TTL 5.0v vs 3.3V
TTL 5.0v vs 3.3V
by JavaRN
Mon Jul 07, 2008 7:06 pm
I was considering connecting a CMU CAM camera to my Robonova for colour blob detection. However while reading the camera specs I came across the following "TTL 0 to 3.3V". I was going to connect the camera as follows:
CAMERA -> MR-C3024
GND -> GND
TX (TTL) -> ETX
RX (TTL) -> ERX
+5V -> +5V (from the upper row where sensors are)
Would the following connections damage the controller or the camera, since the Robonova Controller's TTL is 0..5V?
CAMERA -> MR-C3024
GND -> GND
TX (TTL) -> ETX
RX (TTL) -> ERX
+5V -> +5V (from the upper row where sensors are)
Would the following connections damage the controller or the camera, since the Robonova Controller's TTL is 0..5V?
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by HumanoidFan
Mon Jul 14, 2008 3:23 pm
Hi,
I don't think it will be a problem. I have done this many times with DIFFERENT PCB boards.
To be absolutely sure I would contact the CMU manufacturer and ask.
you could also look at the specs for the CHIP that the robonova is connected to. You should be able to find the maximal Vin.
On the Robonova side, inputing 3.3V signal should not affect the MCU board.
I don't think it will be a problem. I have done this many times with DIFFERENT PCB boards.
To be absolutely sure I would contact the CMU manufacturer and ask.
you could also look at the specs for the CHIP that the robonova is connected to. You should be able to find the maximal Vin.
On the Robonova side, inputing 3.3V signal should not affect the MCU board.
by JavaRN
Mon Jul 14, 2008 4:23 pm
I tried it and it worked. There was a note on the manual which said that the CMUCAM is 5V tolerant for inputs and I didn't see it 
Thanks
ThanksF'dan il-passatemp ghandek bzonn zewg affarijiet - FLUS u HIN. Zewg affarijiet li huma skarsi hafna u li jien minnhom ghandi vera ftit!
by i-Bot
Mon Jul 14, 2008 4:25 pm
With all these interface problems there are two questions:
1) will it do any damage ?
2) will it work ?
For 1) the LPC2106 which I understand is used in the CMU CAM is 5V tolerant which means it can accept up to about 6 V on the inputs. Therefore you are safe to make a direct connection. Other 3.3V devices which are not 5V tolerant must have a a series resistor or a FET to make the interface with 5V, depending on the chips.
For 2) the LPC2106 has TTL compatible inputs (VIH = 2V, VIL = .8V) and TTL compatible outputs (VOH = 2.9V, VOL, .4V). The ATMega128 is not really TTL compatible on the input (VIH = 3V[0.6*Vcc] VOL = 1V[0.2* Vcc] and is TTL compatible on the output (VOH = 4.2V, VOL = 0.7V). It appears the VOH of the CMUCAM misses the VIH of the MR-C3024 by 0.1V. Given the low currents and low speeds I would expect it to work OK in practice. If not a small pull up on the C3024 input to 5V should resolve.
Also understand different versions of the MR-C3024 label the ETX and ERX pins differently. I think the early boards send data from the MR C3024 on ERX and receive on ETX, later send on ETX and receive on ERX. It seems to be the screen print not the board that differs.
1) will it do any damage ?
2) will it work ?
For 1) the LPC2106 which I understand is used in the CMU CAM is 5V tolerant which means it can accept up to about 6 V on the inputs. Therefore you are safe to make a direct connection. Other 3.3V devices which are not 5V tolerant must have a a series resistor or a FET to make the interface with 5V, depending on the chips.
For 2) the LPC2106 has TTL compatible inputs (VIH = 2V, VIL = .8V) and TTL compatible outputs (VOH = 2.9V, VOL, .4V). The ATMega128 is not really TTL compatible on the input (VIH = 3V[0.6*Vcc] VOL = 1V[0.2* Vcc] and is TTL compatible on the output (VOH = 4.2V, VOL = 0.7V). It appears the VOH of the CMUCAM misses the VIH of the MR-C3024 by 0.1V. Given the low currents and low speeds I would expect it to work OK in practice. If not a small pull up on the C3024 input to 5V should resolve.
Also understand different versions of the MR-C3024 label the ETX and ERX pins differently. I think the early boards send data from the MR C3024 on ERX and receive on ETX, later send on ETX and receive on ERX. It seems to be the screen print not the board that differs.
