C8051F120/1/2/3/4/5/6/7
276
Rev. 1.2
23.1. Enhanced Baud Rate Generation
The UART1 baud rate is generated by Timer 1 in 8-bit auto-reload mode. The TX clock is generated by TL1; the RX
clock is generated by a copy of TL1 (shown as RX Timer in Figure 23.2), which is not user-accessible. Both TX and
RX Timer overflows are divided by two to generate the TX and RX baud rates. The RX Timer runs when Timer 1 is
enabled, and uses the same reload value (TH1). However, an RX Timer reload is forced when a START condition is
detected on the RX pin. This allows a receive to begin any time a START is detected, independent of the TX Timer
state.
Timer 1 should be configured for Mode 2, 8-bit auto-reload (see
Section “24.1.3. Mode 2: 8-bit Counter/Timer
with Auto-Reload” on page 287
). The Timer 1 reload value should be set so that overflows will occur at two times
the desired baud rate. Note that Timer 1 may be clocked by one of five sources: SYSCLK, SYSCLK / 4, SYSCLK /
12, SYSCLK / 48, or the external oscillator clock / 8. For any given Timer 1 clock source, the UART1 baud rate is
determined by Equation 23.1.
Where
T1
CLK
is the frequency of the clock supplied to Timer 1, and
T1H
is the high byte of Timer 1 (reload value).
Timer 1 clock frequency is selected as described in
Section “24.1. Timer 0 and Timer 1” on page 285
. A quick ref-
erence for typical baud rates and system clock frequencies is given in Table 23.1 through Table 23.5. Note that the
internal oscillator or PLL may still generate the system clock when the external oscillator is driving Timer 1 (see
Sec-
tion “24.1. Timer 0 and Timer 1” on page 285
for more details).
Figure 23.2. UART1 Baud Rate Logic
RX Timer
Start
Detected
Overflow
Overflow
TH1
TL1
TX Clock
2
RX Clock
2
Timer 1
UART1
Equation 23.1. UART1 Baud Rate
UARTBaudRate
T
1
CLK
256
T
1
H
–
(
)
------------------------------- 1
2
---
×
=
Summary of Contents for C8051F120
Page 2: ...C8051F120 1 2 3 4 5 6 7 2 Rev 1 2 Notes ...
Page 8: ...C8051F120 1 2 3 4 5 6 7 8 Rev 1 2 26 2 Flash Programming Commands 318 26 3 Debug Support 321 ...
Page 16: ...C8051F120 1 2 3 4 5 6 7 16 Rev 1 2 Notes ...
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