Ready, set, optimize!

Alright, finally I had time to grab my solder iron and punish the naughty GSM shield for being such a greedy pig when it comes to efficiency in power consumption.

When we look more closely at the shield schematic, we can see that NETLIGHT and STATUS pins of SIM900 control the transistors which turn on and off the leds, powered from Arduinos +5V input via 300 ohm resistors R21 and R20. There's also the "shield powered" led that is always on when, as the name implies, the shield receives input power, regardless whether the GSM module is powered or not. Very wasteful when thinking about power efficiency, so I started with that first:

Actually I haven't ever done any operations on such a small SMD components before, so after unsoldering the R1 the end result was quite far from satisfactory: both copped pads under the resistor were ripped off because of too high temperature, excess force and prolonged heat exposure. Oh well, not that I need the "shield powered" LED anyway..

Before removing the R1 the power consumption of the SeeedStudio GSM shield was 12.2 mA (shield powered, but SIM900 not turned on) and 23.4 mA (SIM900 in sleep mode) when powered with external +5V lab supply (HP 6632B). After removal of R1 and thus disabling the LED, power consumption drops to 9.1 mA and 20.3 mA, respectively. This brings savings of 3.1 mA. Nice.

Wiser from this incident, I practiced desoldering a little bit with old PC motherboard and found a suitable removal procedure consisting of adding liquid flux, new solder and then removing everything with a solder wick. Then add little bit more solder to both ends of the resistor, heat one end with solder iron while lifting it with tweezers. Then repeat this with the other end and finally the resistor can be removed.

 I removed the R20 and R21 resistors and soldered a breadboard jump wire to bypass the LEDs. This time no solder pads were harmed and thus the procedure is reversible. The transistors Q3 and Q4 now connect our NETLIGHT and STATUS wires to ground when signaled by SIM900, making it posssible not only to put the GSM power and/or activity leds on the enclosure if needed, but also it's now easy to check the power status of the GSM module. Before this, the only way to see if the module is powered was to send AT commands and wait for the answer. Actually this was one of the things that annoyed me about this shield from the start. Luckily now only one digitalRead is needed.

It could have been possible to remove the transistor and tap into the NETLIGHT and STATUS pins of the SIM900 directly, but leaving them behind transistors is safer since the module uses lower voltage than rest of the device. It could have been possible to destroy the whole chip or at least the output pins with software by simply switching the pin mode on Atmega side to output and signaling +5V. Puff, there goes the magic smoke.

Since there isn't a nice way to attach strain relief to the jumper wires,  I used hot glue (again) to attach the proximal ends to the PCB to make sure all the vibration and shaking in the car doesn't cause the cables to come loose.

Now, after bypassing also the STATUS led and rewiring the NETLIGHT led (blinks once every 2800 ms, so not a concern in this case), I've managed to reduce the sleep mode consumption from 23.4 to 10.2 mA! Those LEDs really are gluttonious beasts! Then I tried inputting +12V onto the shield directly via its "low ground current, low-dropout voltage regulator", the MIC29302. Low ground current my ass. Increasing the voltage from +5V to +12V increases the quiescent current (=ground current) linearly from 10.2 mA back to 21.8 mA! And that is even without powering the GSM module! So, in this case it is better to use the 7805 to drop the voltage from +12V to +5V and distribute that to the GSM shield. If someone knows better alternatives, I'm all ears.

All in all, the ChiliCAN now consumes 14.3 mA when all the chips are sleeping (i.e., when there's no CAN traffic and no calls are being made). That constitutes maybe 90% of the time. When the heater is on (or otherwise CAN bus is active), the consumption wanders between 30 and 55 mA. During phone calls it lies around 100-200 mA having maximum peaks of 1.5 A, but those are quite short bursts, having almost nil effect on battery life. If we assume average consumption of 17.2 mA, it would take almost 4 months for ChiliCAN to deplete my 95 Ah car battery to 50% charge. I could live with those numbers :)

Debugging in -25C...

