SPICE simulations for board level design

What trigger me to write this article is base on a question raise by one of trainee engineer in my present company. While I was drawing some schematics file, he asked me did i use SPICE simulations to get the correct circuits or put the correct value. I answer "no". Then he ask back, how do u know it's correct?.

When you do board level design, many sub-block is basically a recycle of sub-block that u had known before or that u already understand. If u understand very well during electronics course at university, u will know how transistor switch circuit is, class A amplifier, class AB amplifier, low pass filter, high pass filter, etc. This is the sub-block that when connected, it makes a circuits that target your applications. You should have a good sense on the how big the value of resistor should be, the kind of transistor u need to put and u actually can do the calculations by hand.The important things is, you must understand the qualitative analysis first. SPICE just help you to get a very very accurate quantitative analysis.

Another reason for not using SPICE is that, board level design consist of using a lot of ICs. The fact is, no IC manufacturer would give u SPICE models for their IC. You can get SPICE models for transistor, diodes, resistors and other passive elements, but u wont get for the ICs. Imagine design consist of using PIC16F84A MCU, I don't think Microchip release the SPICE models for it.

How do u get the correct circuits? For board level design, you always make prototype first. You get the prototype PCB, solder the componets, load the firmware and you power up the board.From the design, u know that it should behave this way, and the voltage or current value at those point should give this and that value. If it doesn't, you desolder some components, solder other components with different value, repeat this few times until you get what you want. Basically that's how it been done.

What is the purpose of SPICE? SPICE is certainly being use in IC design. I had experience working for fabless IC design company to understand this. The reason is, IC prototype is very very much expensive compare to PCB prototype. The IC designer try to get as accurate as possible the design. If the IC wont work, you cannot desolder or cut some wire to make it work. Basically, it just means that the prototype fails. The way IC being design also, make it possible to use SPICE. Inside the IC, it consist a lot of transistors,diodes,resistor connected together. You don't have another IC inside an IC.

What I try to point to is that, you must have very strong understandings of circuit theory and operations. Stuff like kirchoff voltage law, current law, transistor operations,etc should be well understood. In other way to see it is, you should able to run SPICE simulations inside your head before you could actually use the SPICE programs effectively.

First MCU Experience

My first experience doing work using MCU was when I work as engineer in one of Japannese multinational company in Malaysia. I was not involve in any embedded product development. I was a design engineer for department that produce TV tuner. The tuner that we use at that time, use Texas Instrument PLL IC for controlling the local oscillator. It is control using I2C protocol. The device that use to send I2C signal to the tuner came from Japan and we had to buy it quite expensive. No one at that time knew how to make our own.

Later, I met this one customer name Mr Vakku. He was embedded engineer developing firmware for LCD TV for one of the company in Shah Alam. I ask him how to get started using MCU because i know it could solve our I2C tuner problem. He asked me to buy one book title "Programming and Customizing PICmicro Microcontrollers" by Myke Predko. (You can get the book from Amazon.com).

The reason he recommend the book for beginner because the book come with a PCB and software to make the MCU programmer. With that, I went to "jalan pasar", quite famous place for electronics hobbyist in Malaysia to get their electronics components. Then, I assemble and solder the component and the programmer finally done.

The MCU I used is the Microchip PIC16F84A. It was chosen because a lot of example in the book was base on that and you could find huge resource on the net base on the IC. Then, when I got free times I slowly learn and develop the product using the IC. Finally I manage to develop a breadboard design for the I2C PLL controller.

Unfortunately, before I could show the design to my management, I resign from the company because I got better opportunities. Whatever it is, the knowledge that I gain from doing that stuff valuable to me until today.