Oscillator Circuit & Timing in 8051

Oscillator: Crystal between XTAL1 (19) and XTAL2 (18) with 33pF caps to GND, plus internal inverter amplifier. Typically 11.0592 MHz (baud friendly) or 12 MHz (1µs cycle). Generates clock divided by 12 → machine cycle. Can also inject external clock to XTAL1 with XTAL2 floating (CMOS) or XTAL2 for HMOS.

Timing: One machine cycle = 12 oscillator periods = 6 states S1-S6 ×2 phases P1/P2. Most instructions 1-4 machine cycles: e.g., MOV A,Rn 1 cycle (1.085µs @11.0592M), MUL AB 4 cycles. ALE pulses at 1/6 Fosc, \overline{PSEN} active low during external fetch. Timer increments each machine cycle. Baud = (2^SMOD/32)*(Fosc/12/(256-TH1)). Reset requires 2 machine cycles high on RST.