c8051t617 Silicon Laboratories, c8051t617 Datasheet - Page 137

• Manufacturer Part Number: c8051t617
• Description: Mixed Signal Byte-programmable Eprom Mcu
• Manufacturer: Silicon Laboratories

Figure 15.4 shows the typical SCL generation described by Equation 15.2. 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 15.1.
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 15.2 shows the min-
imum setup and hold times for the two EXTHOLD settings. Setup and hold time extensions are typically
necessary when SYSCLK is above 10 MHz.
With the SMBTOE bit set, Timer 3 should be configured to overflow after 25 ms in order to detect SCL low
timeouts (see Section “15.3.3. SCL Low Timeout” on page 134). The SMBus interface will force Timer 3 to
reload while SCL is high, and allow Timer 3 to count when SCL is low. The Timer 3 interrupt service routine
should be used to reset SMBus communication by disabling and re-enabling the SMBus.
SMBus Free Timeout detection can be enabled by setting the SMBFTE bit. When this bit is set, the bus will
be considered free if SDA and SCL remain high for more than 10 SMBus clock source periods (see
Figure 15.4). When a Free Timeout is detected, the interface will respond as if a STOP was detected (an
interrupt will be generated, and STO will be set).
Timer Source
Overflows
SCL
*Note: Setup Time for ACK bit transmissions and the MSB of all data transfers. The s/w
EXTHOLD
delay occurs between the time SMB0DAT or ACK is written and when SI is cleared.
Note that if SI is cleared in the same write that defines the outgoing ACK value, s/w
delay is zero.
0
1
LOW
Table 15.2. Minimum SDA Setup and Hold Times
T
. The actual SCL output may vary due to other devices on the bus (SCL may be
Low
Figure 15.4. Typical SMBus SCL Generation
Minimum SDA Setup Time
1 system clock + s/w delay*
T
low
11 system clocks
– 4 system clocks
T
High
OR
Rev. 0.3
C8051T610/1/2/3/4/5/6/7
Minimum SDA Hold Time
12 system clocks
3 system clocks
SCL High Timeout
HIGH
is typically
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