Adding a GPS Module (hat/shield) on a Raspberry Pi

[Ralph Smith]

I’ve gotten this working on various flavors of the Raspberry Pi, just now verified this on the Pi Zero and on the Model B. Comments below

> On Feb 3, 2018, at 6:15 AM, Shamim Shahriar <shamim.shahriar at gmail.com> wrote:
> 
> Good morning all
> 
> I had been searching for a solution for the last 3 days, and tried various different things based on the google results, but so far no luck. Hope someone from the list can shed some light and maybe point me to the correct direction.
> 
> Here is the scenario:
> 
> Device: Raspberry Pi B (pi 1 model b, the 512M RAM version, or more precisely v1.2)
> GPS Module : Ublox hat that sits perfectly on the Pi, with PPS on GPIO-18
> FreeBSD version: 11.1 RELEASE for RPI-B

Just built and verified on the following:
Device: Raspberry Pi B (pi 1 model b, the 512M RAM version, or more precisely v1.2) — same board
GPS Module: Adafruit Ultimate GPS HAT - connected to UART, PPS is on GPIO 4
FreeBSD Version: FreeBSD 11.1-STABLE #0 r328620 — downloaded from ftp.freebsd.org <http://ftp.freebsd.org/>

This also works with custom builds built using crochet.

> As I understand, the serial ports on the GPIO is activated by default, and if I attach "ONLY" a serial console (USB to serial), I can interact with the Pi through the console. Remove serial console cable, attach the Pi to a display (via HDMI), boot it up -- it works just fine.
> 
> If now I add the GPS module/hat/shield, all I get is series of dots (depending on how soon the GPS finds the satellites and starts sending the data) and the Pi does not boot at all.
> 
> Based on
> https://forums.freebsd.org/threads/59485/
> https://lists.freebsd.org/pipermail/freebsd-arm/2017-March/015845.html
> https://vzaigrin.wordpress.com/2014/12/13/liberation-of-the-serial-console-in-the-freebsd-on-the-raspberry-pi/
> 
> I have managed to get to the point where I can see that if I have only the serial console cable attached, it stops interacting with the boot process after a while (the boot continues, and I can see that on the HDMI output). But that also stops interacting with whatever signal is being sent via that console).
> 
> And if I attach the GPS module now, it refuses to boot and drops me to a loader> prompt
> 
> My understanding (and I might be very very wrong) is, the u-Boot system is now being affected and hence cannot boot.

You are precisely correct, u-Boot is starting and seeing traffic on the serial port and happily thinks you are trying to tell it something.

> I am not new to FreeBSD, had been using it since the 4.4BSD Lite days, but I am seriously baffled by how difficult (nearly impossible) it had been to get rid of the serial console (something so easy and taken for granted on the i386 or other similar architecture). One of the links I found (https://www.cryptomonkeys.com/2014/01/freebsd-raspberry-pi-gps-ntp/) even suggested that I need to create custom image to get PPS working. Well, working PPS comes *after* being able to boot the device with the GPS hat/module/shield on -- and I am still stuck at that level.

I derived the following based on hints from:
https://vzaigrin.wordpress.com/2014/12/13/liberation-of-the-serial-console-in-the-freebsd-on-the-raspberry-pi/ <https://vzaigrin.wordpress.com/2014/12/13/liberation-of-the-serial-console-in-the-freebsd-on-the-raspberry-pi/>
https://github.com/BobBallance/freebsd-gpio-pps/wiki/Regaining-Control-of-the-UART <https://github.com/BobBallance/freebsd-gpio-pps/wiki/Regaining-Control-of-the-UART>

First, you will need to build a version of u-Boot that leaves the serial port alone. From a current ports tree you will build sysutils/u-boot-rpi, but with a few changes enabled. The sysutils/u-boot-rpi port is based on sysutils/u-boot-master. I have created the following patch files for the sysutils/u-boot-rpi port. First files/patch-configs_rpi__defconfig:
--- configs/rpi_defconfig.orig	2018-02-03 16:08:52 UTC
+++ configs/rpi_defconfig
@@ -34,3 +34,4 @@ CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT_OVERLAY=y
+CONFIG_SILENT_CONSOLE=y

and also patch-include_configs_rpi.h
--- include/configs/rpi.h.orig	2017-10-16 17:46:46 UTC
+++ include/configs/rpi.h
@@ -103,10 +103,21 @@
 
 /* Environment */
 #define CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
+#ifdef	CONFIG_SILENT_CONSOLE
+#define	CONFIG_SYS_DEVICE_NULLDEV
+#define	CONFIG_SILENT_CONSOLE_UPDATE_ON_RELOC
+#define	CONFIG_SILENT_CONSOLE_UPDATE_ON_SET
+#define	ENV_DEVICE_SETTINGS \
+	"stdin=nulldev\0" \
+	"stdout=nulldev\0" \
+	"stderr=nulldev\0"\
+	"silent=1\0" 
+#else
 #define ENV_DEVICE_SETTINGS \
 	"stdin=serial,usbkbd\0" \
 	"stdout=serial,vidconsole\0" \
 	"stderr=serial,vidconsole\0"
+#endif
 
 /*
  * Memory layout for where various images get loaded by boot scripts:

Build u-boot using this configuration and place it on the SD card in the MSDOS partition (/boot/msdos). This will keep u-Boot away from the serial port.

Now we need to convince the kernel to do two things: 1 - don’t use the serial port as the console, and 2 - recognize the PPS attached to the GPIO pin. This is done by modifying the FDT. From a machine with source loaded, edit /usr/src/sys/boot/fdt/dts/arm/rpi.dts. My changes are:

--- rpi.dts	(revision 328830)
+++ rpi.dts	(working copy)
@@ -345,6 +345,12 @@
 		};
 	};
 
+	pps {
+		compatible = "pps-gpio";
+		label = "pps";
+		gpios = <&gpio 4 0>;
+	};
+
 	power: regulator {
 		compatible = "broadcom,bcm2835-power-mgr",
 			     "broadcom,bcm2708-power-mgr",
@@ -391,8 +397,10 @@
 
 	chosen {
 		bootargs = "";			/* Set by VideoCore */
+/*
 		stdin = "uart0";
 		stdout = "uart0";
+*/
 	};
 
 	__overrides__ {

You would want to change the 4 to 18 above to reflect the connection of your PPS. Then build the DTB on your build machine by 
# cd /usr/src/sys/tools/fdt/
# setenv MACHINE arm
# ./make_dtb.sh /usr/src/sys /usr/src/sys/boot/fdt/dts/arm/rpi.dts .

Copy the resulting rpi.dtb to two locations: The MSDOS partition (/boot/msdos) and to /boot/dtb/

Finally to get the PPS loaded and recognized at boot time add 'gpiopps_load=“YES”’ to /boot/loader.conf
. The PPS will then be available at /dev/gpiopps0

Hope this helps.

Ralph