NXP Semiconductors OM13260, Руководство по установке, Страница: 42 / 71

NXP Semiconductors
UMxxxxx
Install Guide
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Preliminary 1
— September 6, 2013
© NXP B.V. 2013. All rights reserved.
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Fig 5.34. SPI 2 (Bus1 MCU) connector
Fig 5.35. SPI 0 and SPI 1 (Bus2 LPC Xpresso) connectors
5.10 Logic Probe
Most of the I2C Slaves produce logic signals, on their Input-Output ports. It is necessary to know a logic
state. To facilitate this test there are two LEDs with buffers that may be used as a simple “Logic Probe”
To prevent circuit loading the LEDs are buffered by FETs as shown in figure 5.36. Green LED (D6) is
driven by FET (Q2) when CN11-1 is at or near ground. When CN11-1 is open (or logic 1) the FET is non-
conducting, and the LED is off. Red LED (D7) is driven by FET (Q3) when CN11-4 is at or near ground.
When CN11-4 is open (or logic 1) the FET is non-conducting, and the LED is off. See figure 5.37
Note that the threshold voltage (V
TH
) of the FET is 2.5 – 4.5V, to ensure it operates correctly on both
3.3V and 5V logic levels. The FET Source is tied to 5V, and the Gate must therefore be at 2.5V or lower
relative to ground, to turn on the FET and light the LED.