9.1.1 Signal Integrity in 10G-KR Applications
When configured in "KR Mode", using either the VOD_SEL and MODE pin setting or SMBus register control, the DS100BR111 is designed to operate transparently within a KR backplane channel environment. Installing a DS100BR111 repeater within the KR backplane channel splits the total channel attenuation into two parts. In other words, operating in "KR Mode" implies that signals will pass through the repeater with a linearized output. Ideally the repeater can be placed near the middle of the channel, maximizing the signal-to-noise ratio across the bidirectional interface.
In order to maximize the 10G-KR solution space, the 802.3ap specification calls for an optimization of the Tx partner signal conditioning coefficients based on feedback from the KR Rx ASIC endpoint. This link optimization sequence is commonly referred to as "link training" and is performed at speed (10.3125 Gbps). Setting the DS100BR111 active CTLE to compensate for the channel loss from each of the KR transmitters will reduce the transmit and receive equalization settings required on the KR physical layer devices. This central location keeps a larger signal-to-noise ratio at all points in the channel, extending the available solution space and increasing the overall margin of almost any channel. Suggested initial settings for the DS100BR111 are given in Table 10 and Table 11. Further adjustments to EQx, DEMx, and VODx settings may optimize signal margin on the link for different system applications.
Table 10. Suggested 10G-KR Initial Device Settings in Pin Mode(1)
CHANNEL SETTINGS |
PIN MODE |
EQx[1:0] |
0, 0 |
VOD_SEL |
1 |
DEMx |
0 |
(1) For 10G-KR mode with slow idle-to-active response, the MODE pin should be left floating.
Table 11. Suggested 10G-KR Initial Device Settings in SMBus Modes
CHANNEL SETTINGS |
SMBus MODES |
EQx |
0x00 |
VODx |
100'b |
DEMx |
000'b |
The SMBus Slave Mode code example in Table 12 may be used to program the DS100BR111 with the recommended device settings.
Table 12. SMBus 10G-KR Example Sequence
REGISTER |
WRITE VALUE |
COMMENTS |
0x06 |
0x18 |
Set SMBus Slave Mode Register Enable. |
0x08 |
0x04 |
Enable Output Mode Control for individual channel outputs. |
0x0F |
0x00 |
Set CHA EQ to 0x00. |
0x10 |
0xAD |
Set CHA Output Mode to Linear (10G-KR mode). If link-training is not required, set Reg 0x10 to 0xED. |
0x11 |
0x00 |
Set CHA DEM to 000'b. |
0x16 |
0x00 |
Set CHB EQ to 0x00. |
0x17 |
0xAD |
Set CHB Output Mode to Linear (10G-KR mode). If link-training is not required, set Reg 0x18 to 0xED. |
0x18 |
0x00 |
Set CHB DEM to 000'b. |
0x23 |
0x10 |
Set CHA VOD to 100'b. |
0x28 |
0x00 |
Leave Idle Control at default levels. For SAS/SATA applications, set Reg 0x28 to 0x4C. |
0x2D |
0xB1 |
Set CHB VOD to 100'b. |
9.1.2 OOB (Out-of-Band) Functionality in SAS/SATA Applications
For SAS/SATA systems, a low speed OOB (Out-of-Band) communication sequence is used to detect and communicate device capabilities between host ASIC and link partners. These OOB signals, including COMWAKE, COMINIT, COMRESET, and COMSAS, are a series of burst, idle, and negation times transmitted and detected across the SAS/SATA link. These bursts occur at a rapid rate, with the COMWAKE signal having the most stringent requirement of 106.6 ns active followed by 106.6 ns idle. Normally, if the device is set in 10G-KR mode (MODE pin floating), the device goes idle-to-active in approximately 150 ns. If the device is set to SAS mode (MODE pin tied via 1 kΩ to VDD (2.5 V mode) or VIN (3.3 V mode)), the device goes idle-to-active in approximately 3 to 4 ns. This fast idle-to-active time is critical to pass OOB signaling, and when operating in pin mode, the MODE pin should be tied high. If operating in SMBus slave mode, the user can set Reg 0x28 to 0x4C for this faster idle-to-active response.