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補充一下Type-C目前的規範

在我們把Type-C插上供電端的時候，Source端會有個上升電阻
這個電阻的阻值會有三種狀況，告訴Sink端自己的供電上限

三種狀況為
1. Default current. USB 2.0為500mA, USB 3.0為900mA, dual lane是1.5A
2. 1.5A
3. 3A
相關的阻值為下表
https://i.imgur.com/0ALaBeW.png

所以一個Type-C Port在不支援PD的狀況下
它會有這三種電流的上限，而電壓(按照規定)只能是5V

那怎麼知道我的筆電會支援哪個電流？
這個要看筆電廠商的設計，通常關鍵會看電池的放電能力
但通常會有1.5A，很少會限制在0.9A這麼低
因為充電太慢的話使用者會不爽

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順便跟大家說一個冷知識(?)
這種Type-A to Type-C的線
https://i.imgur.com/dBQMQQw.png

Spec有規定，CC Pin的上拉電阻只可以是56K
https://i.imgur.com/oxk7xPI.png

也就是說，根據規範，這種線的電流上限只能是0.5A或0.9A

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以下附了spec相關的原文，有興趣的可以看一下
或者直接到 https://usb.org 下載全文

The USB Type-C receptacle, plug and cable solution incorporates a
configuration process to detect a downstream facing port to upstream
facing port (Source-to-Sink)  connection for VBUS management and
host-to-device connected relationship determination.

The USB Type-C port configuration process is used for the following:
‧ Source-to-Sink attach/detach detection
‧ Plug orientation/cable twist detection
‧ Initial power (Source-to-Sink) detection and establishing the data
   (Host-to-Device) relationship
‧ Detect if cable requires VCONN
‧ USB Type-C VBUS current detection and usage (注意，在PD之前)
‧ USB PD communication
‧ Discovery and configuration of functional extensions

Three current level advertisements at 5V VBUS are defined by USB Type-C
Current:
‧ Default is the as-configured for high-power operation current value as
   defined by a USB Specification (500 mA for USB 2.0 ports; 900 mA or
   1,500 mA for USB 3.2  ports operating in single-lane or dual-lane,
   respectively)
‧ USB Type-C Current @ 1.5 A
‧ USB Type-C Current @ 3.0 A

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