Summary of Reverse engineering the popular 555 timer chip (CMOS version)
The article reverse-engineers the LMC555 CMOS timer, explaining its tiny transistors, resistors, metal interconnects, and doped silicon regions visible on the die. It describes functional blocks—comparators, current mirrors, flip-flop, discharge transistor—and shows how the 555 operates as an oscillator by charging and discharging an external capacitor between internal 1/3 and 2/3 supply reference voltages set by an internal resistor divider.
Parts used in the LMC555 Project:
- Silicon die with doped N-type and P-type regions
- Metal interconnect layer
- Polysilicon wiring
- MOS transistors (low-power CMOS)
- Resistors formed in silicon
- Internal three-resistor voltage divider
- Upper (threshold) comparator
- Lower (trigger) comparator
- Flip-flop
- Discharge transistor
- External timing capacitor (for oscillator circuit)
- External resistors (for charging/discharging capacitor)
Reverse engineering the popular 555 timer chip (CMOS version)
The structure of the integrated circuit
The photo below shows the silicon die of the LMC555 as seen through a microscope, with the main function blocks labeled (photo from Zeptobars). The die is very small, just over 1mm square. The large black circles are connections between the chip and its external pins. A thin layer of metal connects different parts of the chip. This metal is clearly visible in the photo as white lines and regions. The different types of silicon on the chip appear as different colors. Regions of the chip are treated (doped) with impurities to change the electrical properties of the silicon. N-type silicon has an excess of electrons (making it Negative), while P-type silicon lacks electrons (making it Positive). On top of the silicon, polysilicon wiring shows up as other colors. The silicon regions and polysilicon are the building blocks of the chip, forming transistors and resistors, which are connected by the metal layer.
A brief explanation of the 555 timer
The 555 chip is extremely versatile with hundreds of applications from a timer or latch to a voltage-controlled oscillator or modulator. To explain the chip, I will use one of the simplest circuits, an oscillator that cycles on and off at a fixed frequency.The diagram below illustrates the internal operation of the 555 timer used as an oscillator. An external capacitor is repeatedly charged and discharged to produce the oscillation. Inside the 555 chip, three resistors form a divider generating reference voltages of 1/3 and 2/3 of the supply voltage. The external capacitor will charge and discharge between these limits, producing an oscillation, as shown on the left. In more detail, the capacitor will slowly charge (A) through the external resistors until its voltage hits the 2/3 reference. At that point (B), the threshold (upper) comparator switches the flip flop off turning the output off. This turns on the discharge transistor, slowly discharging the capacitor (C) through the resistor. When the voltage on the capacitor hits the 1/3 reference (D), the trigger (lower) comparator turns on, setting the flip flop and the output on, and the cycle repeats. The values of the resistors and capacitor control the timing, from microseconds to hours.
Read more: Reverse engineering the popular 555 timer chip (CMOS version)
- What version of the 555 does the article analyze?
The article analyzes the LMC555 low-power CMOS version of the 555 timer. - How small is the LMC555 silicon die?
The die is just over 1 millimeter square. - What internal components set the 1/3 and 2/3 reference voltages?
An internal three-resistor voltage divider generates the 1/3 and 2/3 supply reference voltages. - How does the 555 create an oscillation?
An external capacitor charges and discharges between the 1/3 and 2/3 reference voltages, with comparators and a flip-flop controlling output and the discharge transistor to produce oscillation. - What types of transistors are used in the LMC555?
The LMC555 is built from low-power MOS transistors (CMOS). - What visible layers are noted on the die photo?
The metal interconnect layer appears as white lines, polysilicon wiring appears as other colors, and doped silicon regions appear in different colors. - What happens when the capacitor reaches the 2/3 reference?
The threshold (upper) comparator switches the flip-flop off, turning the output off and enabling the discharge transistor to discharge the capacitor. - What happens when the capacitor reaches the 1/3 reference?
The trigger (lower) comparator sets the flip-flop, turning the output on and allowing the capacitor to charge again.
