I recently aquired a SAM7-LA2 board. The board apparently is very outdated but it was on sale and it does support the ARM7TDMI instruction set.
The problem with the board is that it does not come with a power supply. The power requirements are specified at several locations in the user manual. Wierdly enough the manual says it need 4.5V DC on one page and 6V DC on another. Also the polarity of the power supply is not specified! It nowhere explicitly states that the polarity does not matter and that the board contains a rectifier. So I did not know which polarity to use.
Here is what worked for me:
I used a 6V DC power supply and as polarity, I used positive center. The board is running without getting fried immediately.
When it comes to power supplies you have four things to worry about:
- Amount of supplied voltage
- Type of supplied voltage AC / DC
- Polarity of supplied voltage (in case of DC)
- Amount of supplied current
1. For the amout of Voltage, if the circuit needs 6V you have to supply 6V to not damage the circuit. You have to be exact, when it comes to voltage!
2. The voltage is alternating current (AC) when it is consumed out of your wall socket. If your integrated circuit uses directed current (DC) your power supply has to convert AC to DC and output DC. If your circuit uses AC, the power supply has to output AC. If your circuit does not care, it does not matter what the power supply outputs.
3. Polarity is important because if your circuit can only consume DC and within DC a certain type of polarity and you supply the wrong kind, your circuit will be damaged irreversably. Usually a power supply uses a connector that has a pin in the middle which is surrounded by a metal tube on the outside. The pin in the middle is the inner contact the tube is the outer contact. The contacts have to have opposing polarity. If the pin is positive, the tube has to be negative and vice versa. You have to know which polarity setup your circuit requires. Most modern circuits use a rectifier which automatically converts any polarity to the polarity the circuit wants in order to get rid of problems with incorrect polarity. Older circuits may not contain a rectifier and the user is responsible to plug in a power supply having the correct polarity setup. You have to be precise with polarity in order to not irreversably damage your circuit.
4. For current, you have to make sure that your power supply is capable of supplying at least the required amout of current. If your power supply is capable of more current that is usefull but the circuit will only draw as much current as it actually needs at any point in time. So you cannot damage the circuit if you use a power supply that can supply more current. You can compare this to the situation in your gaming desktop PC. Your PC maybe contains a 500 Watt power supply which does not mean that your PC consumes 500 Watt of power at all times. If your graphics card turns itself off, the power consumption drops to maybe 300 Watts. If you play a CPU and GPU intensive game and your graphics card draws power at its max TDP, your PC will maybe use 480 Watts. If you replace your graphics card with a faster model it may draw more power and your 500 Watt powersupply may not able to supply that power causing your PC to turn off during the gaming session. Time to replace the 500 Watt supply by a 800 Watt supply or an even stronger one. Your PC will not take severe damage when there is not enough power to function, it will just stop to function correctly.