps: 0x00000311
mmc_initialize: mmc->capacity = 30777344
```

Unfortunately, at this point the UART seems to cut out (outputting `�`) until about four seconds into the kernel boot sequence. I have no idea why, but this means that dropping into the bootloader shell
is not possible for now.

So, I guess the next step is to get a mainline serial console (and more?) working.