3 Wire Downloads

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Fred Smith

New Member
Hi,

I am trying to learn more about the 3 wire downloading capabilities promoted with picaxe. I am a beginner and most sites I go to mention a MAX232 board. Can ANY PIC with a ANY bootloader do the same 3 wire programming?

Also, the manual mentions a better serial circuit by adding a 180 ohm resistor and a BAT85 diode. I am from the US and when I go to mouser to look for parts they only have a BAT46 shotkky diode. Is the BAT42 a direct replacement for the BAT85?

Thanks...

Thanks
 
hippy

hippy

Ex-Staff (retired)
Most bootloadable PICmicro devices are programmed to use an internal hardware UART ( Universal Asynchronous Receiver-Transmitter ) to communicate, and the chip needs to have the signals inverted from what a PC uses. This is why they need a MAX232 or similar circuit.

The PICAXE's use an alternative method ( bit-banging - a software only UART ) which allows the PICAXE to choose the signal polarity and permits a much simpler and cheaper download arrangement.

Any PICmicro could use the same mechanism as the PICAXE does, but most PICmicro bootloaders I've encountered use the internal UART.

I'm afraid I cannot answer the question on the BATxx diode. It is not essential in all cases so you can build the download circuit without one and use the PICAXE. It's only necessary in cases when reading analogue voltages (READADC/READADC10) when readings may otherwise change slightly between when the download cable is connected and when it isn't.
 
F

Fred Smith

New Member
Hi,

I looked in manual 3 early this morning, It looked like the 28pin X1 version was using the pins 6 & 7 which based on 16F886 is not TX & RX. I must have not figured it out correctly. I will have to go back and look it over again.

Thanks...
 
I

inglewoodpete

Senior Member
Fred, Tx and Rx are names for pins for the basic 16F886 chip. The 28X1 has a PICAXE 'operating system' which changes the way the chip behaves.

One of the changes that the PICAXE O/S makes is that it configures pins 6 and 7 as its "Serial In" and "Serial Out". You're reading too deep.

Edited by - inglewoodpete on 09/05/2007 07:20:08
 
D

Dippy

Moderator
All the BATs you mention should be fine. They are all low cost, low current, cheap'n'cheerful, signal level Schottkys.
If ever in doubt have a read through the appropriate Data Sheets. Very handy things.

Combining hippy and IP; PICAXE is very clver in the download interface.

PICs: As mentioned, the Rx and Tx pins refer to the on-PIC UART/USART serial module which is the 'normal' (arguably) method for programming PICs which have been already PRE-PROGRAMMED with a simple bootloader. You cannot use these pins (to load your programme) unless a bootloader has been put on first (using a proper hardware programmer). As said previously this requires a MAX232 based or transistor inverter circuit for connection to PC. Its quite easy really but still requires more components than the PICAXE I/f. Your PC will require a 'loader' app to 'talk' to bootloader. And this does NOT apply to all PICs either.

PICAXE has a special bootloader/firmware which makes connection easier and as described by hippy above.

Don't confuse the 2 completely different beasties. And you can't just pic'n'mix (lovely pun) from the 2 different 'systems'.
 
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