C8051F360/1/2/3/4/5/6/7/8/9
Figure 18.4 shows the typical SCL generation described by Equation 18.2. Notice that T
is typically
HIGH
twice as large as T
. The actual SCL output may vary due to other devices on the bus (SCL may be
LOW
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 18.1.
Timer Source
Overflows
SCL
TLow
THigh
SCL High Timeout
Figure 18.4. Typical SMBus SCL Generation
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 18.2 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.
Table 18.2. Minimum SDA Setup and Hold Times
EXTHOLD
Minimum SDA Setup Time
Minimum SDA Hold Time
T
– 4 system clocks
low
0
1
3 system clocks
or
1 system clock + s/w delay*
11 system clocks
12 system clocks
*Note: Setup Time for ACK bit transmissions and the MSB of all data transfers. The s/w
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.
With the SMBTOE bit set, Timer 3 should be configured to overflow after 25 ms in order to detect SCL low
timeouts (see Section “18.3.3. SCL Low Timeout” on page 204). 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 18.4). 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).
Rev. 1.0
207
SILICON [ SILICON ]
