Sunday, 26 March 2017

i.MX6SX - Pulse oximetry with MAX30100, SSD1306 and UDOO NEO

In this blog I demonstrate how the cortex m4 on the imx6sx could be used to develop a Pulse Oximeter.

I chose the max30100 because its features an integrated pulse oximetry and heart-rate monitor sensor. Unfortunately Maximi provide little documentation or application notes covering how best to determine heart rate or spo2 values using the data returned from the sensor. The sensor hosts an IR and RED LED that can be adjusted to provide pulses for spo2 and heart rate measurements. As a trade off between adequate data samples, i2c bus speed and post measurement hr/spo2 calculation processing the max30100 was configured to return 100 IR and RED values per second. Below is a graph showing raw data values gathered from the NEO at a 100 readings per second.

For spo2 calculation we have taken a simplified approach of taking the AC components of both signals and determining the ratio. The ratio is referenced in an memory table containing empirical sp02 values. Hence the spo2 isn't clinically accurate, for greater accuracy the table would normally be based on experimental measurements from healthy patients cross referenced against clinically accurate readings.

As shown in the graph the IR values are smoother than RED values possible due to secondary emissions from the RED LED. For this reason the IR value are normally used to determine the heart rate. The heart rate is calculated by feeding the IR values in to a first order 6Hz low pass filter which in turn are used to calculate the time interval between 2 peaks. Sample output of applying the low pass filter is shown below ignore the graph labels, top is IR values, bottom is low pass filter.

I also hooked up a  SSD1306 oled display to the same i2c bus so that the calculated heart rate and spo2 values are displayed. The main challenge of this exercise has been to be ensure the code running on the M4 is as efficient as possible because there are many time critical elements such reading data samples, hr/spo2 calculation and display updates which can interfere with the output results. As with my previous examples on the imx6sx this was developed using the FreeRTOS sdk.

Tuesday, 24 January 2017

Adding a DS3231 Real Time Clock to UDOO NEO/QUAD

Its well know that the in-built RTC on the imx6 processor isn't the best in terms of battery life (performance).  Using an external RTC provides better battery life and fortunately the process isn't too complicated implement. The DS3231 is a popular RTC especially with the RPI community given ease of integration (via I2C) and accuracy. There's a few variations of the DS3231 for the RPI and the one I using is the one below which can be easily sourced.

In the image I have highlighted the pin out to simplify wiring. I'm going to take the UDOO NEO as a example and use I2C4 (alternatively you can use I2C2). For I2C4 wire SDA to pin 35 and SCL to pin 34 on header J5, 3.3v and GND are available on J7. On power up you can verify the DS3231 is visible by executing:

udooneo:~# i2cdetect 3

which should return the DS3231 at address 0x68.

WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-3.
I will probe address range 0x03-0x77.
Continue? [Y/n] Y
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- UU --
20: UU -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Next step is to enable kernel support by enabling the Dallas/Maxim DS1307 driver as below.

Build the kernel and modules (this is important). Lastly we need add the DS3231 to the device tree to I2C4, below is an example,

diff --git a/arch/arm/boot/dts/imx6sx-udoo-neo.dtsi b/arch/arm/boot/dts/imx6sx-udoo-neo.dtsi
index abbf0d8..2ffa6cb 100644
--- a/arch/arm/boot/dts/imx6sx-udoo-neo.dtsi
+++ b/arch/arm/boot/dts/imx6sx-udoo-neo.dtsi
@@ -298,6 +298,11 @@
                compatible = "fsl,fxas2100x";
                reg = <0x20>;
+       rtc@68 {
+               compatible = "dallas,ds1307";
+               reg = <0x68>;
+       };

Rebuild the relevant dtb file depending on your set-up. Deploy the newly generated kernel, modules and dtb to the NEO.

