DS26303短程线路接口单元与LXT384的比较

摘要：本文比较了DS26303和LXT384的不同，特别是提供了如何在已有的LXT384应用中使用DS26303，详细阐述了特性区别、寄存器和硬件设计时的考虑。

概述本文比较了DS26303和LXT384的不同，特别是提供了如何在已有的LXT384应用中使用DS26303。DS26303是一个单3.3V供电的8通道E1/T1/J1短程线路接口单元(LIU)。不需要更改软件它就可以支持LXT384的功能，并且通过扩展的寄存器组它还能提供附加的特性。不需要改变PCB，DS26303就可以用在现有的LXT384应用中，仅仅需要根据应用改变外围元器件值。

特性的区别分为三个不同部分：表1为DS26303具备而LXT384不具备的一些特性；表2为LXT384具备而DS26303不具备的一些特性。表3为DS26303和LXT384共有但是在两个器件上实现不同的特性。

表6到表10为DS26303和LXT384寄存器之间的不同以及DS26303扩展寄存器组提供的附加功能。图1和表11为在现有的LXT384应用中使用DS26303时需要对器件值所做的细微改变。

表1. DS26303不同于LXT384的特性 DS26303 LXT384 Programmable opTIons to clear interrupt status on write or read. Clear on read is default. Not supported. Individual channel control for jitter attenuator:

• Enable/disable
• FIFO depth
• FIFO limit trip

All channels have global control. Internal software-selectable transmit and receive side terminaTIon for 100Ω T1 twisted-pair, 110Ω J1 twisted-pair, 120Ω E1 twisted–pair, and 75Ω E1 coaxial applicaTIons. Not supported. In HPS mode, the transmitter output and the internal impedance of the receiver can be turned off with only the OE pin. Requires that both receivers use the same front-end terminaTIon. Built-in BERT tester for diagnostics. Not supported. Individual channel control for:

• Short-circuit protection
• AIS enable on LOS
• RCLK inversion
• TCLK inversion

All channels have global control. Individual channel-line violation detection. Not supported. Flexible MCLK
See Table 4 for available input frequencies. Not supported. Programmable TECLK output pin (1.544MHz or 2.048MHz) Not supported. Programmable CLKA output pin See Table 5 for available output frequencies. Not supported. Flexible interrupt pin Not supported.
表2. LXT384不同于DS26303的特性 DS26303 LXT384 Uses single optimal value. Capability to select the jitter attenuator bandwidth. — Analog JTAG MLCK Pin Functionality
The DS26303 and LXT384 both require MCLK to for data with clock recovery as well as AIS detection. The MCLK pin of the LXT384 provides additional functionality not present in the DS26303.
LXT384 MCLK held high.

• The LXT384 operates as a simple data receiver. The clock-recovery circuit is disabled and RPOS/RNEG are internally connected to an EXOR that is fed to the RCLK pin for external clock recovery. The PLL recovery circuit is disabled in this mode.

LXT384 MCLK held low.

• RPOS/RNEG and RCLK go to a high-impedance state.

表3. DS26303和LXT384共有的特性区别 DS26303 LXT384 3.3V LIU power only, 5V not provided. 5V LIU power. Non-mux Intel® write address to WRB rising-edge setup time is 17ns. Non-mux Intel write address to WRB rising-edge setup time is 6ns. Expects non-mux Intel read address to be valid when RDB is active. Non-mux Intel read address to RDB rising-edge setup time is 6ns. This might be an error in datasheet because data is out before this setup time. Inactive RDY to tri-state delay time 12ns (max). Inactive RDY to tri-state delay time 3ns (max). Clears the interrupt pin when reading or writing the interrupt status. Clears interrupt pin when reading the status register. Jitter attenuator FIFO depths of 32 bits or 128 bits. Jitter attenuator FIFO depths of 32 bits or 64 bits. Individual channel control for jitter attenuator:

• Enable/disable
• FIFO depth
• FIFO limit trip

All channels have global control.
表4. DS26303 MCLK的选择范围 PLLE MPS1, MPS0 MCLK MHz (±50ppm) FREQS T1 or E1 Mode 0 xx 1.544 x T1 0 xx 2.048 x E1 1 00 1.544 1 T1/J1 or E1 1 01 3.088 1 T1/J1 or E1 1 10 6.176 1 T1/J1 or E1 1 11 12.352 1 T1/J1 or E1 1 00 2.048 0 T1/J1 or E1 1 01 4.096 0 T1/J1 or E1 1 10 8.192 0 T1/J1 or E1 1 11 16.384 0 T1/J1 or E1
表5. DS26303时钟A的选择范围 CLKA3 to CLKA0 MCLK (Hz) 0000 2.048M 0001 4.096M 0010 8.192M 0011 16.384M 0100 1.544M 0101 3.088M 0110 6.176M 0111 12.352M 1000 1.536M 1001 3.072M 1010 6.144M 1011 12.288M 1100 32k 1101 64k 1110 128k 1111 256k
寄存器的考虑事项DS26303包括四个主要的寄存器组。

• 主寄存器组(DS26303和LXT384)
• 二级寄存器组(DS26303独有)
• 独立LIU寄存器组(DS26303独有)
• BERT寄存器组(DS26303独有)

除了一个外，DS26303的主寄存器和LXT384中的寄存器是一样的。LXT384中地址为1Fh的保留寄存器在DS26303中成为其附加寄存器组的地址指针(ADDP)。如果DS26303代替现有的LXT384，不需要修改应用软件。表6为DS26303和LXT384主寄存器组列表。

