question about a reference design

[ican7]

Hi , I would like to include a reference design in to my project and below in the pdf there is a reference schematic for it. I would like to use the EVAL-RS485FDEBZ version. For that particuler reference design there are options and I choose the ADM489A chip. I would like to connect the pins of ADM489A to ATMEGA2560-16AU. My question is can I directly connect the RE,RO,DE,DI pins of ADM489A to the microcontroller?

[robertferanec]

ican7, I understand your question, but by providing you the straight answer I would not really help you So, what you can for example do:

- Find datasheet of the chip (ADM489A)
- In the datasheet, check out power voltage, input voltage and output voltage of the chip for the LOGIC
- Find datasheet of your microcontroller. Check out power voltage, input voltage and output voltage for the IO pins
- Check out if pull ups / downs are not required

I would also recommend you:
- order the kit, power it up, measure the levels on the pins
- connect the pins with your microcontroller board and see if everything works oki

[ican7]

Hi Robert thank you for your reply. I checked those specifications and conclude that I can connect the ADM489 directly to Atmega. And as for the pull up and pull down resistors , it wasn't mentioned on ADM489 datasheet however in Atmega 2560 it states for I/O pins min 20khm and max 50khm pull ups .But I think I don't need them since I want to control both receiver and enable pins via microcontroller. So below I include the places I looked in to from the data sheet. I hope I am correct?

[robertferanec]

Please, let me know then if everything worked oki. I am sure you can do it right. This is exactly how we all learn. Do not worry. Even if something was wrong, you will be able easily to fix it. It is not a complicated design. If you are not sure, I recommend you to buy the boards, connect them, measure them and test them before you design your own board.

Again, I could tell you if it is right or wrong, but it would not help you to learn I am sure you understand.

[ican7]

Hi Robert thank you for your response. Is it possible if you tell me at least the parameters mentioned were correct or not? Is this outlined in one of your course? or is there any guidlines that you could reccomend me to read? Because there are many parameters given in both datasheets and I am not quıte sure If I am looking at the right one... By the way my project involves more modules and at the moment US dollar is fluctioating too much against our currency so for now I will need to make use of the datasheets otherwıse as you said its best to purchase them and test them.

[robertferanec]

CMOS Input Logic Threshold Low, CMOS Input Logic Threshold High, CMOS Output Voltage Low, CMOS Output Voltage High, VCC, Driver Input (DI), Control Inputs (DE, RE), Receiver Output

[ican7]

Hi Robert,
According to Atmel 2560 and the ADM489 datasheets I think that I could connect the ADM489chip straight to Atmel 2560 because the minimum and max values of the voltage logic transition values are within the range both chips specs. But one thing I dont get is that the TYP values are less or more than the min or max values.

ADM489
CMOS Input Logic Threshold Low, VINL
TYP 1.4V ( I dont understand why typical value is more than the max voltage)
MAX 0.8V (maximum voltage that ADM489 still reads as LOW)

CMOS Input Logic Threshold High, VINH
MIN 2.0V ( Minimum value where the ADM489 still reads as HIGH)
TYP 1.4V (I don't understand why the typical value is less then the minimum value)


CMOS Output Voltage Low, VOL

MAX 0.4V ( Max voltage for sending as LOW signal)


CMOS Output Voltage High, VOH

MIN 4.0V ( Min voltage for sending as HIGH signal)

For ATMEL 2560

VIL Input Low Voltage, Except XTAL1 and Reset pin

MIN -0.5V (Maximum negative value where Atmel still reads as LOW)

MAX 0.3VCC for VCC 5V therefor 1.5V ( Maximum positive value where Atmel still reads as LOW)

VIH Input High Voltage,Except XTAL1 and RESET pins

MIN 0.6VCC for VCC 5V therefor 3V
MAX VCC + 0.5 therefor 5.5V (from 3V to 5.5V Atmell will read the signal as HIGH)

VOL Output Low Voltage,Except RESET pin
MAX .9V ( Max voltage for sending as LOW )

VOH Output High Voltage ,Except RESET pin
MIN 4.2V (Min voltage for sending as HIGH )

[robertferanec]

CMOS Input Logic Threshold Low, VINL
TYP 1.4V ( I dont understand why typical value is more than the max voltage)
MAX 0.8V (maximum voltage that ADM489 still reads as LOW)

This means, that in most cases this chip will recognize LOW level as soon as the pin input voltage drops down to 1.4V. However, to be sure the LOW level is ALWAYS correctly recognized, the input voltage must be lower than 0.8V.

CMOS Input Logic Threshold High, VINH
MIN 2.0V ( Minimum value where the ADM489 still reads as HIGH)
TYP 1.4V (I don't understand why the typical value is less then the minimum value)

This means, that in most cases this chip will recognize HIGH level as soon as the pin input voltage rises up to 1.4V. However, to be sure the HIGH level is ALWAYS correctly recognized, the input voltage must be higher than 2.0V.

Than, you have to compare Input vs Output -> Output pin always has to give a level within the tolerance when input pins reads ALWAYS the correct value. So for example if Atmel LOW output is 0-0.9V and Input LOW level for ADM is 0-0.8V, that may in some limiting cases fail (when Atmels output is 0.9V it is outside of tollerance 0.8V for ADM input). In reality, this should be ok, but you know, it is outside of the range what manufacturer guarantees.