1. Sarge – single board computer

At the end of past year my friend David asked me if I could make for him an electronic device for flower watering. The first thought that came was: what a lazy guy 😉 and the second: damn, I forgot about my own flower. One of main feature of this device suppose to be a remote access and control over WiFi so I decided to build a SBC (single board computer) as an universal module for any purpose. It will be running my favorite operating system – linux. A proper software with additional hardware could work as flower watering device. It seems that SBC is way too much for this easy purpose but I’ve found it a quite interesting and educational challenge.


1. Choosing main CPU.

After few hours of searching in web the choice became more clearer and obvious when we come to the point: what kind of electronics parts in single quantity you can buy in Poland if you are not a great, good known and prospering company.

The choice was AT91RM9200 chip from Atmel Company. It’s main features are:

  • ARM9 core,
  • SDRAM controller,
  • SD/MMC controller,
  • Ethernet MAC (MII),
  • 3 xUART’s, 1x UART for debug,
  • 2 x USART,
  • USB host and device,
  • 3 timers and 4 external interrupts sources,
  • full EBI bus – for example for CF card

2. SBC module parameters .
Here are few, most important parameters/features of this SBC:

  • CPU: AT91RM9200,
  • 32 MB SDRAM,
  • SD/MMC card controller onboard,
  • Ethernet (using STE100P PHY chip),
  • 2MBytes of DATAFLASH (for kernel or second stage bootloader),
  • EEPROM for storing useful parameters like ethernet MAC address.
  • uart connector for DEBUG and uploading bootloader,
  • USB host – I’m planning to run WiFi on this,
  • extension connectors – part of EBI – address lines, data WR/RD etc, SPI, I2C, USB Client.

3. Schematic and PCB design – using web resources, others people work and published projects after 2 months of daily 2-3h spare-time work I have an final version of shematic and PCB. The software used for design was Eagle from Cadsoft.

I will publish it later when I get finally the PCB and solder all the parts. The PCB (double sided and lots of SMD components) is on the way – made by external company, I have no doubts that I could not do this myself.

current version of PCB (09.02.2007):

SBC pcb

 

 

 

 

 

UPDATE (19.02.2007):
Today I’ve received PCB’s ordered in Gama company.

PCB parameters:

– dimension: 125 x 80 mm, – double sided – metalized pads and vias, – soldermask , green color, looks beautiful.

Here are some photos (VGA low-fi cam):

PCB 1

PCB 3

PCB 3

PCB 4

PCB 5

Next few days I will spend on soldering. This will be my first SMD soldering, so I will practice on old PC Video Card board.

UPDATE (21.02.2007) -soldering Day 1:

Today I’ve made first soldering. First hundred SMD 0805 capacitors and resistors came smooth and easy. Next step will be more difficult – 208 pins AT91RM9200 chip that’s a challenge !!!.

solder 1

solder 2

Tip of the Day: don’t forget to turn the soldering station off when You’re done.

UPDATE (26.02.2007) -soldering Day 2:

Today I received a package with some parts needed for project. It contained many resistors, capacitors, EMI filters, LED diodes, coils and quartz resonators. The first thing I wanted to solder was coil for impluse switcher power regulator (main +5V supply). After soldering and providing the power it turn out that main impulse switcher is overoscilatting ang releases too much heat. Also the Low Drop Voltage regulators could not give a proper supply voltages. After closer investigation (with magnifing glass) I have found the source of my problems. There was a short-circuit under the one of SMD 0805 capactiors. Since this was a supply filtering capacitor it shorted supply power with the ground singnal – yay, no wonder the impulse switcher was going too far with oscilations, not mention the heat. After that I have added leds diodes for power supply signaling (green,red and yellow for +5V, +3.3V and +1.8V). Having all the power signal lines checked I can continue with more interesting part of the SBC assembly.

Soldering day 2Soldering day 2

 

 

 

 

 

UPDATE (27.02.2007) -soldering Day 3:

It took some effort to solder SDRAM chips (TSOP54). Now I know that the rest of chips which have more pins will be a real difficulty for a person with no SMD soldering experience.

SDRAM

SDRAM

UPDATE (03.03.2007) -soldering Day 4:

Well, soldering is almost finished. Today I’ve sucessfully soldered STE100P (MAC, ethernet) and AT91RM9200 chip. Using SMD soldering flux I was able to solder the chips. Space between the pins in these chips is so small that there was no other way but to make a lot short-circuits during soldering. Hopefully I bought a desoldering-wire before I get to this point – it was very useful and because it was something new to me I needed a bit of time before the successful short-cirtcuits removal.

REST of PCB

REST of PCB

There are still two chips missing MAX3232 (3V to RS-232 converter) and AT24C-XX (I2C serial EEPROM). The first one is needed for serial communication – including boot code upload via debug serial interface. The second will be needed later when we will get to the point where SBC boots the operating system. Check in soon.

UPDATE (06.03.2007)

We are back on the track. Today I’ve finally received the MAX3232 chip and was able to communicate with main cpu via RS232. It is quite well designed and provides a XMODEM upload for main boot-code on startup. The next step is to make a second/third stage bootloader for booting the kernel from MMC/SD card. This will be a real pleasure when all the hardware will pass functionality checks. In few next days I’m going to take a closer look to the U-Boot bootloader. It has an AT91RM9200 already implemented and includes many variations of dev-board’s featured.

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12 Responses to “1. Sarge – single board computer”

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