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NE555: About Astable Operation frequency/ Frequency Divider operation

Hiromu Susami
Intellectual 1400 points

Part Number: NE555

Hi exparts,

Q1. We are considering 555 timer replacement (M51843P=>NE555). From previous schematic, RA=1.5korm, RB=1.5korm, C=0.01uF. Based on equation (4) in data sheet P12, f = 1.44 / {(1.5k + 2 × 1.5k) × 0.01u} = 32kHz. However, looking at the output results of past circuits using the M51843P, it was 16 kHz (RA, RB, and C are unchanged). Do you have any ideas about why this double frequency difference is occurring? Is there a mistake in the formula?

Q2. Does the Frequency Divider(6.3.3) work on the following principles?

  • Once a high is input to S of RS-FF, 1 outputs low (out goes high) until a high is input to R.
  • While OUT is high, C is charged until the voltage at C exceeds 2/3 x Vcc.
  • During this period, even if a pulse is input to the TRIG pin, there is no change in the RS-FF output (it remains low), so the division function is realized by adjusting the time constant. 

Q3. In order to drive a frequency divider, I think it is important to balance the time constant with the period of the input pulse to trig. Is there a formula that needs to be met to achieve this? 

image.png

Regards,
Hiromu

  • +1
    TI__Guru**** 174096 points

    Hello Hiromu,

    The M51843P formula in application example (2) has the same formula as NE555.  The minimum Fmax of M51843P is 10kHz, so the propagation delay might be much higher than NE555 so it might run much slower. NE555 will run a little under 32kHz, but not 16kHz.

    Propagation delay is a factor with these slower timers, so changing Ra, Rb, C is needed when changing timer used.