Microchip ICSP isolation issue

[Electronics4you]

Hi there,

I'm trying to get a grip on the Microchip ICSP for a circuit applying the GP0 and GP1 pins for LEDs. According to the ICSP guidelines, the GP0/GP1 pins must be isolated from the application circuitry during programming, but how to do that if the pins require a low impedance to drive LEDs?

As long as the programmer is able to source the LEDs during programming, the signal is not loaded, thus interfering the signal, right?

 

[Joe G]

think.....JUMPERS and or DIODES

 

[Ian Rogers]

Forward thinking... I tend to use ICSP pins for high input type! On the newer pic16f1828 the ICSP pins are the analogue inputs so they have no bearing!!

Obviously we can't always do it thus... So Joe's right... Jumpers or diodes...

 

[NorthGuy]

You can put a relatvely big resistor going to the base of BJT, which controls the LED. LED will flicker some when you program.

 

[Electronics4you]

Thanks for the quick response...

I'm not fan of jumpers in this case, because the circuit is difficult to reach. How will the diodes have to be connected in this case? Cathode to GP* pins? Then perhaps the LEDs can be sinked instead.

NorthGuy: That's a simple solution, even though requiring a couple of extra components.

 

[Nigel Goodwin]

Or just socket the chip (as you should anyway) and unplug it to do the programming

 

[Joe G]

look at the data sheet, here is one.

 

[Electronics4you]

Nigel Goodwin: The circuit is not for development, so the ICSP is for last minute changes

Joe G: Your schematic shows the isolation circuit as an impedance, which should be high. But in case I need low impedance for the LEDs, the circuit is not preferable. A BJT circuit may be the solution.

 

[Mosaic]

Yes as low as a 1K driving a BJT does the job.

You can reuse ICSP pins to drive a 74hc595 for a 2 digit, 7 seg ment display with 2 tactile input buttons. Just make a piggyback board with a 5 pin receptacle.

I have an LPC 16F690 demo board doing just that right now, as I need the ICSP to update firmware AND do a Pickit2 RS232 uplink to PC.

 

[atferrari]

Forward thinking... I tend to use ICSP pins for high input type! On the newer pic16f1828 the ICSP pins are the analogue inputs so they have no bearing!!

Obviously we can't always do it thus... So Joe's right... Jumpers or diodes...

Ian,

Could you say reasons to prefer one or the other?

 

[Nigel Goodwin]

Nigel Goodwin: The circuit is not for development, so the ICSP is for last minute changes.

Even more reason to simply put the chip in a socket!

 

[Ian Rogers]

Ian,

Could you say reasons to prefer one or the other?

Lets take a recent job!!! If you plan out a circuit... Take both AN0 and AN1 on the pic16f1828 are ICSPDAT and ICSPCLK... Well a joystick is connected to the micro, the connector is modified to suit a programming socket... Forward thinking! You have to disconnect the joystick to program... No contention.

 

[Electronics4you]

atferrari: Jumpers are OK, but it's easier, if the circuit is just designed to avoid them. The diodes can be used, however, as mentioned earlier, what to protect with them - the application circuit or the programming signals from being corrupted?

Nigel Goodwin: I can certainly see the benefits of a socket, but the height requirements only allow for an SMD version. Life is hard

Ian Rogers: Good thinking, if you have such connection

 

[Joe G]

I'll add another way, how about micro-switches (miniature dip switches)

 

[Nigel Goodwin]

Nigel Goodwin: I can certainly see the benefits of a socket, but the height requirements only allow for an SMD version. Life is hard

As it's SMD don't you have room for using a better sized chip?, so you don't need to use the ICSP pins?.

 

[Electronics4you]

It's probably best to spend the time searching for a microcontroller with 2 additional I/O pins instead of designing new circuitry. I'll take that advise

Thanks for all the constructive replies...

 

[Nigel Goodwin]

It's probably best to spend the time searching for a microcontroller with 2 additional I/O pins instead of designing new circuitry. I'll take that advise

It could even be cheaper as well - the improved 16F18xx series are amazingly cheap, yet far better spec.

 

[Electronics4you]

I'll have a look at it, thanks