Jargon Buster

[Cagao]

Hi,

I'm new to PIC programming but slowly getting there. Currently exploring the 12F675 on microchips PICKit 1.

One thing I'm having trouble with is all the jargon in the data-sheets.

Does anyone know of any nice simple descriptions for things. Take the following list as an example...

interrupt-on-change - what's it used for?
Weak pull-up
output 'latch'
port - just another word for a pin?
pins set to Hi/Lo (impedance)

Answers to these directly would be greatly appreciated, but it'd be nice if there was a page or 2 somewhere to help complete newbies like myself.

Thanks,

Rob

 

[dknguyen]

Interrupt on change means just that- when there is a state transition on the pin it activates an interrupt. I am unsure of how this is different from a regular interrupt pin though- unless the difference between the two is one triggers on a transition and the other triggers on a high


weak-pull up- the pull-up resistor has a large value so less currenti s flowign through it which makes it easier to make the line low

output latch- latches the output at a certain logic state
port- it's a bunch of pins that are grouped into the same "port"
pins- hi and low are obvious, high impedence (you can think of impedance as resistance in this case) effetively means the pin has been disconnected from the internal silicon and has no logic state.

 

[evandude]

interrupt-on-change will trigger the interrupt when the assigned pins change from high to low, or vice-versa.

weak pull-up is basically an internal version of adding a pull-up resistor between a pin and +V. the "weak" part refers to it being a very low-current pullup, like using a very high-value resistor. The datasheet would have more detailed info about the specifics if you needed.

a 'port' is a group of I/O pins, containing up to 8 I/O. There are ports A, B, C, etc, depending on how many I/O pins the PIC has. The smaller PICs like the 12F675 just have one port, called "GPIO".

pins set to hi/lo - these are just logic levels. An I/O pin has three usual states - high (output, Vdd), low (output, Vss), and high-impedance (logic input)

output 'latch' has to do with the way the I/O pins are controlled based on the values you put in the associated control registers - someone else can probably provide more insight here.

 

[Cagao]

Thanks for that, I'm slowly getting there!

One thing that I've noticed is throwing me off are the words 'high' and 'low' seem to be thrown in everywhere...

high (output, Vdd), low (output, Vss) - what exactly is the difference?

I come from a very sketchy electronics background, anything apart from ICs and i'm fine.

The Vxx stuff is driving me bonkers right now.

I only seems to be aware of +5v and GND for example.

 

[Cagao]

btw, the weak pull-up has completely lost me, think i'll leave that for another month for now.

 

[Nigel Goodwin]

I only seems to be aware of +5v and GND for example.

High = 5V, Low = GND - easy as that!

 

[dknguyen]

Positive voltage is Vdd or Vcc
Ground is Vss or Vee

dd and ss are paired together and cc and ee are paired together. They have to do with what types of transistors are used inside, but just follow the two above things as far as ICs go (if it's a transistor you will need to know exactly what dd/ss and cc/ee are.)

High and low are just that 5V and GND. 5V and GND are physical, HI and LO are how the software interpret it. It's just that sometimes some hardware interprets GND as HI and 5V as LO, but just assume they don't exist (since it's almsot always 5V = HI and GND = LO).

 

[Cagao]

making more sense, good to know there's nothing obscure in all this!

 

[Nigel Goodwin]

btw, the weak pull-up has completely lost me, think i'll leave that for another month for now.

A 'pull-up' is simply a resistor that pulls a pin to a higher voltage, usually you will have them from +5V to input pins with switches on. The resistor pulls the pin HIGH (5V), and when you press the switch it goes LOW (0V).

The weak pull-up's are current sources within the PIC (not actually resistors, but they work the same), they can be turned ON or OFF in software - so essentially they work like resistors you can switch ON and OFF in software. They are called 'weak' because they can only pull a fairly low current.

 

[Sceadwian]

I think a 'weak' pullup/down could roughly be defined as anything above 10k as a practical resistance the up is to the supply the down is to ground. Most MCU's have pullups in the range of 20-40k mostly as a basic noise prevention method, pulldowns are not as common.

A latch is basically a register that holds information like a buffer, and when it's given a trigger pulse that latch is pushed onto the actual output register (the real world) Exactly what kind of latches and how they're updated depend on the device being used.

A 'port' is the entire input/output buffer, usually 8 bits and usually consisting of routing logic which makes the port either input or output directing the signals to appropriate lines like the supply voltage, or ground, or an input bus, sometimes tied to other internal circuitry, like in the case of a PWM output line which is controlled directly by the CPU itself, or interrupt lines which are I/O lines that trigger internal state flags.

I/O pins on a micro controller generally have three states. Attached to supply voltage (1) attached to supply ground (0) or high impedance (effectively disconnected) A high impedance line is considered 'floating' input lines are in a high Z state.

It's not common usage but even if an I/O line is set to source or sink current the PIN states on that port are still valid, this can actually be used to determine when the resistance on the outside connection has formed a resistor divider that results in a logic state change.
For example if an I/O line was set to source current (1) but the external resistance was significantly less than the internal resistance of the I/O driver then the PIN state would register as a 0. This can be an effective short circuit detector.
Inversely if the output pin was set to sink current and too much was being supplied to it via a low impedance source the pin output would go high.
The high impedance state lets the outside circuit do whatever it wants with as high a resistance as possible.

 

[Sceadwian]

P.S. one of the biggest problems people have with micro controllers is that they have three port registers.
DDRA PORTA and PINA (this is from AVR notation hopefully it's similar to the way PIC's refer to them)

DDR means data direction register. Set to input, or output.

PORT has two functions.
If the pin is set to output it's set to the side of the power lines it is connected to (source or sink 1 or 0)
If the pin is set to input the PORTA register usually contains the pullup enable state. Depends on the micro controller but 1 or 0 (or 0 or 1) is going to say weather the internal pullup resistors are enabled.
The PIN register itself is the only true status register related to the I/O port, it's a direct connection to the I/O lines comparator. If it's 1 the I/O line's voltage is above the minimum garunteed 1 state. If it's 0 the I/O line's voltage is at or bellow the maximum garunteed 0 state. There's a slight fuzzy region inbetween the two states but they I/O lines are almost always schmitt triggered.

 

[Papabravo]

Now that you've learned some basic vocabulary for microcontrollers, I was wondering if you would care to suggest some non-jargon alternatives for these fundamental concepts. I'm all ears.

 

[ericgibbs]

hi cacago,
For reference just did a quick search on google for 'electronic terms glossary'
There are dozens of excellent sites where you could browse for some of
the abbreviations of interest.

Regards
EricG

 

[Sceadwian]

Non jargon alternatives to what phrases? Everything has to be called something, the jargon is just a simple way of saying it. You have to learn it or you'll forever be out of the loop =)