Howdy!

The name of this post kind of says it all. Sitting in the car with a laptop and tapping on the keyboard while exhaling visible moisture which might make me look like a living steam engine. Trying to fix relentlessly the few remaining bugs, hoping that the laptop battery would last long enough, even though part of me wishes that the laptop would generate more heat.  At least the parking heater works!

For the past few days I've been building the remote heater starter and finally ChiliCAN v0.1 was born! It resembles little bit those implementations that I wrote about in the first post, but don't let the version number confuse you. This one speaks CAN and has multiple additional functions in addition to it's raison d'être: flipping on the relay and making the car a warm haven amidst the hostile environment that the Northern Finland is now at this time of the year.

Let's see what the little controller box has eaten: 

Ok,  it's not going to win any beauty contests, but it does what it's supposed to do.

The schematic is quite similar to what Arduino and the CAN shield would be when combined together. On the left is a separate USB breakout board, since it is not needed unless there is a need for debugging, giving manual test commands via the COM port or re-flashing the firmware. So the list of components is quite familiar: Atmega328p, MCP2515 CAN controller, MCP2551 can transceiver. A relay for grounding the AEM control pin (gives better isolation than grounding it directly via a transistor). The 7805 constant +5V voltage regulator + few passive components. Under the ribbon cable is also a SPI header for easy access to debugging with oscilloscope or maybe BusPirate (actually I ordered it a while back, but haven't yet tested it).

There's a extra 5A car fuse, isolated and kept in place with a hot glue, but now it's kind of deprecated since I have a main fuse outside the case for extra safety. Also the connectors are insulated with hot glue, and that with the IP65 protected case should give ample protection against moisture.  I still have to spray the board with a coating of protective lacquer, but that's only after extensive testing.

Then there's the female headers, resembling little bit the headers in Arduino board. And what's with the antenna?

Yup! That's the Arduino GPRS shield based on the SIM900 chip. I didn't have guts to start dismantling it, so I installed the Arduino type headers and laid the shield on top of the main board. I didn't want to use the built-in SMA-connector on the right side of the shield since that would cause both placement problems as well as increase my worries of poor insulation. So I used a left over WiFi SMA->U.FL extension wire from the Asus Maximus V Formula motheboard that I recently installed. I didn't have any use for desktop WiFi, so why not put the nice cable to use. Actually quite many of the parts here are left-overs or parts that have been recycled from old devices. There was also a long WiFi antenna included with the mother board, but I decided to use the one shipped with the GPRS shield.  

Sadly after preliminary testing I had to conclude that the Arduino+GRPS shield+CAN shield combination isn't going to work so nicely for two reasons. Separately they work quite fine, but all the combined code requires more memory than the measly 2Kb that theAtmega328p has to offer. This resulted in numerous random crashes, weird behaviour or just lack of functioning. This problem could be solved with optimization tricks, such as better memory utilization by removing unnecessarily large parameters and variables, and moving some parts out of SRAM to flash memory etc. 

However the much bigger problem would emerge when looking at the power consumption. Even when the sleep mode was enabled (in which text messages and calls could still be received) , the total amount of juice sucked by the device was around 80-90 mA. Although the sleep functionality of Atmega and CAN controller are not yet utilized by my code, the portion of power consumption by the SIM900 is almost half of the total even though it is in sleep mode ! Maybe I'm doing something wrong here, since according to the specs the GPRS module should consume only around 1-2 mA when dozing. 

For now as a heater start signal I will be using the yellow light button on the wireless key fob. Luckily the parking spot for my Volvo is almost right below my apartment window, so it is in the wireless range of the key fob and I will be able to use it to start the heater from the warmth of my house :) 

Pressing the light button 4 times (turning the car interior and exterior lights on-off-on-off in a time period of 10 seconds will start the heater. Turning it off is done in the same manner. I can also check the heater status with the fob: When pressing the light button  (regardless whether the sequence ends up in starting to heater or not), the car headlights are blinked 1-4 times. One blink means heater is off, two blinks: heater is either starting or stopping, three blinks: heater is on, four blinks: there is an error condition.

