On Sat, Jun 26, 2004 at 12:49:15PM -0400, Victor Faria wrote:
> Sorry if this is a stupid question??

Why would you think that?

> But with all the talk about bootloaders is it possible to turn these into a
> single pin loader???

Of course! Tato's loader is a prime candidate because it bit bangs the
serial interface.

> Much like Wouter's. Even if it costs some memory but you gain a pin it may
> be a good trade off!!

There are more advantages:

1) You get to pick the I/O pin.
2) You don't have to tie up the USART.
3) Since it's a bit banged interface you can dynamically adjust the bit rate
   so that the load will work using the internal oscillator even when the temp
   fluctuates.


Wouter's basic idea is sound. A single pin interface is created by tying the
RX and TX pins of the serial interface via a resistor (he shows 330 ohms
on his WLoader schematic here: http://www.voti.nl/wloader). Then the TX
driver output pin is connected directly to a PIC I/O pin. So the PIC I/O pin
can drive TX, receive from RX, with only the minor side effect of everything
from RX being echoed to TX, which is actually a good check for the PC side
loader software.

BTW for bit banging serial Timer 2 is great as a bit rate generator when it's
available. With the period register PR2 set, the timer will auto reset after
the given interface. So you can set it to time out and auto reset after one
bit cell. Then only a single bit needs to be checked to see when to transmit
or receive the next bit from the serial interface.

Lastly the single pin interface will cost only a trivial amount of code over
a 2 pin bit banged interface. The only difference is that you have to turn
the TRIS bit for the pin around from transmitting to received. Otherwise it's
exactly the same. So 2 instructions to save an I/O pin, especially on an 18
pin part? Definitely a winner.

BAJ

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