Input pullup for SN74ALS541

Mark Liang

Other Parts Discussed in Thread: SN74LVC8T245, SN74ALS541

Hello,

We need to connect the output of SN74LVC8T245 to the input of SN74ALS541 in our design. Do we need any weak pullup for the input of SN74ALS541?

Best regards,

Mark

over 8 years ago

0 ShreyasRao over 8 years ago

TI__Guru 57340 points

Hi Mark ,

The inputs of the devices are recommended to be at one of the valid logic levels . A weak pullup or pulldown; if current is not much of a concern can be put in .

0 Clemens Ladisch over 8 years ago

Guru 267600 points

What are the VCCs?

0 Mark Liang over 8 years ago in reply to Clemens Ladisch

Intellectual 740 points

The VCCA and VCCB for SN74LVC8T245 are 2.5V and 5V respectively, data is from A to B. The Vcc for SN74ALS541 is 5V.

Best regards,
Mark

0 Clemens Ladisch over 8 years ago in reply to Mark Liang

Guru 267600 points

With the same VCC, CMOS outputs are compatible with TTL inputs.

A pullup would be needed only in the reverse case (TTL output, CMOS input).

0 ShreyasRao over 8 years ago in reply to Clemens Ladisch

TI__Guru 57340 points

Thanks for your contribution to aid the community in general . Appreciate all your precise inputs.
I agree with your post above regarding the compatibility of CMOS and TTL .
The ALS is TTL compliant with 2V Vih and 2V Voh (4.5V) . The CMOS outputs are 3.8V which can interface with the TTL inputs of ALS . However , the ALS output cant be interfaced to LVC8T245 directly at 5V . In your case , with VccA at 2.5V , this can be done . The Vih is 1.7V for 2.5V Vcc .

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Input pullup for SN74ALS541