Digital Chronometer

Hey! I have a new Digital Chronometer from a Airbus A320, which is similar to the used in the Boeing 737.

The Unit...

Brand: Smith Industries
PN: 2610-13-1

Likely this unit doesn't work with AC power source, so a normal low DC power supply can be used between 24 and 28Vdc.

Lets see inside...

Here you will see the power supply of the device.

This board is providing 5V to the digital bus and aprox -24V (negative) to drive the LCD display.

And this is the processing unit.

Perpendicular to the processing unit and the power supply there is another board that has all the drivers of the LCD display (CD4056B).

This device is also providing an Arinc 429 output with all the information regarding the UTC time, Chronometer Information and Date.

Later I will evaluate the proper Arinc Labels that this unit is sending!!

Repare process...

The first time that I powered up this unit nothing was happening. I checked the power supply and was working nice and the microcontroller (MCU) was receiving the proper power feed.

After further checking I could see that the RESET signal of the MCU was active as pulses all the time, which is not ok... that means that the MCU is being reset periodically not giving it the chance to execute the code inside.

I did a schematic of the reset circuity and it was clear what was happening.

The signal /RES_IN is one pin of the TL7705 which is a Supply Voltage Supervisor used to manage the RESET condition of the MCU.

The circuit above is a very simple oscillator that generates RESET pulses.

If the MCU starts properly, the firmware is going to clear the proper pin which is going to produce the reset oscillator to stop, so then the circuit stops generating RESET pulses.

All of this was working properly, so then after this I could find the mayor issue... the main oscillator of the MCU was not working!!! so that means that the microcontroller was not able to execute any code.

A picture of the broken part:

So I couldn't find the original part, but this oscillator has a standard oscillation frequency (3.6864 MHz), thus it was easy to find a replacement.

Here you can see the board with the new part:

Due the fact that the footprint of the new oscillator was quite different I had to place it in other location... even better because it is closer to the MCU... kidding!

The Pinout...

So this is what I got after tracking the boards:

The Unit working...

First power up looking:

At this point the device is expecting that we introduce the first UTC data in order that the device can start working.

Chronometer function:

This feature starts pressing the button CHR ones.

If this button is pressed again the chronometer stops... pressing a third time will reset the counter.

Unit in normal operation during the flight:

The down-left switch is set in RUN position, that means that the Flight Timer is running, and this value is being shown in the ET display.

A quick video showing how that works :)

Enjoy!!