I have a 2006 Mazda 3. It's a nice car, but the more I learnt about the 3 I get ever so slightly more annoyed with Mazda Australia. There's multiple things that the car "supports" but wasn't sold with in Australia.
One of them (and the easiest of which to enable) is DIS - Driver Information System.
DIS lives on the left side of the LCD above the radio - it's the clock by default. It can display the following:
- Current Fuel Consumption (Averaged over the last few seconds)
- Average Fuel Consumption
- Average Speed
- Remaining Distance (based on fuel in tank and average consumption)
There's an official way to enable this - Hold down a few buttons, turn the key, and it's on ! Unfortunately this only works for a few 3s, and not the base trims, or the later 2006 models (BKs2).
So if you can't follow that guide, or are curious to see how it works I'll explain how that button combination turns it on.
The Mazda 3 uses a CAN bus - this is a network not unlike a conventional computer network (think blue cables) but with a few differences:
- It's a bus - there's no "Hub" or "Switch" - every device on the network is directly connected to each other via a common set of wires. Shorting this bus will stop any device from talking to another.
- There's no "Source" or "Destination" - only an ID, a flag or two, the data, and then a checksum.
The Mazda 3 has two of these buses - one is the "High Speed" bus for the Engine, ABS, Traction Control, Transmission, and Instrument Cluster computers, and the other is the "Medium Speed" bus in the cabin, for the LCD, Radio, Body Control (BCM - door locks, windows etc), Climate Control, and Instrument Cluster computers. The Instrument Cluster will pass some messages from the Engine network into the Cabin network, and may also do some passing the other way.
I'm pretty sure that the DIS is controlled either in the BCM or Instrument Cluster. Regardless, when you are holding down the Buttons, the Radio sends a message onto the Cabin network, and that somehow tells both the LCD and something else that DIS is now on. (from my testing this setting is not stored in the screen, as the screen does not seem to have any permanent memory and forgets everything on power loss, but I can't see any other message responding to the button press)
Messages sent regarding DIS
From both a combination of reading Madox's handy reference  and using my home made Arduino CAN tool  I was able to figure out the following messages relating to DIS and the screen:
- 0x28F: A formatting and misc message for the screen, sets locations of colons and commas, does the CD/ST etc fixed segments on the screen. Also is the state of the buttons on the face of the Radio.
- 0x290: The first 7 characters of the LCD
- 0x291: The last 5 characters of the LCD (depending on the other values, it might be possible to implement scrolling)
- 0x400: The trip computer data above. Funnily enough this data is present regardless of the state of DIS.
- 0x401: Beeps emitted by either the Radio or LCD for the Radio to beep through the main speakers.
When you're holding down CD and AM/FM, the expected combination of bits are flagged in 0x28F's fifth byte (as in, bit-wise the correct bits are 1 while the buttons are being held). However, after the time out occurs, this byte switches to the DIS byte:
0x28F:128, 0, 0, 0, 32, 0, 0, 0 #Radio Face Idle
0x28F:128, 0, 0, 0, 160, 0, 0, 0 #DIS ENABLE
The Radio then writes the "DIS ON" message to the LCD, with 0x290 and 0x291
The LCD responds with an 0x401 message for the beep.
DIS is now enabled. As you can see, I've not found any evidence that the LCD and something else communicate during this process, so I presume that the LCD and BCM/Cluster both read the DIS ON message, and the BCM/Cluster remembers and tells the LCD when necessary.
The SET and CLOCK keys are similar with SET being 0x28F = 40 (+8) and CLOCK being 0x28F = 48 (+16)
The key is however, that the "Radio Face Idle" message MUST BE SENT AFTERWARDS - as these buttons are held to reset the clock/DIS, they only trigger off the release (falling edge) of the button.
- If the original Radio did not permit you to enable DIS through the button trick, the Radio will continue to block DIS usage until ACC power is turned off, and the Radio is removed. Probably something about the initial 0x28F message.
- The MS CAN bus is easily accessible by pushing breadboard cables into purpose designed holes on the harness-side plug for the LCD.
- There's a nice little hole above/behind the centre vents that you can stash your micro, if you wish to leave it there permanently. You can run a USB cable down behind the LCD, Radio and HVAC controls, and then out through the cigarette ash holder to allow easy debugging.
- You can switch the units used on DIS by using the following method:
- Enter test mode on the Instrument Cluster (Hold the Trip reset button, and then switch the key to ON)
- Press the trip button until the IC LCD is on page "60" and is showing "DIS"
- Hold SET to switch the modes (It depends on which page you are on - just hold SET for ~5 seconds per page until the units switch)