On power up the kernel output should include the following lines ( try dmesg | grep ds1307)

[    8.095963] rtc-ds1307 3-0068: rtc core: registered ds1307 as rtc0
[    8.095989] rtc-ds1307 3-0068: 56 bytes nvram

If all is ok we can query the clock for it current time by using the hwclock utility:

udooneo:~# hwclock -r
Tue 24 Jan 2017 12:32:25 PM UTC  -0.858087 seconds

We can sync with the ntp time:

udooneo:~# hwclock -s

On reboots the RTC time may become corrupt with the udooubuntu release to overcome this ntp service needs to be disabled with the following commands:

echo manual | sudo tee /etc/init/ntp.override
timedatectl set-ntp false

The timedatectl command is extremely useful as it provides a complete picture of the system and rtc times. For example to sync RTC with system time:

udooneo:~# timedatectl
      Local time: Fri 2016-01-01 01:18:06 UTC
  Universal time: Fri 2016-01-01 01:18:06 UTC
        RTC time: Tue 2017-01-24 12:40:36
        Timezone: Etc/UTC (UTC, +0000)
     NTP enabled: no
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a
udooneo:~# hwclock -s
udooneo:~# timedatectl
      Local time: Tue 2017-01-24 12:42:03 UTC
  Universal time: Tue 2017-01-24 12:42:03 UTC
        RTC time: Tue 2017-01-24 12:42:03
        Timezone: Etc/UTC (UTC, +0000)
     NTP enabled: no
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

Wednesday, 18 January 2017

i.MX6SX - Prototype VW (VAG) vehicle diagnostic adapter for KWP2000 services (UDOO NEO)

An interesting use case for the i.mx6sx is as vehicle diagnostic (or interface) adapter. In this blog I will demonstrate how we can re-purpose a UDOO NEO as a prototype diagnostic adapter.

The adapter targets VW vehicles supporting KWP2000 services. Typically an adapter requires a real time interface to the vehicle in order to keep the diagnostic session alive after establishing communications with an ECU.  The real time needs can easily be met on the M4 and we can exploit the A9 side to offer data transformation and API services, for example to make the data available a Mobile application. The end goal is demonstrated in the video where a custom developed Android Application retrieves diagnostic information from the vehicle in real time. The application communicates over Wifi with the NEO which in turn is connected to the vehicles OBD-II port.

The application is first used to query the vehicle for a list of available ECUs (modern vehicles can contain tens of ECUs). For each ECU the physical address and overall status is displayed (OK or has DTC errors).

Subsequently after selecting an individual ECU the application retrieves information about the ECU including the short/long coding value (if applicable).

Although the video demonstrate a few KWP2000 services being invoked, its actually possible to invoke most if not all of the services available. Furthermore it could be enhanced to support UDS services.

At a hardware level VW KWP2000 is supported through CAN (some older vehicles use K-Line) and is accessible through the vehicles on board 16-pin OBD-II connector. For the prototype (in the photo to right) the NEO is simply connected to a SN65HVD230 which in turn is connected to the CAN pins of the OBD-II connector.

Typically VW KWP2000 services are supported over VW's proprietary TP2.0 protocol. TP2.0 is used to establish a session and pass datagrams between 2 ECUs, one of which is our case is the NEO and normally referred to as the 'tester'. Implementing TP2.0 is a challenge as accurate timings are required to correctly deal with time out and error scenarios in addition to implementing logic to cater for varying ECU behaviours depending on their age.  Above TP2.0 is the KWP2000 protocol which implements a simpler request response model. As shown the diagram below a complete TP2.0 and KWP2000 stack was developed to run on the M4.

On the A9 side KWP2000 services are exposed through a custom API interface that when invoked communicate with the M4 over a bidirectional link. The A9 allows the data to be enriched and transformed eg XML/JSON before being exposed via a number of network interfaces such as bluetooth, wifi or even ethernet. For the demo its done by enabling a  Wifi Access Point on the NEO. For those sharp eyed readers you will notice the prototype uses a NEO Basic which has no on board wifi support, instead a wifi dongle was plugged into the USB port to create the Access Point.