Voltage Upscale and Downscale
Voltage Upscale and Downscale
(OP)
Hi all,
First post! This place seems pretty friendly and full of helpful resources; I hope I can contribute the same amount that I gain from this site =)
Any how here is my question:
I have a system with three main components:
1. microcontroller with VDD @ 5V
2. a pressure sensor with VDD @ 5V
3. a wireless chip with a VDD @ 3V
In order for me to send signals to the wireless chip from the micro; I would need to downscale the voltage 3V instead of 5V.
I was thinking of a voltage divider but for some reason when I attach a load to my micro controller the output voltage drops to about 4.~ volts instead of the usual 5V.
Also, how can I upscale the voltage to 5V when I use it as an input to the micro (5V VDD) from my wireless chip (3V VDD)?
Any insight would be great on this topic. Thanks!
First post! This place seems pretty friendly and full of helpful resources; I hope I can contribute the same amount that I gain from this site =)
Any how here is my question:
I have a system with three main components:
1. microcontroller with VDD @ 5V
2. a pressure sensor with VDD @ 5V
3. a wireless chip with a VDD @ 3V
In order for me to send signals to the wireless chip from the micro; I would need to downscale the voltage 3V instead of 5V.
I was thinking of a voltage divider but for some reason when I attach a load to my micro controller the output voltage drops to about 4.~ volts instead of the usual 5V.
Also, how can I upscale the voltage to 5V when I use it as an input to the micro (5V VDD) from my wireless chip (3V VDD)?
Any insight would be great on this topic. Thanks!





RE: Voltage Upscale and Downscale
If they are not compatible, there are buffers that take care of the differences in logic levels. Lots of information on the web. Just go for it!
Gunnar Englund
www.gke.org
RE: Voltage Upscale and Downscale
I just needed the right direction; I will do some searching! Thanks a lot for your timely response.
Regards,
Garmond
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
Thx for the link, I've looked it over. I think I will go and pick up some of those chips mentioned in the article.
In the mean time I went to my local electronics shop and picked up
Inverters and Diodes.
Upscale: 3V to 5V. I was thinking of puting the 3V logical high through two inverters that will convert the value to a strong 5V high logical
Downscale: 5V to 3V. I was thinking of place 2 - 3 diodes in series in order to drop the voltage down when the signal reaches the wireless chip.
I know that there will be a loop effect if I use the same pin for input and output on my microcontroller; so thats why a colleague suggested that I use one pin dedicated for input and one pin dedicated to output.
That way the loop will no effect the values when I am reading or writing data.
What do you think of this solution?
Thx,
Garmond
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
I just bought this inverter during lunch and it says that Vmin High is 2V and I can easily output that from my wireless chip since the vdd on that is 3.3V.
What is the problem with using diodes? I was thinking of using a voltage divider BUT this will cause power loss.
Here is the inverter that I got:
h
I think that may work.
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
Refer to the following App note for the MSP-430 for designing resistor dividers. The MSP-430 is a 3v processor that sometimes must be connected to 5 volt logic. This is opposite of you issue but this app note provides a lot of info.
"Interfacing the 3-V MSP430 to 5-V Circuits" TI application Note: slaa148.pdf
RE: Voltage Upscale and Downscale
What micro are you goung to use?
RE: Voltage Upscale and Downscale
Without a pull-down resistor, depending on the wireless chip technology if it's TTL then you can't sink the necessary "0" (low) state input current, so the input sees no signal change. If it's CMOS this is even worse since open circuit or effectively "floating" inputs can cause hefty currents to flow within the chip and may even cause it to be permanently damaged.
If the chip is TTL the pull-down resistor needs to be quite a low value, which in turn may load the 5V microprocessor output too much. CMOS is less of a problem since the resistor can be quite a high value and still get reasonable logic levels. |However, signal rise/fall times may then become a problem because of the overall C-R time constant. If the signal rise/fall is too slow this may cause oscillation within the wireless device logic.
Hope that helps explain the design consequences.
RE: Voltage Upscale and Downscale
The Micro I am using is the ATMEGA16:
http:
I see that the Vih = 0.6 * VCC so about 5.5 * 0.6 = 3.3V
Which is a bit low; but then again my output from the wireless chip is only about 3V; So there may be some trouble with the logic there; I don't want to take any chances.
With an inverter the VinHighMin = 2V which is great for me.
Last Night I confirmed a wireless packet received using voltage dividers instead of the diodes..but I am concerned that this may bite me in power consumption later on.
Thanks for such an interactive thread =)
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
I am using the following sensor:
ADSX030A2 which requires 5V VDD datasheet available here:
http:
For the micro I am using ATMEGA16 not the ATMEGA16L which can go all the way down to 2.5V I think. The micro requires a VDD higher than 3.3V I believe; I looked into that to try to save myself some time.
For the good news: I got was able to download packets sent from the receiver =)
My current implementation is with voltage divider for the downscale and two inverters back to back for the upscale. So far it works well.
I was wondering whether anyone could explain why using voltage dividers to downscale the signal is bad engineering design? Thanks all for the help =)
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
Thanks guys.
RE: Voltage Upscale and Downscale
RE: Voltage Upscale and Downscale
If you are not using the "L" version it is a 5 volt part. Too bad for the other interface.
Are the signals bidirectional between the micro and the RF unit? Or just in one direction?
RE: Voltage Upscale and Downscale
Yeah it is bidirection; only for one signal though; the data pin. But this doesn't pose a problem though =)
RE: Voltage Upscale and Downscale