Internal vs. external crystal

[yossihagag1978]

Hi
1. What are the advantages and disadvantages of using EXTERNAL or INTERNAL
CRYSTAL on pic micro ?
2. I am using CX1SM - 2.0MHz crystal.
My pic18f8722 is on sleep and awakes from int ext.
The problem is the latency from int to ext isr is about 10msec (does it make sense ?).
I solved it by enable "two speed startup".
Is it the right way ? Will temp effect it ?
Regards
Yossi

[Ttelmah]

There isn't an internal crystal.

There is an internal oscillator, that is relatively low accuracy.

The data sheet is your friend here. +/-2% typically at room temperature +/-5% for the chip's whole temperature range.

Your startup time for the oscillator is determined by the loop gain of the oscillator. Adding a resistor across the crystal may well improve this.

_Everything_ is affected by temperature. The question is 'by how much'....

[newguy]

External crystal advantages:
- Higher frequency accuracy. If you need to interface to some external circuitry or external bus (particularly CAN or USB), a crystal is pretty much required.
- Higher frequency accuracy over temperature and supply voltage variation. Although it's possible to get some PICs with ~1% internal oscillator accuracy, that ~1% is specified for a very small temperature and supply voltage "window". Outside that window, accuracy degrades.

Internal oscillator advantages:
- Smaller component count (lower cost).
- Smaller PCB footprint (lower cost).
- Smaller wake up (or power up) to being able to execute code delay. As long as a PLL is NOT involved, of course.
- Usually smaller current draw than having a comparable external crystal.
- Pretty much guaranteed to start & run no matter what. Sometimes crystal circuits can be finicky if you didn't design the PCB 100% correctly or if your crystal changes (originally specified part becomes obsolete or isn't available for whatever reason).

There are probably others. This is just off the top of my head.