Learn USB Library and Interfacing With Pic Microcontroller
Universal Serial Bus, or USB, is a very common method of communication between electronic devices. USB is universal, meaning that it’s standard across all electronic devices, serial, meaning that data is transmitted on only one data line one bit at a time, and is a bus, meaning that all devices are connected through the same parallel electronic wires and therefore require addresses. A simple USB connection requires a host and a device. The host initiates all communication, assigns device addresses, and controls the flow of the data line in the connection. For our purposes, the USB host will be the PC and the USB device will be a PIC chip.
There are many different speeds associated with USB. ยูเอสบี 1.0 introduced data rates of 1.5 Mbits/s (Low-Speed) และ 12 Mbits/s (Full-Speed). With USB 2.0 และ 3.0, data rates of 480 Mbits/s (Hi-Speed) และ 5 Gbits/s (Super-Speed), were added respectively. PIC chips have hardware capable of full speed USB.
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A USB cable has 4 wires: พลัง (แต่เป็นมะเร็งเต้านมชนิดหายาก), ground (สีดำ), D+ (เขียว), and D- (สีขาว). You may notice that many USB plugs and receptacles have 5 หมุด. This fifth pin is called ID, and is often not connected, but has been added to the specifications for use with USB-OTG, or On-The-Go. OTG allows devices to also act as a host. Many PIC chips are compatible with USB-OTG communication, but we will not discuss this topic any further at this point.
The two data lines, D+ and D-, provide information about the state of the USB link.
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Detached State, or SE0 – device is unplugged, both data lines are low, pulled down by 15 kOhm resistors within the host hardware.
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Attached State, Idle State, or J State – the device pulls up the D+ line (for a high-speed device) when it is attached.
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K State – opposite polarity from J State. To communicate, the host or device alternates the device between the J and K states in specific patterns.
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Illegal State, or SE1 – both data lines are high. หากสิ่งนี้เกิดขึ้น, there is a hardware problem with the USB link.
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