Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

PIC ICSP with TMR1 crystal connected to RB6 & 7

Status
Not open for further replies.
Hey guys,

I'm using a 32.768 crystal for a clock application within a project. The crystal gets connected to RB6 & 7 which is the TIMER 1 external clock source but it is also the In Circuit Serial Programming pins.

I want to know if I can leave a both the crystal and the PICKit 2 ICSP connected.
Will I blow the crystal and/or will it interfere with the programming?

I dont really use a simulator and I have the PICkit set to auto update so I just click build in MPLAB and it programs straight away, no disconnecting the ICSP.

Jake.
 
No, you won't blow the crystal.

The crystal is very high impedance at any frequency that isn't a resonant frequency, so it can effectively be regarded as not there. I guess the capacitors are small enough not to upset the ISCP. If you haven't got any capacitors, there is a risk that the crystal will run at an overtone, so I would suggest that you have around 22 pF to ground on each side of the crystal, and a resistor in series with the CKO pin of the PIC. You could also put 1 - 10 kΩ in series with the CKIN pin, which won't make any difference to the running of the oscillator, but will certainly prevent the ISCP being upset by the capacitors.

A watch crystal has an ESR of somewhere around 40 kΩ, and a drive level of 1 μW, so the voltage across it should be less than around 0.2 V at its resonant frequency. Watch crystals have a very high Q, so it takes a couple of seconds for the oscillations to build up, so to get to the point where the crystal were to be damaged, the output of the ISCP would have to have a significant signal at 32.768 kHz, that would have to be sustained for a few seconds. That is highly unlikely. I think that the ISCP runs much faster than that.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top