C8051F987-GMR Silicon Labs, C8051F987-GMR Datasheet - Page 256

• Manufacturer Part Number: C8051F987-GMR
• Description: 8-bit Microcontrollers - MCU 8kB 512B RAM
• Manufacturer: Silicon Labs

Specifications of C8051F987-GMR

• Rohs: yes
• Core: 8051
• Processor Series: C8051
• Data Bus Width: 8 bit

C8051F99x-C8051F98x
23.1. 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 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 “25.1.3. Mode 2: 8-bit Coun-
ter/Timer with Auto-Reload” on page 280). 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 exter-
nal input T1. For any given Timer 1 clock source, the UART0 baud rate is determined by Equation 23.1-A
and Equation 23.1-B.
Where T1
value). Timer 1 clock frequency is selected as described in Section “25.1. Timer 0 and Timer 1” on
page 278. A quick reference for typical baud rates and system clock frequencies is given in Table 23.1
through Table 23.2. Note that the internal oscillator may still generate the system clock when the external
oscillator is driving Timer 1.
256
CLK
is the frequency of the clock supplied to Timer 1, and T1H is the high byte of Timer 1 (reload
Detected
Start
A)
B)
UartBaudRate
T1_Overflow_Rate
Figure 23.2. UART0 Baud Rate Logic
RX Timer
Equation 23.1. 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