Porsche 928 LH Replacement Rev 2
From Francois Louw
Porsche 928 LH Replacement Revisoin 2 Build Page
The discussion forum can be found at Rennlist
To Do list
Here is where I list all the mistakes I need to fix and the optimisations that needs to be done. This will focus primarily on the hardware side of the board because the software will in the end be 99% the same as the prototype version.
I will mark what is done as I complete it.
- Remove LM317 and remove intermediate voltage level
- Fix power supply to op-amps
- Add buffering to analog input signals
- Add more filtering to input analog signals
- Use a divide by 15 * multiply by 3.3 to ensure the input analog signal at CPU pin does not exceed 3.3V
- Fix USB power line
- Replace USB connector
- Make board smaller
- Add 25-pin connector support
- Add aftermarket connector support
- Add more diagnostic LEDs
- Remove serial line
- Add extra digital and analog inputs for engine mods
- Add I2C/SPI interface for future expansion
- Add battery voltage analog line
- Remove external EEPROM lines
- Add header for bootloader
- Improve Silkscreen labels
- Optimise layout for faster component placing
- Add pull-up resistors to TTL signals (EZK Load and Warning Light)
- Fix protection diodes
- Add MAF-Burn function at engine shutdown
- Add test-points
- Add mounting holes
- Change to switching power supply
- Change to surface mount protection diodes
- Redo FET driver circuitry
- Change serial port emulation to HID usb communication
Updates done so far 1 April 2013
SO far I have done major reworking to the second revision of the PCB. All the changes that has been made are marked on the to do list in italics and the still to be done is marked in bold.
Most of the to do list is done. Here are a bit more detail on some of the changes that has been made.
3.3V supply: The supply to the processor has been changed to use a switching supply. The main reason for this change was that the intermediate voltage level of around 8V was no longer used, meaning that using a linear supply to drop straight from 12V to 3.3 was not practical. A simple TI switching circuit is now used.
Analog Inputs: The analog signals has been completely changed and cleaned up. Now there is proper filtering, and the input voltages are managed better allowing better use of the A/D Converter. The changes mainly result from dividing the input voltages by 15 and then multiplying up to a maximum of 3.3V. This will yield more accurate sampling.
FET Drivers: The FET driving circuitry has been optimised. Previosuly a simple NPN transistor was used to pull down the Gate of the FET. Now a proper NPN-PNP pair is used. This results in slightly more ucrrent being drawn, but will yield less heat. The previous circuit heated up the pull up resistors quite a bit.
MAF Burn: The MAF burn has been fixed. Now when the MAF burn signal is sent, the MAF burn will only happen after engine has been shut down. The prototype revision was not able to do this, resulting in no MAF burn feature.
Other Changes: the other changes are general and upon request I can explain in more detail what was done.
After more testing with prototypes I will make a batch of the next revision boards (around 10) and then text to make sure that there are no errors.
Upadtes 3 April 2013
Ok, so almost everything is done one the PCB. I require the pinout and PCB footprint of the 25pin connector for the S3 generation.
I added mounting holes as well as an external bus that can be used for future communications with some other device (Hint, I am thinking of making a carputer that will plug in to the ECU directly)
Also I need to find a replacement male/female connector set that I can use on the PCB. This will be useful if the connector of the harnass is worn out, and need to be replaced.
I think I will try to find a connector that can fit on either the 25 or 35 pin footprint to make life easier.
Revised 20 Apr 2013
Of, here are a few of the things done:
- Board Shrinked
- Mounting Holes Added
- Battery Voltage Analog Signal
- FET Driver Schematic Updated
- Switching Power supply
Here is a low quality image of the layout.
Soon: 25 Pin connector and aftermarket connector.
I have found good connectors from Phoenix Connectors that I like. I will be putting the footprint for a 32pin connector on the PCB sometime. I decided to use a 32pin connector because that suits my PCB layout the best. I will try to make the pinout as close as possible to the 25/34 pin layout to make the change easier.
I shrank the board quite a bit. It is now 123mmx100mm. This will decrease the board cost and will make it easier to fit into aftermarket enclosures. I have found some really good enclosures for PCB up to 135mmx100mm.
A change to surface mount protection diodes reduces the amount of through hole components that I need to solder on.
A switching power supply is made because it will increase efficiency and lower the heat generated. I also dont need an intermediate voltage for the FET drives, meaning that I only need 12V and 3.3V rails.
After the connectors are sorted then the PCB should be finished!