C8051F709-GQR Silicon Labs, C8051F709-GQR Datasheet - Page 223

• Manufacturer Part Number: C8051F709-GQR
• Description: 8-bit Microcontrollers - MCU 8kB 32B EEPROM Cap Sense
• Manufacturer: Silicon Labs

Specifications of C8051F709-GQR

• 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 30.1.The selected
clock source may be shared by other peripherals so long as the timer is left running at all times. For exam-
ple, Timer 1 overflows may generate the SMBus and UART baud rates simultaneously. Timer configuration
is covered in Section “33. Timers” on page 262.
The selected clock source should be configured to establish the minimum SCL High and Low times as per
Equation 30.1. 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 30.2.
Figure 30.4 shows the typical SCL generation described by Equation 30.2. 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 30.1.
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
meet the SMBus Specification requirements of 250 ns and 300 ns, respectively. Table 30.2 shows the min-
Timer Source
Overflows
SCL
LOW
T
Equation 30.1. Minimum SCL High and Low Times
. The actual SCL output may vary due to other devices on the bus (SCL may be
Low
SMBCS1
T
Figure 30.4. Typical SMBus SCL Generation
Table 30.1. SMBus Clock Source Selection
HighMin
0
0
1
1
Equation 30.2. Typical SMBus Bit Rate
BitRate
SMBCS0
=
T
High
T
0
1
0
1
LowMin
=
f
--------------------------------------------- -
ClockSourceOverflow
Rev. 1.0
Timer 2 High Byte Overflow
=
Timer 2 Low Byte Overflow
SMBus Clock Source
--------------------------------------------- -
f
ClockSourceOverflow
Timer 0 Overflow
Timer 1 Overflow
3
1
SCL High Timeout
C8051F70x/71x
HIGH
is typically
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