Cirrus Logic AN83 - Manual (Page 4)

AN83
4 AN83REV3
SCHEMATIC CHECKLIST
Before getting into the meat of the technical refer-
ence manual here is a schematic checklist. It’s pre-
sented here, at the beginning, to help the hardware
designer implement the design quickly and easily.
- No caps across the crystal. The CS8900A
implements these internally.
- 4.99K 1% resistor between pin 93 and pin 94. A
common mistake is the resistor is connected to
Vcc instead of ground.
- RESET is active high, not active low.
- Check addressing.
- On non-ISA systems, if the processor is Big
Endian, it may be beneficial to byte swap the
data lines to minimize byte swapping in
software.
- SBHE (16 bit mode) -- must be low on IO or Mem
address. And it must toggle at least once to put
the CS8900 in 16 bit mode.
- IO and Memory Accesses: SBHE, AEN, etc.
must be stable for 10ns (read) and 20ns (write)
before access.
- IOCHRDY - Generally not connected in non-ISA
bus.
- CHIPSEL (active low). Tie to ground if not using
ELCS.
- Make sure interrupt line is active high. It is best
to put a pull down (10K) on INT line since
selected IRQ line is tristated during software
initiated reset.
- ELCS should be pulled to ground or left floating
if not used.
- EEDataIn should be pulled to ground if not used.
- 10Base-T circuit -- no caps on TX lines between
isolation transformer and 10 Base-T connector.
- 10Base-T circuit -- no center tap caps on
isolation transformer and 10 Base-T connector.
Good to have pads, don’t populate except for
EMI problems.
- Isolation transformer -- start with one that does
not have a common mode choke. If there are
EMI considerations, then use one with common
mode choke. The pin outs are the same. For
3.3V operation, use a transformer with 1:2.5
turns ration on TX and 1:1 on RX like the Halo
TG41-2006N.
- For EMI problems, 1) add choke, 2) add center
tap caps on isolation transformer
- If using a shielded RJ45 connector, make sure
the shield pins are connected to chassis ground.
- AEN connected to ground if not using DMA.
- AEN can be used as an active low chip select if
not using DMA.
- AUI Interface -- use a 1AMP fuse. MAU can use
.5amps even better use a thermistor ("poly
switch"). Also, use a diode so can’t back-drive
from an externally powered MAU. Use a Halo
TnT integrated module to simplify 10Base2
interface.
- TX series termination resistors are R: 24.3 Ohm
1% (8 or 8.2 Ohm 1% for 3.3V)
- RX shunt termination resistor is 100 Ohm
- Put a 68pF shunt across TX on primary side
(560pF for 3.3V)
- Don’t use split analog/digital power and ground
planes.
- Void ground/power plane from transformer to
RJ45
- Put .1uF cap on each supply pin very close to
CS8900
The schematic checklist and the example connec-
tion diagrams to the Hitachi SH3, Cirrus Logic CL-
PS7211 and the Motorola MC68302 microproces-
sors should make clear the necessary the hardware
connections for a wide variety of situations.