Question 10.1: With XTAL = 11.0592 MHz, find the TH1 value needed to have t...
With XTAL = 11.0592 MHz, find the TH1 value needed to have the following baud rates. (a) 9600 (b) 2400 (c) 1200
- XTAL = 11.0592 MHz
Final Answer
The machine cycle frequency of 8051 = 11.0592 / 12 = 921.6 kHz, and 921.6 kHz / 32 = 28,800 Hz is frequency by UART to timer 1 to set baud rate.
(a) 28,800 / 3 = 9600 where -3 = FD (hex) is loaded into TH1
(b) 28,800 / 12 = 2400 where -12 = F4 (hex) is loaded into TH1
(c) 28,800 / 24 = 1200 where -24 = E8 (hex) is loaded into TH1
Notice that dividing 1/12 of the crystal frequency by 32 is the default
value upon activation of the 8051 RESET pin.
11.0592
\fbox {XTAL oscillator} \rightarrow \fbox {÷12} \overset{Machine \ cycle\ freq}{\underset{921.6 \ KHZ \\ }{\longrightarrow}} \fbox {÷ 32 By UART} \overset{28800 \ HZ}{\underset{To \ timer \ 1 \\ To \ set \ the \\ Baud \ rate}{\longrightarrow}}
| TH1 (Hex) | TH1 (Decimal) |
Baud Rate |
| FD | -3 | 9600 |
| FA | -6 | 4800 |
| F4 | -12^{\nearrow ^{\text{TF is set to 1 every 12 ticks, so it functions as a frequency divider}} } | 2400 |
| E8 | -24 | 1200 |
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