c8051t623-g Silicon Laboratories, c8051t623-g Datasheet - Page 165

• Manufacturer Part Number: c8051t623-g
• Description: Full Speed Usb Eprom Mcu Family
• Manufacturer: Silicon Laboratories

C8051T622/3 and C8051T326/7
20.1. Enhanced Baud Rate Generation
The UART0 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 20.2), 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 “23.1.3. Mode 2: 8-bit Coun-
ter/Timer with Auto-Reload” on page 205). The Timer 1 reload value should be set so that overflows will
occur at two times the desired UART baud rate frequency. Note that Timer 1 may be clocked by one of six
sources: SYSCLK, SYSCLK/4, SYSCLK/12, SYSCLK/48, the external oscillator clock/8, or an external
input T1. For any given Timer 1 clock source, the UART0 baud rate is determined by Equation 20.1-A and
Equation 20.1-B.
Where T1
value). Timer 1 clock frequency is selected as described in Section “23. Timers” on page 202. A quick ref-
erence for typical baud rates and system clock frequencies is given in Table 20.1 through Table 20.2. The
internal oscillator may still generate the system clock when the external oscillator is driving Timer 1.
172
CLK
is the frequency of the clock supplied to Timer 1, and T1H is the high byte of Timer 1 (reload
Detected
B)
A)
Start
UARTBaudRate
T1_Overflow_Rate
Figure 20.2. UART0 Baud Rate Logic
RX Timer
Equation 20.1. UART0 Baud Rate
Timer 1
TH1
TL1
=
Overflow
Overflow
Rev. 1.1
1
-- -
2
=

T1_Overflow_Rate
------------------------- -
256 TH1
T1
–
CLK
2
2
UART
RX Clock
TX Clock