I finished soldering the basic power and LED circuits and so started doing incrementdal testing. I first use crocodile clips to test by hand if all the LEDs can switch on and off. I was surprised to find that the LEDs are dimmer than usual. I thought perhaps this batch of cheap LEDs are poor quality. Anyway all LEDs work OK. Then I used my 20+ year old Sanwa multitester to check the IC power. I found the Vcc only 3.0V instead of 3V3, no wonder the LEDs are dim.
Then I by chance found the MCP23s17 surface is finger burning hot. I imediately switched off power and checked if I had the wrong numbering of pins. Then I realized that I made a very big careless mistake. I had the wrong polarity of power pins!
In my college and technician years. I almost always used the convention Vcc and Gnd. But this time I have Vss and Vdd. I thought SS is bigger than DD, so SS must be positive and DD negative or ground. But then I immediately understood that my casual guess is very wrong, and the convention has the oppostive polarity I guesses.
To make sure, I wikied "Vdd" and found that it is symbol used in military standard mil-std-498 dated 1994, no wonder I did not learn that in college ages ago.
I now also know that Vcc/Vee is for BJT, and Vdd/Vss is for FET.
One more lesson learnt is that the analog mutlimeter is not that precise in measuring voltages, though it is still very handy to check open/ short circuits.
To remind that the once burning hot IC might be damaged, I put a big white dot on its package surface.
IC power supply pin From Wikipedia, the free encyclopedia
Almost all integrated circuits (ICs) have at least two pins that connect to the power rails of the circuit in which they are installed. These are known as the IC's power supply pins. However, the labeling of the pins varies by IC family and manufacturer.
| Typical supply pin labeling | ||||
|---|---|---|---|---|
| BJT | FET | |||
| VCC | VDD | V+ | VS+ | Positive supply voltage |
| VEE | VSS | V− | VS− | Negative supply voltage |
The simplest labels are V+ and V−, but internal design and historical traditions have led to a variety of other labels being used. V+ and V− may also refer to the inverting (−) and non-inverting (+) voltage inputs of ICs like op amps.
Sometimes one of the power supply pins will be referred to as ground (abbreviated "GND"). In digital logic, this is nearly always the negative pin; in analog integrated circuits, it is most likely to be a pin intermediate in voltage between the most positive and most negative pins[citation needed].
While double subscript notation, where subscripted letters denote the difference between two points, uses similar looking placeholders with subscripts, the double letter supply voltage subscript notation is not directly linked (though it may have been an influencing factor)[1][2]
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