DS125RT410_15 Datasheet, PDF (15/58 Pages) Texas Instruments – Low-Power Multi-Rate Quad Channel Retimer

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DS125RT410
SNLS459A – APRIL 2013 – REVISED OCTOBER 2015
For the 10-GbE and 1-GbE mode listed in Table 2, two frequency groups are defined. These two frequency
groups are referred to as Group 0, for 1 GbE, and Group 1, for 10 GbE. This same frequency group structure is
present for all frequency modes, but for some modes the expected frequency for both groups is the same. The
expected PPM count information for Group 0 is set in registers 0x60 and 0x61. For Group 1, it is set in registers
0x62 and 0x63. For both groups, the PPM count tolerance is set in register 0x64.
The value of the PPM count for either group is computed the same way from the expected data rate in Gbps,
R_Gbps. The PPM count value, denoted N_PPM, is computed by Equation 1.
N_PPM = R_Gbps × 1280
(1)
As an example we consider the PPM count setup for 10 GbE and 1 GbE. The expected PPM count for Group 0,
which in this case is 1 GbE, is set in registers 0x60 and 0x61. The expected VCO frequency for 1 G is
10.0 G. The actual data rate for 1 GbE, which is 8B/10B coded, is 1.25 Gbps. With a VCO divide ratio of 8, which
is the divide ratio automatically used by the retimer for 1 GbE, this yields a VCO frequency of 10.0 GHz.
We compute the PPM count as in Equation 2. This is a decimal value. In hexadecimal, this is 0x3200.
N_PPM = 10.0 × 1280 = 12800
(2)
The lower-order byte is loaded into register 0x60. The higher order byte, 0x32, is loaded into the 7 least
significant bits of register 0x61. In addition, bit 7 of register 0x61 is set, indicating manual load of the PPM count.
When this is complete, register 0x60 will contain 0x00. Register 0x61 will contain 0xb2.
For the example we are considering, Group 1 is for 10 GbE. Here the actual data rate for the 64/66B encoded
10-GbE data is 10.3125 Gbps. For 10 GbE, the retimer automatically uses a divide ratio of 1, so the VCO
frequency is also 10.3125 GHz. For 10 GbE, we compute the expected PPM count as in Equation 3. Again, this
is a decimal value. In hexadecimal, this is 0x3390.
N_PPM = 10.3125 × 1280 = 13200
(3)
The lower order byte for Group 1, 0x90, is loaded into register 0x62. The higher-order byte, 0x33, is loaded into
the 7 least-significant bits of register 0x63. As with the Group 0 settings, bit 7 of register 0x63 is also set.
When this is complete, register 0x62 will contain 0x90. Register 0x63 will contain 0xb3.
Finally, register 0x64 should be set to a value of 0xff. This is the PPM count tolerance. The resulting tolerance in
parts per million is given in Equation 4.
Tol_PPM = (1 × 10^-6 × N_TOL) / N_PPM
(4)
In this equation, N_TOL is the 4-bit tolerance value loaded into the upper or lower four bits of register 0x64. For the
example we are using here, both of these values are 0xf, or decimal 15. For a PPM count value of 12800, for
Group 0, this yields a tolerance of 1172 parts per million. For a PPM count value of 13200, for Group 1, this
yields a tolerance of 1136 parts per million.
These tolerance values can be reduced if it is known that the frequency accuracy of the system and of the
25-MHz reference clock are very good. For most applications, however, a value of 0xff in register 0x64 will give
robust performance.
For all the other standards listed in Table 2 the expected PPM count for Group 0 (registers 0x60 and 0x61) and
Group 1 (registers 0x62 and 0x63) will be set the same, since there is only one VCO frequency for these
standards. The expected PPM count and tolerance are computed as described previously for 10 GbE and 1
GbE. The same values are written to each pair of PPM count registers for these standards.
As is the case with the standards-based mode of operation, the expected PPM count value and the PPM count
tolerance must be written to registers 0x60, 0x61, 0x62, 0x63, and 0x64. These are computed exactly as
described above for the standards-based mode of operation. Since the frequency-range-based mode of
operation uses both Group 0 and Group 1 with the same expected PPM count, the same values should be
loaded into the pairs of registers 0x60 and 0x62, and 0x61 and 0x63.
As an example, suppose that the expected data rate is 8.5 Gbps. The VCO frequency for the frequency-range
based mode of operation is also 8.5 GHz. So we compute as in Equation 5. This is a decimal value. In
hexadecimal this is 0x2a80.
N_PPM = 8.5 × 1280 = 10880
(5)
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