摘要：本文比較了DS26303和LXT384的不同，特別是提供了如何在已有的LXT384應用中使用DS26303，詳細闡述了特性區(qū)別、寄存器和硬件設計時的考慮。

概述

本文比較了DS26303和LXT384的不同，特別是提供了如何在已有的LXT384應用中使用DS26303。DS26303是一個單3.3V供電的8通道E1/T1/J1短程線路接口單元(LIU)。不需要更改軟件它就可以支持LXT384的功能，并且通過擴展的寄存器組它還能提供附加的特性。不需要改變PCB，DS26303就可以用在現(xiàn)有的LXT384應用中，僅僅需要根據應用改變外圍元器件值。

特性的區(qū)別分為三個不同部分：表1為DS26303具備而LXT384不具備的一些特性；表2為LXT384具備而DS26303不具備的一些特性。表3為DS26303和LXT384共有但是在兩個器件上實現(xiàn)不同的特性。

表6到表10為DS26303和LXT384寄存器之間的不同以及DS26303擴展寄存器組提供的附加功能。圖1和表11為在現(xiàn)有的LXT384應用中使用DS26303時需要對器件值所做的細微改變。

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共有的特性區(qū)別

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的選擇范圍

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時鐘A的選擇范圍

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

寄存器的考慮事項

DS26303包括四個主要的寄存器組。

• 主寄存器組(DS26303和LXT384)
• 二級寄存器組(DS26303獨有)
• 獨立LIU寄存器組(DS26303獨有)
• BERT寄存器組(DS26303獨有)

除了一個外，DS26303的主寄存器和LXT384中的寄存器是一樣的。LXT384中地址為1Fh的保留寄存器在DS26303中成為其附加寄存器組的地址指針(ADDP)。如果DS26303代替現(xiàn)有的LXT384，不需要修改應用軟件。表6為DS26303和LXT384主寄存器組列表。

表6. DS26303和LXT384的主寄存器組

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

為了利用DS26303的附加特性和靈活性，必須在所有LXT384應用的源代碼中添加程序。DS26303中的ADDP寄存器的地址是1F (十六進制)，這個地址在LXT384中是保留的。ADDP被用作指針來訪問不同的寄存器組。表7為DS26303寄存器組列表和訪問不同的寄存器組時所需要的ADDP值。

表7. DS26303地址指針選擇

ADDP7 to ADDP0 (Hex)	Bank Name
00	Primary Bank
AA	Secondary Bank
01	Individual LIU Bank
02	BERT Bank

表8為二級寄存器組包含的寄存器列表，表9為獨立LIU寄存器組包含的寄存器列表，表10為BERT寄存器組包含的寄存器列表。

表8. DS26303的二級寄存器組

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的獨立LIU寄存器組

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寄存器組

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

硬件考慮事項

不用改變PCB就可以用DS26303替換現(xiàn)有應用中的LXT384。需要做的是根據目標應用改變外部器件值。圖1為DS26303推薦的網絡端接電路，表11為DS26303正確端接時需要的器件值。

發(fā)送器

LXT384要求發(fā)送端電阻串聯(lián)接入TTIP和TRING輸出，建議這些電阻應該為0Ω (T1 3.3V模式)，11Ω (E1 75Ω同軸)或者11Ω (E1 120Ω雙絞線)。DS26303不要求電阻，所以所有模式中的電阻都應該為0Ω。當禁止脈沖成形時，LXT384要求有一個DC隔離電容。DS26303不要求DC隔離電容，因此，如果現(xiàn)有LXT384應用的PCB電路中有電容可以用一個0Ω的電阻替換。

接收器

在接收側，LXT384要求端接阻抗為12.4Ω (T1 3.3V模式)，9.31Ω (E1 75Ω同軸)或者15Ω (E1 120Ω雙絞線)。當使用外部阻抗模式時，DS26303的所有情況都要求用15Ω端接電阻；當使用DS26303的軟件選取阻抗匹配模式，不需要任何電阻。LXT384要求使用1kΩ電阻同RTIP和RRING管腳串聯(lián)。如果DS26303采用軟件選取端接/阻抗匹配模式，這些1kΩ的電阻可以用0Ω電阻代替。


圖1. LIU前端電路圖

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)