Cirrus Logic AN205 - Manual (Page 5)

AN205
5
does not send out any link pulses on the 10BaseT port. The 10/100 hub is sending out Fast Link Pulses.
The CS8900A doesn’t interpret Fast Link Pulses as a valid signal so it never switches to 10BaseT
mode. When you clear the AutoAUI/10BT bit, the CS8900A will then send Normal Link Pulses which
will cause the 10/100 hub to switch to 10BaseT mode.
25) The data sheet says that we should wait 10 ms after a reset to access registers. The data sheet also
indicates that InitD is set and SiBusy is clear reset when reset is complete. Does this mean that I can
poll the Self Status register through the I/O Map instead of creating some sort of firmware delay to
determine when the controller's other registers can be accessed?
Yes, you can poll Register 16 (self status) instead of insert delay time. In fact this is a better way than
inserting a delay. INITD=high will tell you when initialization is complete. SIBUSY=0 will let you
know that the EEPROM is being accessed.
26) I've written my own driver for the CS8900A. Eventually the chip stops receiving data, and all I get
are RxMiss events. What can cause this?
If you issue a skip command before reading the status and length of the frame the internal buffers may
not clear properly. So, before issuing a skip command, read the RxStatus and RxLength of the current
frame.
27) I get a lot of TxUnderRun errors, but I’m not using early transmits. I didn’t think you could get this
kind of error without using early transmits.
The TxUnderRun error can occur if you do not limit the transmit path of your driver. You should
modify the driver so that only one transmit is allowed to be in progress at a time. Set Tx Interrupts using
the TxCFG register. Then use the TxEvent to indicate that the current Tx operation is complete before
starting the next one. You can see how this can be done in our sample drivers.
28) Is the CS8900A-CQ3 5 V tolerant?
The CS8900A-CQ3 and -IQ3 are not 5 V tolerant. There are voltage protection diodes on all I/O lines
that will turn on whenever the voltage on the I/O line is greater than V
dd
+ .7V or less than V
ss
- .7V.
29) I'm laying out my board and I need to know what kind of traces to use on the transmit and receive
lines. Are there different requirements for 3.3 V or 5 V operation?
The recommended trace characteristics are the same for 3.3 V and 5 V operation. TX+ and TX- should
be 25
Ω single-ended transmission line traces to the transformer. RX+ and RX- should be 50 Ω single-
ended transmission line traces to the transformer. Traces from the transformer to the RJ45 should be
50
Ω. Keep the TX series resistors, cross over cap and RX terminating resistor close to the CS8900A.
Note that in 3.3 V mode the TX series resistors are either 7.9
Ω or 8.1 Ω. One would also try to make
the TX+/- traces the same impedance as the series resistors. However, the traces get exceedingly wide
and the 25
Ω is a good compromise.