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17.4.7
Accuracy of UCLK48
This section describes the accuracy of the UCLK48 clock.
■ Accuracy of UCLK48
USB Function Ver1.1 has two standards for D+ data and D- data signals.
A crystal oscillator module must be used as the 48 MHz clock input source of the UCLK48 clock.
If PLL is used, take special care regarding clock frequency accuracy.
❍ Data rate standard (long-term standard)
The device (entire circuit not limited to an LSI circuit) must satisfy the following: accuracy of
-2,500 ppm to +2,500 ppm (-0.25% to +0.25%).
The USB standards require an accuracy of -2,500 to +2,500 ppm for the 12 MHz clock. However,
the USB function macro program requires the 48 MHz clock to have a higher accuracy than -
2,500 ppm to +2,500 ppm. This is because it selects the clock edges synchronized with data
from the clock edges of the 48 MHz clock and uses the selected clock edges as the 12 MHz
clock.
For this reason, the UCLK48 clock needs a higher accuracy than -2,500 ppm to +2,500 ppm.
❍ Short-term standard
The device (entire circuit not limited to an LSI circuit) must satisfy the following jitter requirements:
•
Jitter within -1 ns to +1 ns until the first crossing point
•
Jitter within -2 ns to +2 ns until the second crossing point
Accuracy within -1 ns to +1 ns from each edge of the 48 MHz clock is required because the D+
and D- signals may alter at every edge.
CHAPTER 17 USB FUNCTION
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