Chips And Smoke

      With Sparkfun's massive “Freeday” becoming more popular every year, more and more people get access to materials they normally wouldn't be able to afford or have not considered worth the price. The last Freeday went along something along the lines of spending hours on the computer filling out captcha's with the futile hope that eventually, after pressing the submit button, instead of having another captcha on your screen, a gift certificate pops up. After spending nearly six hours on the computer furiously filling out captcha's a $100 gift certificate popped up. After debating the purchase of a large assortment of various doodads or a single expensive item, I eventually reached the conclusion that the coupon should be spent on a hot air station. After a few days, the box was sitting in the basement waiting to be used for the next project.

      Never having used a hot air station before, my garbage bin grew bigger and bigger, containing charred boards, PCB's with bubble on them, and IC's that were burnt to a crisp. From frustration of constant hot air failures, I decided to devote an entire day to de-soldering what ever I can get my hands on with this new hot air station. The goal was not just to get new chips, but also gain some experience, and decrease the size of ever growing junk bin. In the end I had tons of new goodies that I can use for future projects, such as some good RAM IC's, voltage regulators, FLASH memory, inductors, and even a few odd components that just looked interesting.

      First de-soldered chip is a DS21S07A from Dallas Semiconductor. It is a couple of termination resistors with some control logic in one IC used for SCSI termination. Although it was not designed for this, it can possibly be used as a couple of controllable pull down resistors for data or control lines. Off the top of my head I cannot really think up of much uses for this chip, but the chances of finding a application for this in the future are high, so this will be kept in a drawer somewhere and eventually be forgotten about anyways.

      Next up is a ST93C46, or according to the chip itself, C46CM1. It is a 1 Kbit EEPROM that is able to be organized as 64 addresses each with 18 bits of storage, or 128 addresses with 8 bits of storage by holding the ORG pin high or low. The package is very simple, with just eight pins, and the interface seems similar to SPI but I see no mention of SPI in the data sheet, regardless there is a clock pin, data in and data out, Chip select, the ORG pin, and the usual power and ground pin, with this being a 5v part. It does register above 3v as logic high on the input though, so you can use your standard PIC's with 3.3v IO, but logic high on the output is VCC, in this case being 5v, so you would need a voltage translator or even a simple resistor divider for transmitting from this EEPROM.

      Lastly, there was an ASIC on the board by Adaptec, probably just serving as the SCSI to PCI converter. Next to it was a 40 Mhz oscillator, which I was unable to find any data sheet for, and a 4 pin male header. A standard red led was also found, not much needs to be said about that. I was unable to find the resistance of the current shunt (metal tab) nor any data sheet, but it will still be kept. The few chips that have stickers on them are most likely ROM's, and will be looked at closer at a later time.

      As processors chug a large amount of power, even the old Pentium 2's, a voltage regulator is needed on the motherboard. Using a linear voltage regulator would be extremely inefficient because these processors are designed to operate at under two volts while pulling a large amount of current, upwards in the 20's. Somewhat recent processors, such as the Q6600 or Pentium 4 required from 60 to 100 amps at a voltage of 1.4 when running at stock. A linear voltage regulator would have to take the lowest voltage rail with a large amount of current capability, such as the 12v rail, and then have to lower that to 1.4 volts while supplying 60 amps, effectively turning it into a nice space heater. Due to this, buck switching voltage regulators are used, which have a much larger efficiency at large currents than linear voltage regulators, often times running with multiple phases to give the lowest voltage ripple, faster response time to constantly changing current demands, less strain on each inductor and mosfets, and power dissipation being more spread out.

      In this case, it is a single phase, synchronous, buck switching converter. The large amount of capacitors are for keeping the voltage ripple as low as possible, and acting as decoupling capacitors for the processor located in the green slot towards the top. Generally, as the current demand increases, voltage ripple required to be lower, or switching frequency is lower, the amount of capacitance needed increases. The copper wire looped around a gray ring is used as the inductor for the switching converter.

      This being a switching synchronous buck voltage regulator means it uses two mosfets (2SK2885), often two N-Channel mosfets, as the switches to connect ground and VSS to the inductor. These mosfets must have low RDS-ON (resistance when fully on) and low gate capacitance to decrease transition times to keep power dissipation as low as possible, especially when dealing with such a large amount of current. You can easily see which of the pins are the gates for each mosfet by the thin trace connected to it.

      The voltage regulator (HIP6004A) is a normal run of the mill chip in this category except for one item. You can control the output voltage using voltage identification pins. These are five pins which are pulled up or down, controlling what voltage the internal DAC creates, which is then sent to a comparator, basically determining what output voltage you want. Five pins allow for 32 steps between two voltages, with the ten lowest steps disabled, enabling you to control the voltage in 0.1v steps for 2.1v to 3.5v, and .05v steps for 1.8v - 2.05v.

      The GD75232D is a RS-232 transceiver, containing transmitting and receiving drivers. It is used for translating the voltage between the external RS232 12v bus and the internal, much lower voltage bus, as providing protection against ESD and noise on the external bus entering the very delicate circuits of the computer. Not much else needs to be said about this IC. Drivers and receivers are commonplace for external buses, and can be found on every computer which employs such buses.

      Looking closely, you can spot a second spot for another transceiver not occupied. This is a sign that this board may have had different versions being sold using the same PCB in order to save costs, with one of the differentiators being more serial ports. This can be seen in nearly all electronics today, such as phones being shipped with different amounts of storage for different prices but otherwise remaining identical.

      The TL74F32D is a quad OR gate logic IC. Not much to say since this is a standard logic chip, it runs on five volts, and the datasheet has only three pages for the actual ic itself, with the remaining three pages either mostly empty or meant for sale information. You ca spot the decoupling capacitor on the left of the IC.