What is the fastest clock speed a 555 timer can go?

 What is the fastest clock speed a 555 timer can go?

It depends. There are or have been about 20 different variations of 555 compatible chips. Bipolar transistor (aka TTL) and MOSFET being the biggest distinction, but some are optimized for speed, some for low voltage, some for low quiescent and running current.

So which one? Choose the one you wish to use, then look up the specific datasheet for that specific part number from that particular manufacturer.

NE555
SE555
TLC555
ICM7555
etc.

The original NE555 timer and NE556 dual timer from Signetics was only rated to work up to 100kHz. That is a very conservative number, as you’d expect from a datasheet, as I commonly used them up to 500kHz. Texas Instruments now makes a nearly identical chip that continues to use the NE555 designation.

Other varieties can go much higher in frequency. Here is a list on Wikipedia that shows just about every variation, with frequency up to 5MHz and supply voltage as low as 0.9V, although not in the same chip.

555 timer IC derivatives - Wikipedia

Note that no minimum frequency is given. This is going to depend on factors like leakage currents in the timing capacitor. Long times require a very high charge/discharge resistance and a large capacitor, however, larger capacitors tend to have more leakage.

Tantalum capacitors have a lot less leakage current than aluminum electrolytic capacitors. Modern ceramic capacitors have very low leakage and come in types with much more capacitance than possible in the past, on the order of 10 to 20uF.

However, those high value, low ESR, low leakage capacitors tend to be very voltage sensitive, losing capacitance at relatively modest voltages. Resulting in rather unpredictable timing.

A company in Australia is or was programming 8 pin microcontollers to act like an enhanced 555 chip in several varieties.

The CSS555 is a programmable 555 timer with internal EEPROM. The Control Voltage pin 5 must be kept above 1V to avoid triggering programming mode, so it is not fully compatible with all circuits. But then, you can’t just drop a CMOS 555 into a circuit designed for a TTL 555 or vice versa and expect zero changes.

http://www.customsiliconsolutions.com/downloads/Revised%20Standard%20products/CSS555_App_Note1_Serial_Interface.pdf