NE555: Function Table

Amy Luo

Part Number: NE555

Hi team,

According to the functional Block Diagram of the NE555, which is an RS trigger internally, the output of the RS trigger should be indeterminate when the R and S pins are both high 1.Why does the truth table give NE555 outputs that are Irrelevant to the threshold pin status when TRIG voltage < 1/3VCC?

Best Regards,

Amy Luo

over 3 years ago

0 Chris Featherstone over 3 years ago

TI__Mastermind 25425 points

Hello Amy,

Good question! The pin priority will be for trigger in this scenario regardless of the level of the threshold voltage. Since trigger is less than 1/3 VCC the output will be high. The pin priority is reset takes first priority and the trigger pin takes second priority. Let me know if I can help with any other questions.

Best Regards,

Chris Featherstone

0 Amy Luo over 3 years ago in reply to Chris Featherstone

TI__Expert 6895 points

Hi Chris,

You mean the NE555 internal circuitry integrates a priority circuit so that when trig voltage < 1/3VCC, the NE555 outputs a high level regardless of the threshold voltage. Is this right?

0 Chris Featherstone over 3 years ago in reply to Amy Luo

TI__Mastermind 25425 points

Hello Amy,

Yes this is correct. The priority by design is given to the trigger input regardless of the level of the threshold voltage.

Best Regards,

Chris Featherstone

0 Amy Luo over 3 years ago in reply to Chris Featherstone

TI__Expert 6895 points

Thank you.

0 Amy Luo over 2 years ago in reply to Chris Featherstone

TI__Expert 6895 points

Hi Chris,

Can you provide the internal circuit or block diagram of the NE555, especially this priority circuit

Thank you.

Amy

0 Chris Featherstone over 2 years ago in reply to Amy Luo

TI__Mastermind 25425 points

Amy,

I am unable to provide the internal circuit for this device, however it can be viewed as a basic SR Latch as shown below. The Q(Not) output is taken and the final output will be high for the 555.

Best Regards,

Chris Featherstone

Clock & timing

Clock & timing forum

NE555: Function Table