I am using the CS8900 in a design and it works very well except I have one problem. Switches don't recognize full duplex when I put the 8900 in full duplex mode. If it's a managed switch then I can configure the switch and tell it that the link is full duplex. I'm wondering if switches are supposed to auto detect this or if 10Mbps ethernet automatically defaults to half duplex. Does anyone have any thoughts on this.
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This is one of the common and often overlooked problems with Ethernet link negotiation. If both side use autonegotiation they can handle out link speed and duplex mode. This works reasonably well these days. If both sides are manually configured you just get what you configure. If you configure one side manually while the other uses auto-config, the auto-config side can not determine the link settings by means of auto-negotiation. It then falls back to a mode where it detects at least the link speed. It can however not find out whether the link is full duplex or not. I don't know what the preference is. Anyway you might end with a link where one partner is in full duplex mode, while the other runs in half duplex. The half duplex side will see a lot of collisions in that case and perform unnecessary retransmissions. It may seem that the link is working but this combination is illegal and causes a lot of problems.
Possible solution for your problem: Use autonegotiation if your PHY supports it. If you manually configure your PHY go for half-duplex. It seems to be the default if auto- negotiation fails.
Yours
- Rene
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Thanks for the reply. It seems that the CS8900A is not capable of negotiating a full duplex connection with the switch. Therefore is the switch isn't managed then you can't run full duplex. So I guess I need to make the device default to half duplex.
In addition to Rene's excellent reply...
NWay auto negotiation (10/100, full/half) was developed for 100Mb Ethernet, and IIRC it's not part of the 10Mb spec. In general, 10Mb devices do not detect or negotiate duplex settings; if you're fortunate to have a device and switch that support 10Mb full-duplex, you can hard-code both ends and really boost performance.
Full-duplex negotiation is one of the bigger reasons we moved our project to the ASIX 10/100 NE2000 chip from the RTL8019AS. If you're creating a 10Mb product, you really have no option but to fix it in half-duplex mode; customers will never get it configured right, hubs won't work, etc.
Richard
Thank you. I did a little more research on the web and found that 10Mbps use NLP (Normal Link Pulses) and 100Mbps uses F(ast)LP to signal for negotiating. It appears that duplex negotiation is part of the FLP spec. So negotiating 10Mbps duplex is out of the question. Our product currently ships with default half duplex, but can be configured to full by a knowledgable user. I just wanted to make sure that I wasn't overlooking anything.
We already have the 100Mbps design completed, working, and waiting for production. But if there was a better way to handle duplex in the 10Mbps I wanted to provide an upgrade for current customers. All is much clearer now.
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