Design inconsistancies for propeller redesign

kittmaster

I'm trying to lay out the USB FT232BM using the demo board schematic and also referencing propeller prop schematic. If you look at the two images, they use the same chip, disregard bm vs rq, the pin assignments are the same, just different package. Notice that one has 150 ohm resistors in the path of the serial rxd/txd while the other doesn't. The led's current control resistor is different, two seperate schematics for the same circuit......

I need to know what the right hardware sequence is so I can build the prototype right the first time. Any guidance????

Phil Pilgrim (PhiPi)

The 150 ohm resistors are just to protect against bus conflicts, and they're probably a good idea. I'm guessing the upper schematic used a quad resistor pack, so all four values are the same.

BTW, the "BM" and "R" versions of the FT232 are completely different chips. For a start, the BM version requires an external crystal, whereas the R version does not. There are other significant differences as well. If you can manage the fine lead pitch, I highly recommend the R version, since it requires fewer external components.

-Phil

hippy

One current limiting resistor for both LED's is probably just component saving. The 100R against 150R will mean more brightness. Current limited through a single R means the LED's will be dimmer when both are on. Behaviour which is probably acceptable and current doesn't increase when two LED's are on.

Ideal choice of R depends on brightness required which in turn depends on LED characteristics. Two R's will allow different coloured LED's with different characteristics to be 'balanced', but it probably isn't a necessity so one will do the job.

kittmaster

Well at this point the company is going to be sending me the BM IC's which are bought and paid for, so my next question is:

This is going to need an max232 between these two devices correct? Or will it just straight up link to the propeller? I'm leaning to max232 because of the TTL transition.

Is this right?

Phil Pilgrim (PhiPi)

No MAX232 needed. The FT232 uses logic levels (non-inverted) for communication with attached devices. (I guess the "232" part of "FT232" is a misnomer.)

-Phil

hippy

According to the datasheet; "Integrated Level Converter on UART interface and control signals The previous devices would drive the UART and control signals at 5V CMOS logic levels.The new device has a separate VCC-IO pin allowing the device to directly interface to 3.3V and other logic families without the need for external level converter I.C.’s"

So, assuming there's no need to invert the polarity of the signals it should be a direct connection. 5V signals would only require suitable current limiting resistors. Current limiting resistors as per the top FT232RQ circuit diagram could be added for additional protection anyway, especially should RX into the Propeller become an output under program control.