Interfacing MC9S08GT60 SCI to a FTDI USB Adapter...

[ehestigoni]

Hi Guys,

I'm getting a bit confused about the voltage compatibilities when interfacing the GT60 MCU to an FTDI SERIAL-USB transceiver... That's because I don't understand what exactly TTL and CMOS mean.

The voltage levels of the MCU IO ports, according to the datasheet, are (VDD = 3.3V):

Vout Hi                 VDD - 0.5 ( so =2.8V)

Vout Low             0.5V

Vin  Low               0.35 x VDD ( so = 1.15)

Vin  Hi                  0.7 x VDD  (so = 2.31)

The FTDI "cable" I am intending to use is on

http://www.ftdichip.com/Products/EvaluationKits/TTL-232R-3V3-PCB.htm

I read the app note AN2433 but I'm still confused...

And the confusion is that acording to the App Note, I will need to use a resistor between the  MCU SCI and the TTL port of the FTDI device.

However, the FTDI Datasheet (in attach) says that the input threshold is 1.2V and the output voltage levels are 2.8V (Hi) and 0.4V (Low)...

So, can I just plug it in without burning something?

The other FTDI chip I've been using (FT232BM) works just fine with no resistors, as there is a pin VCC_IO that is supplied with 3.3V and it limits the IO voltage values to 1.2V, 2.7 and 0.4.

As the Voltage levels are the same for the old chip and this new cable I want to use, Is there anything else that I need to be aware ?

Just to make sure.

Cheers mates,

Ed.

DS_TTL232R3V3.pdf

Message Edited by t.dowe on 2009-10-20 01:22 PM

[bigmac]

Hello Ed,

If the FTDI device and the MCU are supplied from the same 3.3 volt regulator, i.e. derived from the USB interface, the inclusion of series resistors won't be necessary.  However, if the two devices are supplied from separate regulators, and using two different power sources, I would tend to include series resistors, perhaps 2k2 or 4k7, to be sure.

Regards,

Mac