T-Flash (microSD) card has become popular in the digital market with its high performance, small size, large capacity and moderate price. It has become the most popular type of expansion card in mobile phones.
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The TF decks commonly used in mobile phone design are of the Push and Hinge types. The Hinge TF card holder needs to be opened before it can be removed. Therefore, it is usually installed on the motherboard under the battery. The TF card can be inserted or removed after the battery is removed. The Push type TF card holder can be accessed from the side, usually used for the side opening of the mobile phone, and can be hot-swapped TF card; when the space under the battery is tight, the Hinge type TF card holder cannot be used, and the Push type TF card holder can also be used. Installed inside, side access.
The T card allows hot swapping, and the T card can be inserted or removed when the deck is powered. In order to prevent damage to the card during hot plugging, the T card and the card holder are all designed to ensure.
In the gold finger on the back of the T card, the power supply pins (4, 6) are 0.3 mm longer than the signal pins (1, 2, 3, 5, 7), and the power pins (4, 6) in the card holder are longer than other signals. foot. In this way, when the card is inserted, it can be ensured that the power supply pin is in contact with the signal pin in advance to receive power; when the card is pulled out, the power supply pin is powered off later than its signal pin. This effectively ensures the safety of T-card hot swap.
Several detection methods of T card
Since the T card can be hot swapped, the user may actively insert and remove the card during use; it may also cause the T card to loosen due to unintentional vibration and shaking, which requires the mobile phone interface to promptly indicate to the user. This requires the detection of the presence or absence of a T-card (referring to electrical contact).
Mechanical detection - using the switch in the deck
One method is to use the mechanical event of the insertion and removal of the T card for detection, which requires designing a detection switch on the T card holder. There are two types of T-card detection switches: normally open and normally closed. The corresponding definitions are as follows
SanDisk introduced the implementation of a normally open detection switch:
The T card slot has a status signal connected to the processor (usually as an interrupt input) to indicate if a card is inserted, and the status signal is externally pulled up to a high level by a resistor. A special switch is designed inside the card holder, one end is connected to the status signal, and one end is connected to the ground.
When no card is inserted, the switch is open and the status signal is high; when a card is inserted, the switch is turned on, the status signal and the ground signal are navigated, and the status signal goes low. It is possible to know if there is a card insertion depending on the level change of the status signal.
The special switch in the T card holder consists of a mechanical spring A and a spring B. When no card is inserted, the two shrapnels are not in contact, which is equivalent to being disconnected; when a card is inserted, the shrapnel B is squeezed and comes into contact with the shrapnel A, and the switch is turned on.
It should be pointed out that the TransFlash specification does not stipulate that the card holder must have a detection switch, and there is no provision for the detection switch. Therefore, in the circuit design, it is necessary to inquire the specification of the deck used, to understand the implementation of the detection switch, and to handle it flexibly according to the requirements of the interrupt level of the processor.
Circuit Detection - Using CD/DAT3
Inside the T card, the CD/DAT3 (pin 1) signal has a pull-up resistor (50Kohm) that can be used for card detection. During data transfer, CD/DAT3 is used as a data line, at which point the processor should send a specific command (ACMD42) to disable this pull-up resistor.
MTK's MT6226 baseband processor can implement such a detection function, which is the recommended circuit given in the manual. However, this design requires a resistor to be placed on the board, and an electronic switch is required to perform on-off control of the connection of the resistor to the CD/DAT3 signal line. Due to the complexity of this design, the author has learned that few manufacturers use this reference design.
Software Detection - Timeout
The processor issues a corresponding command to the T card, and the T card should respond accordingly. If the response time expires (Timeout), the T card may not exist or the T card may be damaged. Using the software Timeout to detect the existence of the T card requires a certain amount of resources to be wasted, and the reflection time is obviously behind the first two methods.
In the actual product design, the above detection methods should be comprehensively utilized. Imagine if you only use mechanical detection, if the user inserts a thick piece of paper, will it also trigger the interface to prompt the T card to be inserted? Prompt T card insertion, at least should be detected after the T card insertion is detected and the initialization is successful.
Hinge T card detection
The Hinge T card generally needs to be removed after the battery is removed, so there is no hot plugging problem, and the Hinge T card holder does not have a mechanical detection device. Some resources of the mobile phone can be executed without a T card. Therefore, if the T card is not installed when the user is turned on, the interface does not need to prompt the user that the T card does not exist. When the user enters some resources that must be executed by the T card (such as music playing, photography), the mobile phone can use the circuit detection or software detection mechanism to determine whether the T card exists. If it does not exist, the user should be prompted to insert the T card.
The Push type T card that can be picked up after the battery has to be removed is also in line with the above discussion.
Push type T card detection
For the Push type T card with the side opening of the mobile phone, the detection problem is more complicated due to the randomness of the user's hot plug T card event.
When the user uses a resource that can be executed without a T card, the T card is inserted and removed, and the interface should not give a prompt, otherwise the current resource usage of the user will be affected.
However, when the user enters some resources that must be executed by the T card (such as music playing, photography), the mobile phone can use mechanical, circuit, software and other mechanisms to determine whether the T card exists. If it does not exist, the user should be prompted to insert T. card.
If the user immediately inserts a T-card, the prompt should also be given immediately. If the user suddenly pulls out the T card during the resource execution process, the interface should immediately give a corresponding prompt.
Since the Push type T card generally has a mechanical detection switch, the mechanical detection method can be used to quickly know the event of the user inserting and removing the card, prompting the user in time.
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