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Original question:

SN74221: SN74221NE4 regarding the pulse width related query

SN74221: SN74221NE4 regarding the pulse width related query

vinay kattela
Prodigy 70 points
Part Number: SN74221
Other Parts Discussed in Thread: CD74HCT221, , SN7400, SN74LV221A, SN74LVC1G123

Hi,

With reference to the datasheet link in TI :  timing is purely based on external components.

Once fired, the outputs are independent of further transitions of the A and B inputs and are a function of the timing components, or the output pulses can be terminated by the overriding clear.

https://www.ti.com/product/SN74221/part-details/SN74221N?keyMatch=SN74221

In the past there was similar part discussion in the forum, following link for reference: overall we thought the timing variation is due to the external component tolerances.

https://e2e.ti.com/support/logic-group/logic/f/logic-forum/1189597/sn74221-sn74221ne4-regarding-the-pulse-width-related-query/4482820?tisearch=e2e-sitesearch&keymatch=%25252520user%2525253A550546#4482820

  ln(2) R56(1%) C26(2%) min typical max
TP27 0.693147181 1210 1.00E-08 8.14E-06 8.39E-06 8.64E-06
  ln(2) R54(1%) C28(10%) min typical max
TP32 0.693147181 3320 1.00E-07 2.05E-04 2.30E-04 2.56E-04

What unclear is, the first multivibrator output at TP27 is very consistent and observed around 8us.

Where as second multivibrator output at TP32 does not match with theoritical calculation range and the timing varies as we vary the trigger frequency?

Based on previous conversations, I did try with better cap tolerance 1% : still there is no change in the output response.

  ln(2) R54(1%) C28(1%) min typical max
TP32 0.693147181 3320 1.00E-07 2.28E-04 2.30E-04 2.33E-04

Herewith I did try to check with 6 different boards, where the output response is consistent and again the timing does vary with trigger frequency.

pls note : here is till 4Khz (230us timing), i just tried to capture timings later to 4khz just for reference only.

Following response is same with 10% or 1% cap at C28 ....

  • 0
    Prodigy 70 points

    I did replace U1 IC with  CD74HCT221, with same 10% Cap in place, 1% resistor in place.

    Only change was the U1 IC part number,  CD74HCT221 IC works as expected, output is very consistent with variation in the trigger frequency.

    It will be really good to understand from TI design team why such variation exists on the SN74221 part.

    I see they do have internal comparator voltages different, like SN74221 cap discharges to a negative reference(as you vary trigger frequency, the discharge level was not consistent) and the part CD74HCT221 just discharges to 0 and charges back to 3.36V, there was no variation on the charge level, seems that makes it to provide consistent output timing.

  • 0 in reply to vinay kattela
    TI__Guru* 96862 points

    Hello,

    It seems that you have certainly done your due diligence with testing this device. I'm afraid we won't be much help here. Although we continue to provide the legacy SN7400 series logic devices to anyone who wants to use them, we can only provide limited support for these devices.

    It will be really good to understand from TI design team why such variation exists on the SN74221 part.

    I expect it's related to the design of the device, as well as the process on which it was built (one of the first IC semiconductor technologies every developed).

    The SN74221 device was developed about 40 years ago (one of my databooks shows 1983 as the original publishing date), so there isn't a single person left at TI who was present for that development, and our computer systems generally only hold data back about 25 years (ie back to when computers were broadly available). Basically, we have as much information as you do at this point - just using the datasheet, existing app notes, and bench testing to go from.

    I would recommend that you use a more modern device like the SN74LVC1G123 or the SN74LV221A for one-shot applications. These have much more consistent performance.