A ttl delay question (redux) :o)

[rgb]

Hi,

I need to have some ttl level delays (i'm using 74LS type of components).

Something around 1msec.

So I guess a RC with 1k and 1microF should be ok ...

--- R -------- Schmitt Trigger 74ls14 ---
C
|
--------------


But wait a sec, (I try to understand in my old ttl databook what is
really a simple gate (transistors and all that stuff ... )

I guess I should use something more like

74ls04 ---- R --------- R -------- Schmitt Trigger 74ls14
C
|
------------------------------------------

cause gate can also sink current ...
The front inverter serve as buffer for other gates...


Am I right ? Is that the way to do delay like 1ms ?

 

[John Larkin]

On Mon, 27 Oct 2003 12:58:31 -0500, rgb <rgb@blah.com> wrote:

Hi,

I need to have some ttl level delays (i'm using 74LS type of components).

Something around 1msec.

So I guess a RC with 1k and 1microF should be ok ...

--- R -------- Schmitt Trigger 74ls14 ---
C
|
--------------


But wait a sec, (I try to understand in my old ttl databook what is
really a simple gate (transistors and all that stuff ... )

I guess I should use something more like

74ls04 ---- R --------- R -------- Schmitt Trigger 74ls14
C
|
------------------------------------------

cause gate can also sink current ...
The front inverter serve as buffer for other gates...


Am I right ? Is that the way to do delay like 1ms ?

You don't need the second R, but the first R should be lower in value
to give a reliable logic low... the 'LS input tries to pull itself up
(sources current), and there's not a lot of margin. The delay will be
fairly non symmetric on rise/fall with 'LS; an HC-series schmitt
trigger has zero input current and more symmetric thresholds. Much
better if both the driver (your LS04) and the Schmitt are CMOS.

John