Timer IC

The section on application circuits in the data sheet of the uPC1555 has the following: Note 9: Keep the trigger pulse width smaller than the output pulse width. What happens when the trigger pulse width is longer than the output pulse width?

Both the uPC1555 and the uPD5555 do not function as timers. They do not operate normally, for example the same output as the input pulse is output.

In the uPC1555 Data Sheet, there is an example of a monostable multivibrator. In this example, it says, "The output pulse width (delay) is determined by the expression t = 1.1·R₁·C₁." How is the coefficient 1.1 determined?

The expression is calculated assuming inverse operation under the ideal conditions of a pin voltage of 0 V when the capacitor discharges and a threshold pin input voltage of 2/3 Vcc.
When a low level is input to the trigger pin, the internal latch inverts turning the discharge pin off. The capacitor's charge voltage then rises. When this voltage reaches 2/3 Vcc, the internal latch inverts back by 2/3 Vcc applied to the threshold pin, causing the discharge pin to revert to its original ON state. This process is known as a monostable operation.
The expression "capacitor pin voltage = (1-exp(-t/C₁·R₁))Vcc^*" rises to 2/3 Vcc as follows:
1-exp(-t/C₁·R₁)=2/3
exp(-t/C₁·R₁)=1/3
t/C₁·R₁=ln3=1.09861...≈1.1
t=1.1·C₁·R₁

*Note   For more information on this expression, see the "Time constant" FAQ in the Background Knowledge section.

(2007/01)

The data sheets of the uPC1555 and uPD5555 describe astable multivibrator examples, stating that "when the output voltage is high, the charge time is t₁ = 0.693·(R₁ + R₂)·C₁", but how is the 0.693 coefficient obtained?

Take the case of the astable multivibrator of the uPC1555 as an example.
The expression is calculated under the assumed ideal conditions that the output pin signal is repeatedly inverted by repeatedly charging and discharging of capacitor C₁ between a trigger pin input voltage of 1/3Vcc and a threshold pin input voltage of 2/3Vcc.
When the trigger pin voltage drops to 1/3Vcc or lower, the internal latch inverts, turning the discharge pin off, and the charge voltage of capacitor C₁ rises. When this voltage reaches 2/3Vcc, the internal latch pin is inverted by 2/3Vcc applied to the threshold pin, turning the discharge pin on, and the charge voltage of capacitor C₁ drops. When it reaches 1/3Vcc, the internal latch is inverted, turning the discharge pin off, and the charge voltage of capacitor C₁ rises.
Regarding the application circuit example shown in Fig. d of the data sheet, the charge and discharge times are obtained as follows.

<Charge Time>
The charge time being the time it takes for t₃ to change to t₄ in Figure 3 below, t₃ and t₄ are obtained and then ( t₄ - t₃ ) is calculated as the charge time.
Designating the pin voltage of capacitor C₁ (in other words, the voltage of pin 6) as Vc and the charge supply voltage as Vs, based on the time constant formula*, the following expression is obtained:
   Vc=Vs{1-exp(-t/CR)}   -----(1)
First, substituting t = t₃ , Vc = 1/3Vcc, Vs = Vcc, R = R₁ + R₂, and C = C₁ in expression (1) above, time t₃ , which is the time it takes for the difference in potential between the two pins of the capacitor to change from 0 V to 1/3Vcc, is calculated as follows:
   1/3Vcc=Vcc{1-exp(-t₃/((R₁+R₂)·C₁))}
1-exp(-t₃/((R₁+R₂)·C₁))=1/3
exp(-t₃/((R₁+R₂)·C₁))=2/3
t₃/(R₁+R₂)·C₁=ln(3/2)=0.405
Therefore, t₃ = 0.405·(R₁ + R₂)·C₁.

Next, substituting t = t₄ , Vc = 2/3Vcc, Vs = Vcc, R = R₁ + R₂, and C = C₁, in expression (1) above, time t₄ , which is the time it takes for the difference in potential between the two pins of the capacitor to change from 0 V to 2/3Vcc, is calculated as follows:
   2/3Vcc=Vcc{1-exp(-t₄/((R₁+R₂)·C₁))}
1-exp(-t₄/((R₁+R₂)·C₁))=2/3
exp(-t₄/((R₁+R₂)·C₁))=1/3
t₄/(R₁+R₂)·C₁=ln3=1.098
Therefore, t₄ = 1.098·(R₁ + R₂)·C₁.

