Smart solution for single-button power on/off and reset in tablet design

summary:

As more and more smartphones and tablets are designed with built-in batteries, how to perform a hardware reset of the system when the system software freezes has become an increasingly prominent design issue.

STMicroelectronics' STM65xx family of smart-reset ICs enables designers to remove the traditional reset button and the access hole that hides the reset button on the body, enabling not only a double-button long-press reset, but also the single-button power on/off and reset solution popular in smartphones and tablets.

Keywords: Smart ResetTM, Smartphone, TabletPC

1 Introduction

With the application of a large number of emerging data services, the power consumption level of smartphones and tablets has increased significantly, resulting in a significant reduction in standby time. In order to extend the standby time, the design of built-in batteries has become more and more popular. This is because half of the volume of lithium batteries is occupied by their structural parts. If the battery is built into the body of smartphones and tablets, the volume of lithium battery structural parts can be saved, thereby greatly increasing the battery capacity in the same or even larger volume. In this way, the battery capacity has indeed been greatly increased, but a new problem has also arisen - if the software system of smartphones and tablets is stuck during the application process, how to reset the system?

Mechanical reset devices to remove card machine conditions are usually backward compared to the main function of the product. To prevent accidental reset of the device, most manual reset buttons (if any) are hidden inside the body. Because the reset button is difficult to reach, removing the battery has become a very common solution. However, this practice is not only poor user experience and increased costs, but may also damage the system, for example, causing the loss of important data.

So, how to perform a hardware reset of the system in smartphones and tablets with built-in batteries? This article introduces a solution for hardware intelligent reset, which can not only realize the intelligent reset of double-button long press in smartphones and tablets, but also realize the intelligent solution of single-button power on/off and reset popular in smartphones and tablets.

2. Mechanisms and risks of powering on/off and resetting smartphone and tablet application platforms

In the mainstream platforms of today's smartphones and tablets, there is usually an architecture of an application processor (Application Process / Baseband, hereinafter referred to as AP) plus a power management chip (Power Management Unit, hereinafter referred to as PMU), as shown in Figure 1.

Figure 1. AP + PMU hardware architecture in smartphones and tablets

In this hardware architecture, a power switch pin is provided on the PMU and is connected to a mechanical switch on the body (hereinafter referred to as Power_Key).

When the mobile phone is in the off state, press Power_Key to pull the power switch pin of PMU to ground, which will start the PMU power-on process: PMU starts LDO to power AP, and sends a hardware reset signal to AP at the same time. When the AP software system is started, a PS_HOLD signal is sent back to pull the PS_HOLD pin of PMU high, and it is always kept at a high level in the working state; if AP fails to pull the PS_HOLD pin high within a certain period of time (Tpshold time), it indicates that AP startup fails, and PMU automatically performs the power-off process. It is usually required that there is a certain relationship between Power_Key and PS_HOLD signals, that is, the Power_Key signal must be kept at a low level until the PS_HOLD signal is driven high by AP, as shown in Figure 2. This is because, if the AP power-on initialization fails and the PS_HOLD signal cannot be pulled high within the set time Tpshold, Power_Key is still maintained at a low level to ensure that PMU will be triggered to power on again, thereby ensuring successful power-on.

Figure 2. Timing relationship between the PMU's Power_Key and PS_HOLD signals

When the phone is powered on, press the Power_Key to pull the power switch pin of the PMU to the ground. The PMU will send an interrupt to the AP. The AP will respond according to the interrupt request and pull the PS_HOLD pin to the ground. The PMU will automatically power off.

There is an obvious hidden danger in this mechanism: when the AP system software is stuck, it will not be able to respond to the power-off interrupt request sent by the PMU, and it will not be able to shut down or reset. The possible solution is as follows: set a key switch S1 at the input end of the PS_HOLD pin of the PMU. When S1 is pressed, the PS_HOLD signal is pulled down to ground, triggering the power-off process of the PMU, as shown in Figure 3.

Figure 3. Manual reset solution in the AP + PMU hardware architecture

This solution is feasible, but S1 needs to be hidden in a small hole that is not easy to trigger. Normally, users cannot touch the reset switch S1. In addition to the bad user experience and increased design costs and risks, this solution also has a problem - the current popular smartphones or tablets are designed with only one mechanical button, which is the switch Power_Key connected to the PMU power switch pin. In this design, Power_Key and S1 cannot be set together. The reason is shown in Figure 4.

Figure 4. Schematic diagram of the AP + PMU hardware architecture where the power on/off button and reset button cannot be combined into one

When the system is in shutdown state, if Power_Key is pressed short, PMU will trigger the power-on process. When the AP is powered on and started, the PS_HOLD signal will be pulled high. At this time, no matter whether the button is pressed or released, the PS_HOLD of PMU can be pulled high in time through R2/C1/R1 within the Tpshold time, and there is no problem in the successful power-on of the system. When the system is in the boot state, if Power_Key is pressed, the PS_HOLD signal is immediately pulled low, and PMU will enter the power-off process. At the moment the button is released, the system may be in a certain stage of the power-off process or the power-on process, which may eventually lead to unpredictable results such as possible shutdown and possible system reset, which is unacceptable for product design, as shown in Figure 5. More importantly, with such a design, the system cannot implement the software shutdown function at all. Therefore, in this circuit design, Power_Key and S1 cannot be set together.

