SN74LVC16T245: SN74AHCT126

yaniv rosenberg

Part Number: SN74LVC16T245
Other Parts Discussed in Thread: SN74LVCR16245A

Hello

i have a problem using SN74AHCT126DGVR,SN74LVC16T245DGVR

i have 2 types of discrete line:

1). configurable discrete which is connected to both SN74AHCT126DGVR (used as output driver),SN74LVC16T245DGVR(used as input buffer/voltage converter, both ICs are 5V (VCCB) and the control line/Fpga input is 3.3V(VCCA) . once the line is set to output ,the input buffer is 'Z' and the other way around

2). Output discrete which is driven by SN74LVC16T245DGVR, 5V (VCCB) 3.3V(VCCA)

when both configuration set to output , configuration 1 give me 7v on the output as "high" , and on configuration 2 i see normal levels. what is the reson for that? is it related to the different logic famelies(HCT and LVC)?

configuration 1:

configuration 2 :

over 5 years ago

0 Karan Kotadia over 5 years ago

TI__Genius 16725 points

Hello,

Can you provide me with scope shots? Why do you have 5V pull up resistors at the output from the VPX? I don't see anything specifically wrong with the schematic so if I can get scope shots maybe I can help narrow down the problem.

Thanks!
- Karan

0 yaniv rosenberg over 5 years ago in reply to Karan Kotadia

Prodigy 70 points

Hello

the pullups are for the inputs

SN74AHCT126DGVR is enabled when the DISC_CONF_1 is needed as Output ,but when DISC_CONF_1 function as input then SN74AHCT126DGVR is disabled and then the pullup is needed..

regarding the scope:this 50MHz test driven to 24ohm resistive load

0 Karan Kotadia over 5 years ago in reply to yaniv rosenberg

TI__Genius 16725 points

Hello,

So it looks like your signal has over shoot and under shoot. I have drawn lines from ground to the 3.3V so you can clearly see this.

To solve this issue, you can add a damping resistor or a capacitor, or a mix of both to increase the load and lower the ringing.

Another option would be to use a part like the SN74LVCR16245A. This part has a built in series resistor that would slow the edges of the output and reduce the switching noise.

Thanks!

-Karan

0 yaniv rosenberg over 5 years ago in reply to Karan Kotadia

Prodigy 70 points

thank you for your replay

why do we get this over shoot? we tried to add series resistor and it did not improve the outpit, still overshoot

thanks

yaniv

0 Karan Kotadia over 5 years ago in reply to yaniv rosenberg

TI__Genius 16725 points

Hi Yaniv,

The effects of parasitic capacitance and inductance on the board can be considered a type of LC resonance.
The following are methods to possibly reduce your overshoot and undershoot.

1) Increased external gate resistance. What series resistor value did you use?
2) Smaller gate drive circuit output capacitance. What is the load capacitance?
3) Reduced wiring parasitic inductance of the gate drive circuit. If your traces are too long, they will cause ringing on the output. Can you provide me with some layout of your design?

When the resistance is small the overshoot/undershoot peak value will increase and prolong the ringing decay time.
Also, a higher capacitance will slow down the switching speed.
And the inductance should be as small as possible.

Thanks!
- Karan

0 yaniv rosenberg over 5 years ago in reply to Karan Kotadia

Prodigy 70 points

hello

we do not have a series resistor so at first step your advice with the IC you wrote

regarding the layout i will check how to attach this

thanks

yaniv

0 Karan Kotadia over 5 years ago in reply to yaniv rosenberg

TI__Genius 16725 points

Hello Yaniv,

Do you still have a question or have you figured out your issue?

Thanks!
- Karan

0 yaniv rosenberg over 5 years ago in reply to Karan Kotadia

Prodigy 70 points

hello

i am waiting for the buffers with the series resistors and i might have more questions

thank you

0 yaniv rosenberg over 5 years ago in reply to yaniv rosenberg

Prodigy 70 points

Dear Karan
sorry but i got confused .

the over/undershoot is on the "OUTPUT" lines "DISC_OUR_1"..and not on "DISC_CONF_1"

the problem is on the output of the voltage translator that has no pullups

0 Karan Kotadia over 5 years ago in reply to yaniv rosenberg

TI__Genius 16725 points

Hello Yaniv,

Your setup looks correct. I will tell you the same thing, try to add a series resistor to the output. Please let me know what load is being driven. If there is mismatch on the output, there will be ringing which causes this overshoot and undershoot you see.

Thanks!
-Karan

0 yaniv rosenberg over 5 years ago in reply to Karan Kotadia

Prodigy 70 points

hello

i do not know what is the other side (customer's) load, the demand is that VOL(<0.8), VOH(>2.4V) of this line will support TTL levels of 5V

0 Karan Kotadia over 5 years ago in reply to yaniv rosenberg

TI__Genius 16725 points

Hi Yaniv,

Have you tried the series resistor? Any luck?

Thanks!
- Karan

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SN74LVC16T245: SN74AHCT126