表6. DS26303和LXT384的主寄存器组 Address (Hex) DS26303 LXT384 00–15 Primary Registers Registers 16–1E Reserved Reserved 1F ADDP (Address pointer for additional register banks). This register must be set to point to the desired register bank.
00h) Primary Bank
AAh) Secondary Bank
01h) Individual LIU Bank
02h) BERT Bank Reserved
为了利用DS26303的附加特性和灵活性，必须在所有LXT384应用的源代码中添加程序。DS26303中的ADDP寄存器的地址是1F (十六进制)，这个地址在LXT384中是保留的。ADDP被用作指针来访问不同的寄存器组。表7为DS26303寄存器组列表和访问不同的寄存器组时所需要的ADDP值。

表7. DS26303地址指针选择 ADDP7 to ADDP0 (Hex) Bank Name 00 Primary Bank AA Secondary Bank 01 Individual LIU Bank 02 BERT Bank
表8为二级寄存器组包含的寄存器列表，表9为独立LIU寄存器组包含的寄存器列表，表10为BERT寄存器组包含的寄存器列表。

表8. DS26303的二级寄存器组 Address (Hex) Register Name 00 Single-Rail Mode Select 01 Line-Code Selection 02 Not used 03 Receiver Power-Down Enable 04 Transmitter Power-Down Enable 05 Excessive Zero-Detect Enable 06 Code-Violation-Detect Enable Bar 07–1E Not used 1F Set to AAh for access to Secondary Register Bank
表9. DS26303的独立LIU寄存器组 Address (Hex) Register Name 00 Individual JA Enable 01 Individual JA Position Select 02 Individual JA FIFO Depth Select 03 Individual JA FIFO Limit Trip 04 Individual Short-Circuit-Protection Disable 05 Individual AIS Select 06 Master Clock Select 07 Global-Management Register 08–0F Reserved 10 Bit-Error-Rate Tester Control Register 12 Line-Violation Detect Status 13 Receive Clock Invert 14 Transmit Clock Invert 15 Clock-Control Register 16 RCLK Disable Upon LOS Register 1E Global-Interrupt Status Control 1F Set to 01h for access to Individual LIU Register Bank
表10. DS26303的BERT寄存器组 Address (Hex) Register Name 00 BERT Control Register 01 Reserved 02 BERT Pattern Configuration 1 03 BERT Pattern Configuration 2 04 BERT Seed/Pattern 1 05 BERT Seed/Pattern 2 06 BERT Seed/Pattern 3 07 BERT Seed/Pattern 4 08 Transmit-Error Insertion Control 09–0A Reserved 0C BERT Status Register 0D Reserved 0E BERT Status Register Latched 10 BERT Status Register Interrupt Enable 11–13 Reserved 14 Receive Bit-Error Count Register 1 15 Receive Bit-Error Count Register 2 16 Receive Bit-Error Count Register 3 17 Receive Bit-Error Count Register 4 18 Receive Bit Count Register 1 19 Receive Bit Count Register 2 1A Receive Bit Count Register 3 1B Receive Bit Count Register 4 1C–1E Reserved 1F Set to 02h for access to BERT Register Bank
硬件考虑事项不用改变PCB就可以用DS26303替换现有应用中的LXT384。需要做的是根据目标应用改变外部器件值。图1为DS26303推荐的网络端接电路，表11为DS26303正确端接时需要的器件值。

发送器LXT384要求发送端电阻串联接入TTIP和TRING输出，建议这些电阻应该为0Ω (T1 3.3V模式)，11Ω (E1 75Ω同轴)或者11Ω (E1 120Ω双绞线)。DS26303不要求电阻，所以所有模式中的电阻都应该为0Ω。当禁止脉冲成形时，LXT384要求有一个DC隔离电容。DS26303不要求DC隔离电容，因此，如果现有LXT384应用的PCB电路中有电容可以用一个0Ω的电阻替换。

接收器在接收侧，LXT384要求端接阻抗为12.4Ω (T1 3.3V模式)，9.31Ω (E1 75Ω同轴)或者15Ω (E1 120Ω双绞线)。当使用外部阻抗模式时，DS26303的所有情况都要求用15Ω端接电阻；当使用DS26303的软件选取阻抗匹配模式，不需要任何电阻。LXT384要求使用1kΩ电阻同RTIP和RRING管脚串联。如果DS26303采用软件选取端接/阻抗匹配模式，这些1kΩ的电阻可以用0Ω电阻代替。


图1. LIU前端电路图

表11. LIU前端值 Mode Component 75 Coax 120 Twisted Pair 100/110 Twisted Pair Tx Capacitance Ct 560pF (typ). Adjust for board parasitics for optimal return loss. Tx Protection Dt International Rectifier: 11DQ04 or 10BQ060 Motorola: MBR0540T1 Rx Transformer 1:2 TFr Pulse: T1124 (0°C to +70°C) Tx Transformer 1:2 TFt Pulse: T1114 (-40°C to +85°C) Tx Decoupling (ATVDD) C1 Common decoupling for all eight channels is 68µF. Tx Decoupling (ATVDD) C2 Recommended decoupling per channel is 0.1µF. Rx Decoupling (AVDDn) C3 Common decoupling for all eight channels is 68µF. Rx Decoupling (AVDDn) C4 Common decoupling for all eight channels is 0.1µF. Rx Termination C5 When in external impedance mode, Rx capacitance for all eight channels is 0.1µF. Do not populate if using internal impedance mode. Rx Termination Rt When in external impedance mode, the two resistors for all modes is 15.0Ω ±1%. Do not populate if using internal impedance mode. Voltage Protection TVS1 SGS-Thomson: SMLVT 3V3 (3.3V transient suppressor)