I have found that the Lyrtech C6457EVM always transmits Ethernet packets when running at the expected 1GHz core clock. However, if the core clock is dropped to 600MHz then sometimes the initial DHCP discover / request and gratuitous ARP will not appear on the wire. This problem occurs about 1/3 of the time.
We also have our own product that uses a single C6457 and a Vitesse VSC8221 PHY and have experienced similar problems: Ethernet transmissions are faultless when running at a DSP core speed of 1GHz (indeed 840MHz seems OK) but problems appear at about 600MHz.
I believe there is an NDK Ethernet packet transmission problem that relates to DSP core clock speed.
This post is related to the more specific http://e2e.ti.com/support/dsp/tms320c6000_high_performance_dsps/f/112/p/64135/230984.aspx#230984 but I wanted to raise this post because I believe that the problem is more generic - not only our hardware but it also affects the Lyrtech C6457EVM.
Therefore, it could only be the C6457 NSP or the NDK 2.0 itself.
I also wondered if it could be a silicon issue with the C6457 DSP itself because of the following:
Our product uses a 60MHz board clock. Running at a PLL multiplier of anything under about 12 or 13 is unreliable. Above this and the DHCP discover / gratuitous ARP never seems to fail. It just so happens that x12 PLL gives 720MHz CORECLK or 720/6 = 120MHz EMAC peripheral clock and x13 PLL gives 130MHz EMAC peripheral clock. I wondered if for some reason the EMAC inside the DSP needs a peripheral clock >= 125MHz only because the SGMII input clock is also 125MHz. Coincidence?
There is also something else which I have not taken any further: When the EMAC peripheral clock which is SYSCLK5 (chip_clk6) is close to the 125MHz SGMII clock input to the DSP the software does not run properly at all - the StdOut messages that usually appear - SGMII reset / config, registration of EMAC etc - do not appear at all.
i.e.
EMAC peripheral clock < 125MHz gives unreliable Ethernet TX about 1/3 of the time
EMAC peripheral clock close to 125MHz gives unreliable Ethernet TX about 1/3 of the time and sometimes software does not run as described above
EMAC peripheral clock > 125MHz gives Ethernet TX that seems completely reliable - typically I run 30 software restarts and 30 times the DHCP discover / request and gratuitous ARP are sent successfully.