Here's the device ready for testing! Leftmost green led is for status. Next (yellow) is CAN traffic indicator (actually it doesn't blink for every CAN frame, since then it would just stay lit when connected to the car because of the huge amount of CAN traffic going on all the time. Instead, it blinks only when sending messages or receiving ones that have some inherent importance to us.).The third LED is a heater status indicator and fourth one is an error LED. I might later print labels and other stuff on the case with silk screen technique.

Here is the wiring harness for extending the AEM connector, nicely sleeved for making it look less like a botch job for what it really is. I tried for quite many hours to find suitable male/female connectors compatible with the 4-pin one that AEM and its harness has, but without any result. So I ended up having 4 separate connectors, one for each AEM pin. 

Then there are two DB9 standard CAN connectors. Leftmost has 12V connected to its pin #9 and is meant for connecting the ChiliCAN, and the right one is an extra debugging/tracing/logging CAN cable carrying only CAN-L, CAN-H and ground wires. Then there's extra fuse housing and the screw terminal to put the wires together. That + numerous cable ties give pretty good strain relief. 

I have to remind you that this is still going to be temporary installation as I may not even keep the AEM beyond the first few days of testing, so this not-so-elaborate setup will have to do for now.  Ok, so maybe I'm going to keep these in place until when outside temperatures are easier to count in celcius degrees than Kelvins.

Here's the final setup. Installing these separate dangling wires to AEM connector with  "relative safety" requires either that the negative wire from the car battery is detached or at least that the AEM fuse is removed. Also good electrical insulation tape is necessary. Did I mention botch job?

Later I might bypass the AEM connector altogether and connect the ChiliCAN directly to the Rear Electronic Module (REM) where the AEM wiring is originated.

Inspired by the stuff the guys at the Swedish Volvo forum have been doing, I too decided to add menu feature to the ChiliCAN. The menu can be accessed by keeping the cruise zero button pressed for two seconds.  The +/- buttons are used in navigating and another zero button press makes a selection. The return button exits the menu.

Using cruise buttons doesn't interfere with the normal cruise functionality unless we have turned it on before entering the menu. And even then the cruise buttons work normally and are not hijacked in any way.  Another, more familiar way of using the DIM menu would have been with SWM control stalk, but then we would have been forced to hijack the control stalk and I'm not sure if that can be done via CAN bus.

Heater can be turned on and off via the menu. Here it's status is shown (off / on / starting / stopping / error). It's handy to see the battery voltage without having to go crawl under the hood (or in case of S80, the trunk) with a multi meter. The various bits of information are queried by the ChiliCAN from CEM via diagnostic messages.

 It can be quite informative to see the coolant temperature here, since it gives rough estimation of how cold the engine is and whether you should start the heater. This makes it much easier to do rough guesses of how long should you keep the heater on to get the cabin and the engine comfortable. And of course, this isn't limited to the diesel/petrol parking heater, but could be used in conjunction with any additional heaters. When my Ardic broke down a while back, I got an electrical block heater (coolant heater) installed, but I wasn't (and still I'm not) quite sure how effective it is in hard metrics. With ChiliCAN I can now easily be able to see how well (or badly)  it really works.

One feature to tinker in the future would be the enabling of the water pump in Ardic in conjuction with the electrical coolant heater. That would give more even spread of the heat, since the block heater doesn't have its own water pump and is just heating one part of the coolant circulation, hoping that the difference in thecoolant temperature itself moves the liquid around a bit.

 Here we can see the GSM module status. Code isn't included in the firmware yet, since the memory optimization hasn't been done yet.


There is still quite a lot of things to do, but priority is now in lowering the energy consumption. After that, I might start thinking about having features little bit like what Volvo has to offer in their newest models: Volvo On Call mobile app :)