| 摘要 |
PURPOSE: A microcomputer is provided to interface data transmission and receipt between a host and the microcomputer through a USB signal line. CONSTITUTION: A personal computer(100) and a USB interface circuit(30) in the microcomputer(10) are connected to each other through a USB cable and a hub(101) is provided therebetween. A pair of USB signal lines(110, 111) are pulled down through pull-down resistors(112, 113). When the USB cable is connected through a connector to a substrate(116) having the microcomputer(10) mounted thereon, for example, one of the USB signal lines(D) is pulled up to a power potential VDD through a pull-up resistor(114) provided in the substrate(116) and a predetermined power potential(for example, 3.3 V) is supplied to the microcomputer(10) through a regulator(115). Consequently, the personal computer(100) detects that the state of the USB signal line(111) is changed from a low level to a high level and knows that the USB cable is connected to the microcomputer(10). In other words, a pair of USB signal lines(110, 111) serve as transmission paths for communication data, and furthermore, inform the personal computer(100) of the connection state of the device. Moreover, a change in the state of the USB signal lines(110, 111) simultaneously informs the personal computer(100) of the type of the device. More specifically, the device has a high speed if the USB signal line(110) is changed from the low level to the high level, and has a low speed if the USB signal line(111) is changed from the low level to the high level. The personal computer(100) detecting the change in the state of the USB signal line(110) transmits a bus reset signal. On the other hand, the microcomputer(10) carries out initialization corresponding to the power ON. In the case in which the communication data are transmitted immediately after the bus reset signal, they cannot be normally received because a system clock is still unstable.
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