C8051F544-IMR Silicon Labs, C8051F544-IMR Datasheet - Page 191

• Manufacturer Part Number: C8051F544-IMR
• Description: 8-bit Microcontrollers - MCU 50 MIPS 8 kB 1kB LIN 2.1 SPI UART I2C
• Manufacturer: Silicon Labs

Specifications of C8051F544-IMR

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

The SMBCS1–0 bits select the SMBus clock source, which is used only when operating as a master or
when the Free Timeout detection is enabled. When operating as a master, overflows from the selected
source determine the absolute minimum SCL low and high times as defined in Equation 20.1. Note that the
selected clock source may be shared by other peripherals so long as the timer is left running at all times.
For example, Timer 1 overflows may generate the SMBus and UART baud rates simultaneously. Timer
configuration is covered in Section “23. Timers” on page 227.
The selected clock source should be configured to establish the minimum SCL High and Low times as per
Equation 20.1. When the interface is operating as a master (and SCL is not driven or extended by any
other devices on the bus), the typical SMBus bit rate is approximated by Equation 20.2.
Figure 20.4 shows the typical SCL generation described by Equation 20.2. Notice that T
twice as large as T
extended low by slower slave devices, or driven low by contending master devices). The bit rate when
operating as a master will never exceed the limits defined by equation Equation 20.1.
Setting the EXTHOLD bit extends the minimum setup and hold times for the SDA line. The minimum SDA
setup time defines the absolute minimum time that SDA is stable before SCL transitions from low-to-high.
The minimum SDA hold time defines the absolute minimum time that the current SDA value remains stable
after SCL transitions from high-to-low. EXTHOLD should be set so that the minimum setup and hold times
Timer Source
Overflows
SCL
LOW
T
Equation 20.1. Minimum SCL High and Low Times
. The actual SCL output may vary due to other devices on the bus (SCL may be
Low
SMBCS1
0
0
1
1
T
Figure 20.4. Typical SMBus SCL Generation
HighMin
Table 20.1. SMBus Clock Source Selection
Equation 20.2. Typical SMBus Bit Rate
BitRate
SMBCS0
0
1
0
1
=
T
High
T
LowMin
=
f
-------------------------------------------------
ClockSourceOverflow
SMBus Clock Source
Timer 0 Overflow
Timer 1 Overflow
Timer 2 High Byte Overflow
Timer 2 Low Byte Overflow
Rev. 1.1
=
-------------------------------------------------
f
ClockSourceOverflow
3
1
SCL High Timeout
C8051F54x
HIGH
is typically
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