CY28346
Two-Wire SMBus Control Interface
Serial Control Registers
The two-wire control interface implements a Read/Write slave
only interface according to SMBus specification.
Following the acknowledge of the Address Byte, two additional
bytes must be sent:
The device will accept data written to the D2 address and data
may read back from address D3. It will not respond to any
other addresses, and previously set control registers are
retained as long as power in maintained on the device.
1. “Command code” byte
2. “Byte count” byte.
Although the data (bits) in the command is considered “don’t
care,” it must be sent and will be acknowledged. After the
Command Code and the Byte Count have been acknowl-
edged, the sequence (Byte 0, Byte 1, and Byte 2) described
below will be valid and acknowledged.
Byte 0: CPU Clock Register[2,3]
Bit
@Pup
Pin#
Description
7
0
Spread Spectrum Enable. 0 = Spread Off, 1 = Spread On
This is a Read and Write control bit.
6
0
0
CPU Clock Power-down Mode Select. 0 = Drive CPUT(0:2) to 4 or 6 IREF and drive
CPUC(0:2) LOW when PD# is asserted LOW. 1 = Tri-state all CPU outputs. This is only
applicable when PD# is LOW. It is not applicable to CPU_STP#.
5
4
3
35
3V66_1/VCH Frequency Select, 0 = 66M selected, 1 = 48M selected
This is a Read and Write control bit.
Pin 53 44,45,48,49,5 CPU_STP#. Reflects the current value of the external CPU_STP# (pin 53) This bit is
1,52 Read-only.
Pin 34 10,11,12,13,16 Reflects the current value of the internal PCI_STP# function when read. Internally PCI_STP#
,17,18
is a logical AND function of the internal SMBus register bit and the external PCI_STP# pin.
Frequency Select Bit 2. Reflects the value of SEL2 (pin 40). This bit is Read-only.
Frequency Select Bit 1. Reflects the value of SEL1 (pin 55). This bit is Read-only.
Frequency Select Bit 0. Reflects the value of SEL0 (pin 54). This bit is Read-only.
2
1
0
Pin 40
Pin 55
Pin 54
Byte 1: CPU Clock Register
Bit
7
@Pup
Pin 43
0
Pin#
43
Description
MULT0 (Pin 43) Value. This bit is Read-only.
6
53
CPUT/C(0:2) Output Functionality Control When CPU_STP# is Asserted. 0 = Drive
CPUT(0:2) to 4 or 6 IREF and drive CPUC(0:2) LOW when CPU_STP# asserted LOW.
1 = three-state all CPU outputs. This bit will override Byte0,Bit6 such that even if it is 0,
when PD# goes LOW the CPU outputs will be three-stated.
5
4
3
2
1
0
0
0
1
1
1
44,45
48,49
51,52
44,45
48,49
51,52
CPU2 Functionality Control When CPU_STP# is Asserted LOW. 1 = Free Running, 0 =
Stopped LOW with CPU_STP# asserted LOW. This is a Read and Write control bit.
CPU1 Functionality Control When CPU_STP# is Asserted LOW. 1 = Free Running, 0 =
Stopped LOW with CPU_STP# asserted LOW. This is a Read and Write control bit.
CPUT0 Functionality Control When CPU_STP# is Asserted LOW. 1 = Free Running, 0 =
Stopped LOW with CPU_STP# asserted LOW. This is a Read and Write control bit.
CPUT/C2 Output Control. 1 = enabled, 0 = disable HIGH and CPUC2 disables LOW. This
is a Read and Write control bit.
CPUT/C1 Output Control. 1 = enabled, 0 = disable HIGH and CPUC1 disables LOW. This
is a Read and Write control bit.
0
CPUT/C0 Output Control. 1 = enabled, 0 = disable HIGH and CPUC0 disables LOW. This
is a Read and Write control bit.
Notes:
2. PU = internal pull-up. PD = internal pull-down. T = tri-level logic input with valid logic voltages of LOW = < 0.8V, T = 1.0 – 1.8V and HIGH = > 2.0V.
3. The “Pin#” column lists the relevant pin number where applicable. The “@Pup” column gives the default state at power-up.
Rev 1.0,November 24, 2006
Page 3 of 19
SPECTRALINEAR [ SPECTRALINEAR INC ]
