Saturday, June 25, 2011

Using the BU2090FS LED shift driver

I do a lot of scavenging for components. Whenever I get my hands on junk electronics I whip out my soldering iron, a desoldering pump, some pliers, and a pair of tweezers to see what I can extract from it. Scavenging is a long, tedious and sometimes painful process involving singed hair and burnt fingers, but every once in a while I come across a really nice component from an old TV or washing machine. I’ve learned a lot of desoldering (and some soldering) skills, and managed to save a few bucks by scavenging for components.

IIRC, I found the BU2090FS on a VCR board. I managed to desolder the SSOP16 chip by throwing the entire PCB into a toaster. I let the board cook for a few minutes and then started pulling off parts with my tweezers. The chip is quite solderable by hand, but I had no 0.8mm SSOP boards, so I superglued the chip onto veroboard and had a go at hand soldering the thing.

BU2090F-E2

Now you’re probably thinking that hand soldering a 16 pin SSOP is a daunting job, but its not really that hard. All you need is a fine point soldering tip and a steady hand. A little bit of magnification and good lighting helps as well. All in all it took me less than 15 minutes to make the DIY SSOP-to-dip ‘adaptor’.

DSC01430 DSC01425
Left: the ‘adaptor’
Right: the chip can be seen below the wires

The ‘2090 is a 12-bit latched shift register, and is primarily used to drive LEDs. It runs off 5v and can sink upto 25mA per channel. Coding for the chip wasn’t difficult at all. Serial data can simply be clocked in and you’re ready to go.

Below is a ‘scope capture of the data to make output Q8 and Q0 low. Data is fed in MSB-first. Keep in mind that the outputs are active low since this is a sinking driver.

scope

 Oscilloscope capture

Data and clock lines idle low. According to the datasheet data is sampled every rising clock. A shift occurs after every new bit of data is clocked in. Data is latched after all 12 bits are sampled by raising data high and then pulling clock low.

I used CCS-C on a PIC16F882 to test the chip. An excerpt of the driver is below:

#define clk pin_c5
#define data pin_c4

void writedata (int16 var)
{
    int8 loop;

    var<<=4;   

    for(loop=0;loop<11;loop++)
    {
        output_bit(data,var&0x8000);
        output_high(clk);
        output_low(data);
        output_low(clk);
        var<<=1;
    }
    output_bit(data,var&0x8000);
    output_high(clk);
    output_high(data);
    output_low(clk);
}

 

DSC01427  DSC01428
The BUF2090 mounted for prototyping

3 comments:

  1. How did ya exactly make the adapter??

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  2. The 'adapter' is a piece of veroboard with pin headers soldered on. I stuck the chip in the middle of the board with CA glue and soldered fine wires directly onto the legs. Very similar to what Dave Jones has done here (http://www.eevblog.com/2011/06/21/eevblog-181-dead-bug-prototype-soldering/), but the orientation of my SSOP16 chip is not dead-bug.

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