Factory image files

The non-free files holding the GPS infos/code are the following:


gps.xml parameters

We have tried to change some parameters in gps.xml to see how it behaves:

Parameter Original Changed to Result
acPortName /dev/s3c2410_serial1 /dev/s3c2410_serial42 The chip wasn't "booted"
gpioNStdbyPath /sys/class/sec/gps/GPS_PWR_EN/value /sys/class/sec/gps/GPS_PWR_EN/value2 The chip was booted
gpioNResetPath /sys/class/sec/gps/GPS_nRST/value /sys/class/sec/gps/GPS_nRST/value2 The chip was booted

After all, it seems that when the gpsd binary is running without the gps.s5pc110.so library, the chip isn't started (our test utility doesn't work) whereas when the library is running and connects to the socket when it is created by starting gpsd, the chip is booted.

gps.s5pc110.so will actually order bootup via the socket, when the gps is requested by the Android framework. When it's not used anymore, it will request poweroff as well.


According to the logs obtained from gpsd, the chip seems to be using the MEIF protocol at first, then a patch is sent and it starts using another protocol, which doesn't seem related to MEIF according to the logs (there are basically no more references to MEIF after uploading the patch). However, as we have no information about what MEIF is (it's a binary proprietary undocumented protocol), these are just guesses.
We decided to implement the first protocol under the name MEIF, but it could also be some sort of BCM4751-specific bootloader protocol that is in charge of making the patch upload.

The GPSD component is in charge of translating the second protocl to standard NMEA that is sent to the gps.s5pc110.so lib via the /dev/socket/gps Unix socket, created by GPSD.


Here is a list of the devices that are known to use the BCM4751 chip:

Device Vendor BCM4751 revision
Nexus S Google/Samsung 4751A1 or 4751A2
Galaxy S I9000 Samsung 4751A2
Galaxy Tab P1000 Samsung ?
Galaxy Tab 8.9 P7300/P7310 Samsung 4751A2
Nexus 7 Google/Asus ?

The bCM4751 chip exists under the following revisions: 4751A0, 4751A1, 4751A2, 47511A0

Free software implementation

On January 2012, the work to write a free software implementation that could handle the BCM4751 chip was started.
The main target is the Nexus S, even though it should work with few changes on other BCM4751 devices.

The source code is available at: https://git.replicant.us/PaulK/bcm4751/

Current status

Part Status Comments
Serial setup DONE Magic is: termios.c_cflag = 0x800018b2;
MEIF parsing DONE
MEIF dispatch DONE
MEIF patch upload DONE Nexus S and Galaxy S patches differ


Name Task Arguments
bcm4751_gpsd Main utility, boots the chip, send the patch, switch protocol None
bcm4751_test Deprecated utility, can be used for poweroff stop: poweroff the chip
bcm4751_hal Acts as the framework: permits to trace gps.s5pc110.so None
bcm4751_daemon Acts as (a fake) gpsd to the lib None
bcm4751_lib Acts as (a fake) lib to gpsd None

BCM4751 gpsd

This is where MEIF is implemented. It currently does the following:
  • Serial setup
  • Autobaud
  • MEIF reader loop
  • MEIF parsing
  • MEIF dispatch
  • MEIF patch upload
  • Protocol switch (sends unknown bytes in the second protocol to get a response)
  • Response dump

Sample output log:

Turning the GPS on...
Opening the GPS serial...
Sending autobaud...
Read 17 bytes
Read 32 bytes
MEIF message: MEIF_STATE_REPORT_MSG with 18 bytes of data:
[0000]   01 00 00 00 01 00 00 00   00 00 00 00 00 00 00 00   ........ ........
[0010]   1A 00                                               ..
Got a STATE_REPORT message

Read 23 bytes
Read 32 bytes
Read 16 bytes
Read 7 bytes
MEIF message: MEIF_CONFIG_VALUES_MSG with 70 bytes of data:
[0000]   02 00 01 00 01 00 40 00   01 00 02 00 00 00 00 00   ........ ........
[0010]   01 00 02 00 00 00 00 00   00 00 06 00 81 11 00 09   ........ ........
[0020]   07 07 D9 07 42 52 4F 41   44 43 4F 4D 00 00 00 00   ....BROA DCOM....
[0030]   00 00 00 00 34 37 35 31   41 31 00 00 00 00 00 00   ....4751 A1......
[0040]   00 00 00 00 B3 05                                   ......
Got config values:
    vendor: BROADCOM
    product: 4751A1

Sending the first part of the patch...
Sending 2054 bytes!
MEIF message: MEIF_SEND_PATCH_MSG with 2046 bytes of data:

Read 14 bytes
MEIF message: MEIF_NACK_MSG with 6 bytes of data:
[0000]   03 00 03 00 0F 00                                   ......
Got a NACK message

Read 12 bytes
MEIF message: MEIF_ACK_MSG with 4 bytes of data:
[0000]   04 01 0B 00                                         ....
Got an ACK message

Sending the second part of the patch...
Sending 706 bytes!
MEIF message: MEIF_SEND_PATCH_MSG with 698 bytes of data:

Read 12 bytes
MEIF message: MEIF_ACK_MSG with 4 bytes of data:
[0000]   05 02 0D 00                                         ....
Got an ACK message

Ready to switch protocol!
Sending unknown bytes!
Read 12 bytes:
[0000]   FE 00 FD 40 00 00 F1 B1   12 20 67 FC               ........ ..g.

BCM4751 patch

In order to use the same protocol as the non-free gpsd, a patch needs to be sent. It is hardcoded in the non-free gpsd binary.
Note that we don't know what that patch exactly is nor what it does. In any case, it must be considered as the propriety of Broadcom (or Samsung maybe) and falls under the non-free gpsd license.

Here are notes on how to extract the patch from various non-free gpsd binaries:
Device Source GPSD MD5 Offset Length dd command
Nexus S CM 9.0.0 4a6c0027e530b5b8a346153a355ef8e3 0x15DDEA 2738 bytes dd skip=1433066 count=2738 if=gpsd of=bcm4751a1.fw bs=1
Galaxy S CM 9.1.0 4a6c0027e530b5b8a346153a355ef8e3 0x15E89E 6406 bytes dd skip=1435806 count=6406 if=gpsd of=bcm4751a2.fw bs=1

The bcm4751_gpsd utility will attempt to read the patch from /data/bcm4751a1.fw or /data/bcm4751a2.fw

Post protocol switching

Sending this string:
many times makes some other string appear on the serial port...

fe 00 fd 0f ff 07 06 00 00 01 54 fc
fe 00 fd 0f ff 08 06 00 00 01 1c fc

Here's the decoding of the first bytes:
ff00 = 8bytes
fe00 = 12bytes
fe01 = 16bytes
fe02 = 20bytes
fe03 = 24bytes
fe04 = 28bytes
fe05 = 32bytes
fe06 = 36bytes
fe07 = 40bytes

  • fc seem an end marker and 54/c1 a checksum....
  • 07,08 seem a sequence number, if I do it right after booting the chip it starts with 01
run that python program:
print "\xfe\x00\xfd\x6f\x3a\x01\x00\x00\x00\x00\x34\xfc" 

like that
python foo.py > serial.txt

adb push serial.txt /sdcard/

on target:
hexdump -C /dev/s3c2410_serial1

cat /sdcard/serial.txt > /dev/s3c2410_serial1

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