參數(shù)資料
型號: Si3232
廠商: Electronic Theatre Controls, Inc.
英文描述: DUAL PROGRAMMABLE CMOS SLIC WITH LINE MONITORING
中文描述: 兩個可編程的CMOS用戶接口與在線監(jiān)測
文件頁數(shù): 51/128頁
文件大?。?/td> 2327K
代理商: SI3232
Si3232
Preliminary Rev. 0.96
51
Refer to "2. Typical Application Schematic" on page 17.
The pulldown resistor on the SDO pin is required to
allow this node to discharge after a logic high state to a
tri-state condition. The discharge occurs while SDO is
tri-stated during an 8 kHz transmission frame. The value
of the pulldown resistor depends on the capacitance
seen on the SDO pin. In the case of using a single
Si3232, the value of the pulldown resistor is 39 k
. This
assumes a 5 pF SDO pin capacitance and about a
15 pF load on the SDO pin. For applications using
multiple Si3232 devices or different capacitive loads on
the SDO pin, a different pulldown resistance needs to
be calculated.
The following design procedure is an example for
calculating the pulldown resistor on the SDO pin in a
system using eight Si3232 devices. A pullup resistor is
not allowed on the SDO pin.
1. The SDO node must discharge and remain discharged for
244 ns. The discharge occurs during the Hi-Z state;
therefore, the time to discharge is equal to the time in Hi-Z
time minus the 244 ns.
2. Allow five time constants for discharge where the
time constant, t = RC
3. SDO will be in Hi-Z while SDI is sending control and
address which are each 8 bits. Using the maximum
SCLK frequency of 16.13 MHz, the SDO will be in
Hi-Z for 16 / 16.13 MHz = 992 ns.
4. We want to discharge and remain discharged for
244 ns. Therefore, the discharge time is:
992 ns – 244 ns = 748 ns
5. To allow for some margin, let’s discharge in 85% of
this time. 748nS x 85% = 635.8 ns
6. Determine capacitive load on the SDO pin:
a.Allow 5 pF for each Si3220 SDO pin that
connected together.
b.Allow ~2 pF/inch (~0.8 pF/cm) for PCB trace.
c.Include the load capacitance of the host IC input.
7. For a system with eight Si3220 devices, the
capacitance seen on the SDO pin would be:
a.8 x 5 pF for each Si3220 = 40 pF
b.Assume 5 inch of PCB trace: 5inch x 2 pF/
inch = 10 pF
c.Host IC input of 5 pF
d.Total capacitance is 55 pF
8. Using the equation t = RC, allowing five time
constants to decay, and solving for R
a.R = t / 5C = 635.8 ns / (5 x 55 pF)
b.R = 2.3 k
So, R must be less than 2.3 k
to allow the node to
discharge.
Table 31. SPI Control Interface
7
BRDCST
Indicates a broadcast operation that is intended for all devices in the daisy chain. This is
only valid for write operations since it would cause contention on the SDO pin during a
read.
6
R/W
Read/Write Bit.
0 = Write operation.
1 = Read operation.
5
REG/RAM
Register/RAM Access.
0 = RAM access.
1 = Register access.
4
Reserved
3:0
CID[3:0]
This field indicates the channel that is targeted by the operation. The 4-bit channel value is
provided LSB first. The devices reside on the daisy chain such that device 0 is nearest to
the controller, and device 15 is furthest down the SDI/SDU_THRU chain. (See Figure 26.)
As the CID information propagates down the daisy chain, each channel decrements the
CID by 1. The SDI nodes between devices will reflect a decrement of 2 per device since
each device contains two channels. The device receiving a value of 0 in the CID field will
respond to the SPI transaction. (See Figure 27.) If a broadcast to all devices connected to
the chain is requested, the CID will not decrement. In this case, the same 8-bit or 16-bit
data is presented to all channels regardless of the CID values.
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