C8051F120 Datasheet, PDF (270/356 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
19.4.2. Clock Rate Register
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Reset Value
00000000
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
SFR Address: 0xCF
SFR Page: 0
Bits7-0:
SMB0CR.[7:0]: SMBus0 Clock Rate Preset
The SMB0CR Clock Rate register controls the frequency of the serial clock SCL in master
mode. The 8-bit word stored in the SMB0CR Register preloads a dedicated 8-bit timer. The
timer counts up, and when it rolls over to 0x00, the SCL logic state toggles.
The SMB0CR setting should be bounded by the following equation , where SMB0CR is the
unsigned 8-bit value in register SMB0CR, and SYSCLK is the system clock frequency in
MHz:
SMB0CR < _288 – 0.85 ⋅ ^S----^Y----^S--4-^C----^L----^K--_ ⁄ 1.125
The resulting SCL signal high and low times are given by the following equations, where
SYSCLK is the system clock frequency in Hz:
T_LOW = 4 × (256 – SMB0CR) ⁄ SYSCLK
T_HIGH ≅ 4 × (258 – SMB0CR) ⁄ SYSCLK + 625ns
Using the same value of SMB0CR from above, the Bus Free Timeout period is given in the
following equation:
T_B
FT
≅
10
×
4-----×_-----(_{--2---5---6-----–----}S_----M_-----B_----0---C_-----R_----)_---+-----1--
SYSCLK
Figure 19.9. SMB0CR: SMBus0 Clock Rate Register
270
Rev. 1.3