Figure 5. Timing sequence of the AP + PMU hardware architecture where the power on/off button and the reset button cannot be combined into one

In order to correct the defect that the PMU itself does not have a dedicated hardware reset input pin and needs to use the PS_HOLD signal to pull low to reset, the new PMU began to introduce a dedicated RESET_IN reset pin, allowing external circuits to reset the PMU through this pin. However, there is still a problem here - the PMU specification requires that the power on/off button and the reset button must be physically separated and cannot be set on the same button. The reset button needs to be hidden in the inspection hole on the body, and the single-button power on/off and reset solution cannot be implemented.

So, is there a hardware solution that can combine the power on/off button and the reset button into one, realizing a smart solution for single-button power on/off and reset in smartphone and tablet computer designs? [page]

3 Smart solutions for single-button on/off and reset in smartphone and tablet designs

STMicroelectronics' STM65xx series of smart reset chips have two or one inputs that can be connected to two or one function keys on the device. If the two keys are pressed simultaneously or individually for a certain period of time (the length of time can be set or selected according to the model), the reset chip will send a reset signal to the main processor. The two or one inputs and delay setting function of the reset chip combine the "normal function" of the key and the "system reset function" of the key, and can effectively prevent the device from being accidentally reset.

In the design of smart phones and tablet computers, the current popular design is the single-button power on/off and reset design, that is, there is only one mechanical button on the entire body, which carries the functions of power on/off and card reset. The STM6513 in the STM65xx smart reset chip series can perfectly implement this function. Designers only need to connect the SR0 and SR1 input pins of the STM6513 to the Power_Key (for the design that requires a double-button long-press reset, just connect /SR0 and /SR1 to different function keys respectively), /RST2 to the reset input pin of the AP, and RST1 to the PS_HOLD pin of the PMU, so that the smart solution of single-button power on/off and reset in the design of smart phones and tablet computers can be easily implemented, as shown in Solution 1 in Figure 6.

Figure 6. Smart solution for single-button on/off and reset using STM65131

When the system is in shutdown state, if Power_Key is pressed short, PMU will trigger the power-on process. When the AP is powered on and started, the PS_HOLD signal will be pulled high, and the system will power on successfully without any problems. Since Power_Key is pressed short in the design, the delayed reset function of STM6513 (optional, for example, 8 seconds) will not be triggered.

When the system is in the power-on state, if the Power_Key is pressed for more than one time (optional, such as 8 seconds), /RST2 outputs a low-level effective reset signal to the AP, and the RST1 pin outputs a high-level signal. Due to the existence of the line-and function circuit composed of two diodes on the PS_HOLD input pin of the PMU, when the AP is reset, the RST1 output by the STM6513 will remain high (the trec of RST1 can be set as needed through the external capacitor pin of the STM6513) until the AP drives the PS_HOLD pin high. In this way, when the system is reset, only the AP is reset by the STM6513, and the PMU is not actually powered off, which can ensure that the system reset is successful. In addition, since the PMU is not powered off during the system reset process, the cached data is not lost, and the function of saving user application data when the system crashes can also be realized.

Some designers may prefer that the PMU can also be restarted during the system restart process. For such designers, it is also possible to only use the /RST2 pin of the STM6513 to connect to the PS_HOLD pin of the PMU (for PMUs with RESET_IN, it can be connected to the RESET_IN pin), as shown in Solution 2 in Figure 7. When the system is in the power-on state, if the Power_Key is pressed for more than one time (optional, such as 8 seconds), /RST2 outputs a low-level valid reset signal to pull down the PS_HOLD signal of the PMU. Since the trec of /RST2 is fixed (for example, 210ms), that is, /RST2 will change to an output high-impedance state after the reset signal maintains a low level for 210ms, thereby releasing the PS_HOLD signal of the PMU. The PS_HOLD of the PMU will be completely controlled by the state of the PS_HOLD output pin of the AP. Since the Power_Key is still at a low level at this time, the PMU will be triggered to power on again, and finally the power-on is successful.

Figure 7. Smart solution 2 using STM6513 for single-button on/off and reset

For designers who use Option 2, a more cost-effective solution is to use the new STM6519 chip from STMicroelectronics. This chip is a single-button delayed reset chip. The reset delay time is selected by the model. There is only one /RST reset output signal. It uses a UDFN6 or UDFN4 1.0x1.45mm package, as shown in Figure 8.

Figure 8. Smart solution for single-button power on/off and reset using the STM6519

Using STMicroelectronics STM6513 or STM6519 smart reset products, the following single-key power on/off and system reset processes can be achieved:

In the shutdown state, short press the button to power on;
in the power-on working state, under the premise that the AP system software is not stuck, short press the button, and the AP will display "Return? Shutdown?" on the screen for the user to choose - if you confirm to return, then return; if you confirm to shut down, the AP will pull PS_HOLD low, the PMU will enter the power-off process, and finally shut down. In the case of AP system software stuck, long press the button (optional, for example 8 seconds), the system will perform a hardware reset and restart.

4 Summary

This article first introduces the reset mechanism and hidden dangers of the AP+PMU hardware architecture on smartphone and tablet platforms, and then explains the use of STMicroelectronics STM6513 and STM6519 smart reset chips to achieve double-button long-press reset, especially the smart solution of single-button power on/off and reset popular in smartphones and tablets.

STMicroelectronics' STM65xx family of smart-reset chips enables product designers to remove traditional reset buttons and hidden access holes on the body of the device, saving costs, reducing design risks, and improving user satisfaction.

References

[1] Data sheet of 'STM6513 Dual push-button Smart Reset with dual reset outputs and user-selectable setup delay', Rev2.0, Jun 2010; http://www.st.com/internet/analog/product/241463 .jsp ;