Re: [Usbpicprog-technical] Proposed schematic for future compat with 2.5V devices

[Frans S. <fra...@gm...>]

2008/10/9 Francesco Montorsi <f18...@ya...>

> Hi,
>
> Frans Schreuder ha scritto:
>
>>    Pull-down resistors on the drain pins of the two new PMOS may be
>>    necessary to ensure a good zero voltage level.
>>
>>    This should allow to use UPP to program also 2.5V devices, isn't it?
>>    Do I miss something?
>>
>>
>> Thanks for thinking with me about the hardware, but I can't agree with
>> your version of the circuit for a couple of reasons:
>>
>>    * With these mosfets you have made an open-drain circuit which can
>>      pull the data / clock line towards 5V / 2.5V. What about if you
>>      want to send a "0"?
>>
> right; I thought that pull-down resistors could be used for that.
>
>     * I don't like the size of the circuit you have added. usbpicprog
>>      must be kept as simple as possible in terms of hardware. The L317
>>      and all the jumpers and stuff make the circuit too complex to control
>>    * You are losing the pin-direction of the data line, with a mosfet
>>      you can "write" to the data line, but how will you read the level?
>>
> indeed. This is the killer reason for the proposed schematic.
> Actually I "forgot" about the fact that the PGD dataline must be
> bidirectional :/ Sorry!!
>
>     * I have already been thinking about a revision with just a
>>      resistive voltage divider, this way you achieve the same with just
>>      2 additional resistors connected to 2 other pins of the PIC. (You
>>      don't need all this additional stuff :P )
>>
> The problem of a resistive divider is that if the current drawn from the
> "middle node" (the node connecting the two resistors) is comparable with the
> current that the UPP is sourcing, then it ceases to be a simple resistive
> divider and the voltage at the middle node drops.
> A solution with the resistive divider may be the attached one: if the
> jumper is present, then the two resistors attached to the UPP's PIC divide
> by two the voltage; the buffer is used to avoid problems if the
> PIC-to-program draws too current.
> Since a buffer is used, using an IC which packages four opamps, another
> opamp may be used as a non-inverting amplifier with gain = 2; this way when
> you're programming a 2.5V PIC, the UPP-PIC gets back TTL levels, instead of
> 0/2.5V levels.
> The nice thing is that since the op-amps are always powered with 0/+5V, the
> UPP-PIC always get back 0/5V TTL levels from the PIC-to-program, without the
> need of a further jumper.
> I hope I was clear :)
>
> Something similar (not shown in the attached schematic) may be done for PGC
> clock line (using a single opamp since that's not a bidirectional line,
> AFAIK).
>
> How does it look?
>
1: I am not going to change the hardware that much right now, let's first
investigate the resistive divider thing
2: the pins don't draw current, so we CAN use resistive dividers
3: stop doing so difficult, I don't want to use more components and
certainly no opamps / voltage regulators and all that stuff


>
>
>     What about dynamic characteristics? I don't know which data rate
>>    should the clock and data line be able to handle...
>>
>> If you use long wires ( > 40cm or something), the wires become the
>> limiting factor. Else, it's the clock frequency of usbpicprog but since it's
>> an synchrounous system, the software determines wether it wants to use
>> higher or lower data rates. I am also testing this while writing the
>> software.
>>
> ok; btw using op-amps as suggested above the data rates shouldn't be a
> problem (op-amps up to 20Mhz are very cheap).
>
> THanks,
> Francesco
>
>
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