hi,

i have read the icsp guide in the microchip website (http://ww1.microchip.com/downloads/e...Doc/30277d.pdf)

i focused on the 16f8x programming. there is a circuit at the bottom of page 2-21 (figure 1: typical application circuit). i am very noob in icsp/isp and i would like to learn using it... i have several questions that really need clarification:

- is the diode found in mclr/vpp already enough?
- can a simple tri state buffer be used as an isolation for rb7 and rb6? i found this (http://focus.ti.com/lit/ds/symlink/sn74lvc2g240.pdf)
- if i use tri-states, is there a way to automatically switch the gates to its its hi-z state during programming? or i simply need to place jumpers/dip switches to make it hi-z?

if there is further information that may be useful, plese feel free to add...

thanks!

 

Yes, The 14 volt programming voltage will reach the chip directly while the diode prevents the 14V from getting to the power rail.

If you put it to H-zi when the circuit is operating it shouldnt interfere, I think it should work. On the other hand why not connect RB6 - RB7 to something that won't react when the device is powered off. For example, if you use RB6 - RB7 to drive leds and those leds are not powered during programming, then they won't react to programming signals.

You could modify your ICSP plug to have an extra pin, wich for example, provides a 5V signal. You could then use this pin to switch the tri-state. Plug the programmer on the board, power it on and the tri-state will switch automatically. I use the same system.

I would replace the resistor between VDD and the pic with a diode. The resistor thing will work, but only if nothing draws a load from your pic. If you , for example, drive 5 leds of your pic, then the current used by these leds eventually come from the VDD line, the current required will generate a voltage drop across the resistor. It will quickly become to high and the pic will shut down.

 

speaking of isolation, i was just puzzled by this circuit:
(http://www.jdm.homepage.dk/icsp.htm)

is this circuit good enough for icsp?

 

That's basically the same as the circuit in microchip's App note. though, the VDD system is better, using a diode in stead of a resistor, like i said

 

what about the resistors found in the rb6 and rb7?

 

Like i explained in the previous post, if you use these 2 pins to drive something, a transistor perhaps, then it will work fine, the 2 10K resistors work nice as base-current limiting resistors, and because the rest of the application should be turned off while programming there isn't anything that will respond to the programming signals.

If you want full I/O functionality from those pins it would be better to add some kind of isolation, such as your tri-state for example