SN74AVC4T774: JTAG voltage level transceiver

Asaf Abraham

Prodigy 150 points

Part Number: SN74AVC4T774
Other Parts Discussed in Thread: SN74AXC4T774, TXB0104, , TXB0108

Hello,

I need to convert the JTAG signals from 3.3V to 1.8V levels.

I thought of using the SN74AVC4T774RGYR.

Can this device do the job?

Is there any other device which can do this convertion?

Thnaks,

Asaf

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 243180 points

This device is suitable, but consider the newer SN74AXC4T774.

0 Rami Mooti1 over 3 years ago

TI__Mastermind 23373 points

Hey Asaf,

I'd recommend the SN74AXC4T774 as an upgraded to the AVC part with better performance. Only has a smaller RSV package.
The TXB0104 is another option. This one is auto-bidirectional so you won't need see the direction controlled pins (DIR) that are used to set the direction of the I/O pins as you do in the AXC so the pin count can be smaller, therefore reducing the size of the device as well. Caveat here is less drive strength in the TXB than the AXC.

Here's an FAQ on recommendations for level shifting devices based on your application and voltage level.
The voltage translation buying guide is also useful and provides more detail.

Thanks,
Rami

0 Asaf Abraham over 3 years ago in reply to Clemens Ladisch

Prodigy 150 points

Hi,

Thanks for the answer.

I've made a comparison between the SN74AXC4T774 and the SN74AVC4T774.

It seems that the SN74AXC4T774 has larger propagation delays.

Why is the SN74AXC4T774 more recommended than the SN74AVC4T774?

Best regards,

Asaf

0 Asaf Abraham over 3 years ago in reply to Rami Mooti1

Prodigy 150 points

Hello there,

Thanks for the answer.

Since I have few components connected to the JTAG, I thought of using the TXB0108RGYR.

I saw there are differences at the switching timings between these devices.

The differences are not major, but I want to make sure that the TXB0108RGYR will do the job properly.

Best regards,

Asaf

0 Rami Mooti1 over 3 years ago in reply to Asaf Abraham

TI__Mastermind 23373 points

Asaf,

The propagation delay Prop delay is higher in the AXC because of the delay due to the suppression feature. However, the device can receive the next bit even before the output has finished switching the previous bit. So propagation delay isn't a reliable method to check the speed of the device. The timing characteristics of the device are more limited by the output driver. AXC has slightly stronger driven outputs.
TXB0108 will work if the low drive strength meets your requirements, but note that this isn't a switch so you can't select the output if you're trying to connect 1 JTAG device to 2.

Rami

0 Asaf Abraham over 3 years ago in reply to Rami Mooti1

Prodigy 150 points

Hi Rami,

Thanks for your replys.

I looked at the TXB0108. I need to drive a Xilinx FPGA which has about 15k input pull up resistance.

According to the TXB0108 it needs minimum 50k pull up resistor due to its' low drive strength.

1. I didn't see this limitation at the SN74AVC4T774. Does it have also this limitation?

2. Can you reccomend other device than TXB0108 which can do the job, for buffering JTAG?

Best regards,

Asaf

0 Rami Mooti1 over 3 years ago in reply to Asaf Abraham

TI__Mastermind 23373 points

Asaf,

The AVC wouldn't have this limitation. But I would recommend going with the AXC version of most any AVC device you select.
What exactly are you pulling up? Usually 4 JTAG lines (TCK, TDI, TMS, and TDO) are level shifted in JTAG application and JTAG is pushpull so typically you wouldn't need a pullup.
If you're looking for an 8 bit level shifter that can handle your setup and you have to have the 15k pullups so the TXB wouldn't work, the best option would be 2 x SN74AXC4T774 which allows for individual direction control of the pins.

Rami

0 Asaf Abraham over 3 years ago in reply to Rami Mooti1

Prodigy 150 points

Hi Rami,

Thanks for your most helful answer.

Ii drive Xilinx FPGA which has a 15k intenal pull up.

If you dont mind, I'll ask again - why use the SN74AXC4T774 and not the SN74AVC4T774 ?

Accpet it is newer.

I ask since we already have the SN74AVC4T774 at out system, and a new device will take time to buy.

Best regards,

Asaf

+1 Rami Mooti1 over 3 years ago in reply to Asaf Abraham

TI__Mastermind 23373 points

Asaf,

Either one is acceptable. If the AVC is already in your system and you don't have time to switch it out, it will still be suitable.
The AXC is faster, has a wider supply range and guarantees glitch free power up. [app not for AXC].

Thanks,
Rami

0 Asaf Abraham over 3 years ago in reply to Rami Mooti1

Prodigy 150 points

Rami,

Thanks for your answers.

Your answers were very helpfull.

Best regards,

Asaf

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SN74AVC4T774: JTAG voltage level transceiver