Finally, since t₁, which is the time it takes for the charge of the capacitor pin voltage (= 1 - exp (-t1/(( R₁ + R₂) C₁))) Vcc ^*) to change from 1/3Vcc to 2/3Vcc, is determined as t₁ = t₄ - t₃ , the following expression is obtained:
   t₁=1.098·(R₁+R₂)·C₁-0.405·(R₁+R₂)·C₁=0.693·(R₁+R₂)·C₁  -----(2)

The coefficient can therefore be calculated as 0.693.

<Discharge time>
During discharge, the discharge current flows from C₁ through R₂, so that, obtaining discharge time t₂ in a similar manner with R = R₂, the following expression results:
   t₂=0.693· C₁· R₂  -----(3)

Therefore, oscillation period T being the sum of charge time t₁ and discharge time t₂, the following expression is obtained from expressions (2) and (3):
   T= t₁+ t₂=0.693· C₁·(R₁+2 R₂)  -----(4)

* Caution: For more information on this expression, see the "Time constant" FAQ in the Background Knowledge section.

(2007/01)

I am using the uPD5555. The output signal may or may not be produced just after power-on. Why?

It internally has a flip-flop that is created by positively feeding back two stages of inverters.
The state of this flip-flop is undefined just after power is applied.
The unexpected output is produced when this flip-flop is in the triggered status.
Countermeasure:
  The flip-flop can be reset if the reset pin is held low when power is applied.

A resistor is connected to the control pin to fine-tune the oscillation frequency of the uPD5555. Is there any problem?

Although the timer can be used in that way, note the following points.
1) The trigger voltage changes if this pin is manipulated.
2) A voltage to be supplied to this pin is limited.
  Refer to the Data Sheet for the voltage range.
3) This pin is not an input pin.
  A voltage is generated by using three resistors of approximately 200 kΩ.
  Note the impedance (about 130 kΩ) of the pin.

Can I replace the uPD5555 (CMOS) timer IC with the uPC1555 (bipolar) timer IC, which has the same pin assignment?

The functions of the two ICs are the same, but the uPC1555 differs in terms of electrical specifications due to its bipolar characteristics.
Be sure to evaluate the uPC1555 carefully before use, paying special attention to the cautions described in the data sheet.

(2006/03)

Can the IC be damaged or any other problems occur if Trigger and Reset are input simultaneously?

No. There is no problem with simultaneous Trigger and Reset. The device is designed to give priority to Reset when Trigger and Reset are input simultaneously.

(2007/09)

In a monostable multivibrator configuration, if a trigger is input again during a high-level pulse output, does a retrigger operation occur?

No. A retrigger operation does not occur.

(2007/09)

Why is the actual pulse width different from the pulse width given in the data sheet, and why is the actual oscillation frequency different from that given by the theoretical equation?

As described in FAQ timer-0201 or timer-0202, the theoretical equation assumes ideal performance by the capacitor and other elements, uses the typical threshold voltage (2/3V_DD), and does not take propagation delay into consideration. Therefore, values obtained with this equation are approximate. When greater precision is required for the output pulse width, it is recommended that you determine the values of R and C on the basis of measuring every data (and, when applicable, consider using semi-fixed elements).
The main factors that can cause actual values to differ from the theoretical values are:

• Effect of IC propagation delay: Especially in the case of high-speed pulses.
• Variation in threshold voltage: The threshold voltage is set by internal voltage-dividing resistor. Resistance varies within the range of relative error.
• Effect of capacitor leakage current: Depending on the utilized capacitor, the actual pulse width can be larger than the theoretical value due to the effect of capacitor leakage current, especially for long pulse settings.

(2007/09)

How are the internal reference voltages provided?

As shown in the figure, the control voltage and trigger reference voltage are obtained by resistance division.
The resistance value is 200 kΩ x 3 for the uPD5555 and uPD5556, and the relative precision is about 2 to 3% (reference values).

(2007/09)

What is the range for external voltage applied to the control pin?

The voltage applied to the control pin should be within the range specified in the data sheet: 2 to 4 V (at V_DD = 5 V).

(2007/09)

What is the purpose of the capacitor connected to the control pin?

This is to prevent misoperations caused by external noise.
The capacitor connected to the power supply pin is important for stability. Be sure to locate a capacitor between the power supply and GND.

(2007/09)