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Design Considerations

In addition to the design considerations found in the RC56D/RC336D Designer’s Guide (Order No. 1154), the following should

be taken into account when designing a USB solution.

Series Resistor Requirement for Impedance Matching

Per USB rev. 1.0 specification, the impedance of the differential driver must be between 29

Ω and 33 Ω should be connected

to each USB line; i.e., on D

This condition can be seen on an oscilloscope as excessive overshoot and undershoot, and will introduce errors and lower

throughput.

Pullup Resistor Requirement

The USB specification requires a pullup resistor to allow the host to identify which devices are low speed and which devices

are high speed in order to communicate at the appropriate data rate. For R8292, add a 1.5K

Ω pullup to 3.0V – 3.6V, such as

pin 53 (ECAP). These pullup resistors should be placed as close to the R8292 as possible. If ECAP is used, a 1 µF capacitor

should be connected between ECAP and GND.

Noise Considerations

The R8292 high-speed logic can produce noise spikes on power and signal lines. Decoupling capacitors and transient

absorbers will keep noise to a minimum. Connect 0.1µF bypass caps between VCC/AVCC and each VSS pin.

Noise spikes on XTAL1 and XTAL2 can disrupt the timing of the R8292. Place the crystal and capacitors near the device and

connect with short, direct traces. This will help minimize coupling between other digital circuits and the crystal. Additionally,

place guard rings around the oscillator circuitry and ground the metal crystal case.

Power-on Reset

To automatically generate a reset when power is applied, connect the RST pin to VCC through a 0.33 µF capacitor. When

VCC is applied, the RST pin rises to VCC, and does not decay below the threshold before the crystal stabilizes, plus two

machine cycles.

Excessive LED Current

Pins LED3:0 (36, 37, 43, 44) were designed to drive LEDs connected directly to VCC. The LED driver is too strong, causing

the device to sink excessive current. When all of the LED drivers are turned on at the same time, the current sinking

capability of the device will be exceeded, causing excessive heating and reducing reliability of the device. By using a 250

Ω

resistor in series with each LED, to avoid this problem.

Electrical Characteristics and Timing

For R8292 electrical characteristics and timing information, refer to Intel “8x931AA/8x931HA Universal Serial Bus Peripheral

Controllers”, Order Number: 273108-002.

R8292 power consumption is 70 mA active and 40 mA idle.