Accurately compute TH (High byte) and TL (Low byte) register values for Intel 8051-based microcontrollers.Supports standard 12‑clock architecture, all timer modes, and provides actual delay error analysis.
The Intel 8051 microcontroller integrates two 16-bit timers/counters (Timer0 and Timer1). These timers operate in four distinct modes. Our calculator focuses on the three most practical modes: Mode0 (13-bit timer), Mode1 (16-bit timer), and Mode2 (8-bit auto-reload). Each mode offers a unique balance between maximum delay range and resolution, critical for embedded system design.
Core Formula: Timer count = Desired Delay / Machine Cycle
Machine cycle (standard 8051) = 12 / Crystal Frequency (in MHz) [µs]
Initial value = Maximum count – Required counts;
TH = (Initial >> bits) , TL = Initial & mask
13‑bit timer (TH0: 8 bits, TL0: lower 5 bits). Maximum count = 8192. Suitable for short delays.
Initial value = 8192 – counts, TH = value/32, TL = value % 32 (only LSB 5 bits used).
16‑bit timer (TH0 & TL0 combined). Maximum count = 65536. Most flexible for delays from microseconds to milliseconds.
Initial = 65536 – counts, TH = initial >> 8, TL = initial & 0xFF.
8‑bit timer with automatic reload. Maximum count = 256. Excellent for generating accurate, repetitive interrupts (e.g., baud rate, PWM).
Reload value = 256 – counts, TH = TL = reload value.
Engineers designing real‑time systems rely on exact timer calculations. For instance, using 12MHz clock, a 1ms delay in Mode1 requires TH=0xFC, TL=0x18. The actual delay becomes 1000µs with zero error (perfect match when counts integer). For non‑integer counts, our calculator provides error analysis so you can decide if external calibration is needed.
Note on rounding: To ensure the actual delay never exceeds the desired timeout (critical for timeout routines), the required count is always rounded down (floor). If you require a minimum delay (e.g., pulse width must be at least N µs), manually increase the desired delay value accordingly.
| Timer Mode | Resolution | Max Count | Max Delay @ 12MHz (T_mc=1µs) | Use Case |
|---|---|---|---|---|
| Mode 0 (13‑bit) | 13 bits | 8192 | 8.192 ms | Short timing, legacy systems |
| Mode 1 (16‑bit) | 16 bits | 65536 | 65.536 ms | General delays, PWM, Input capture |
| Mode 2 (8‑bit auto-reload) | 8 bits | 256 | 256 µs | Baud rate generation, repetitive interrupts |
For longer delays, combine timer interrupts with a software counter. This calculator helps you choose the optimal mode.
The HC-SR04 requires a 10µs trigger pulse and measures Echo pulse width. Using 11.0592MHz crystal, to generate 10µs delay accurately, Mode2 (auto-reload) with counts = 10µs / 1.085µs ≈ 9.22 → using nearest integer count=9 gives actual 9.765µs, error ≈ -2.35%. The calculator helps you decide proper rounding and compensation. Meanwhile, echo measurement uses Mode1 (16-bit) to capture up to ~65ms range — good enough for 11m distance.
freq_actual / 12) or multiply desired delay by 12 beforehand. This yields correct reload values for 1T cores without modifying formulas.
freq_entered = actual_freq / 12 (or scale desired delay ×12). This gives correct reload values for 1T cores. See the 'Common Pitfalls' section for details.