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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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PIC18F2X 데이터시트(PDF) 62 Page - Microchip Technology |
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PIC18F2X 데이터시트(HTML) 62 Page - Microchip Technology |
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62 / 496 page  PIC18(L)F2X/4XK22 DS41412C-page 62 Preliminary 2010 Microchip Technology Inc. 4.4 Brown-out Reset (BOR) PIC18(L)F2X/4XK22 devices implement a BOR circuit that provides the user with a number of configuration and power-saving options. The BOR is controlled by the BORV<1:0> and BOREN<1:0> bits of the CONFIG2L Configuration register. There are a total of four BOR configurations which are summarized in Table 4-1. The BOR threshold is set by the BORV<1:0> bits. If BOR is enabled (any values of BOREN<1:0>, except ‘00’), any drop of VDD below VBOR for greater than TBOR will reset the device. A Reset may or may not occur if VDD falls below VBOR for less than TBOR. The chip will remain in Brown-out Reset until VDD rises above VBOR. If the Power-up Timer is enabled, it will be invoked after VDD rises above VBOR; it then will keep the chip in Reset for an additional time delay, TPWRT. If VDD drops below VBOR while the Power-up Timer is running, the chip will go back into a Brown-out Reset and the Power-up Timer will be initialized. Once VDD rises above VBOR, the Power-up Timer will execute the additional time delay. BOR and the Power-on Timer (PWRT) are independently configured. Enabling BOR Reset does not automatically enable the PWRT. The BOR circuit has an output that feeds into the POR circuit and rearms the POR within the operating range of the BOR. This early rearming of the POR ensures that the device will remain in Reset in the event that VDD falls below the operating range of the BOR circuitry. 4.4.1 DETECTING BOR When BOR is enabled, the BOR bit always resets to ‘0’ on any BOR or POR event. This makes it difficult to determine if a BOR event has occurred just by reading the state of BOR alone. A more reliable method is to simultaneously check the state of both POR and BOR. This assumes that the POR and BOR bits are reset to ‘1’ by software immediately after any POR event. If BOR is ‘0’ while POR is ‘1’, it can be reliably assumed that a BOR event has occurred. 4.4.2 SOFTWARE ENABLED BOR When BOREN<1:0> = 01, the BOR can be enabled or disabled by the user in software. This is done with the SBOREN control bit of the RCON register. Setting SBOREN enables the BOR to function as previously described. Clearing SBOREN disables the BOR entirely. The SBOREN bit operates only in this mode; otherwise it is read as ‘0’. Placing the BOR under software control gives the user the additional flexibility of tailoring the application to the environment without having to reprogram the device to change BOR configuration. It also allows the user to tailor device power consumption in software by eliminating the incremental current that the BOR consumes. While the BOR current is typically very small, it may have some impact in low-power applications. 4.4.3 DISABLING BOR IN SLEEP MODE When BOREN<1:0> = 10, the BOR remains under hardware control and operates as previously described. Whenever the device enters Sleep mode, however, the BOR is automatically disabled. When the device returns to any other operating mode, BOR is automatically re-enabled. This mode allows for applications to recover from brown-out situations, while actively executing code, when the device requires BOR protection the most. At the same time, it saves additional power in Sleep mode by eliminating the small incremental BOR current. 4.4.4 MINIMUM BOR ENABLE TIME Enabling the BOR also enables the Fixed Voltage Reference (FVR) when no other peripheral requiring the FVR is active. The BOR becomes active only after the FVR stabilizes. Therefore, to ensure BOR protection, the FVR settling time must be considered when enabling the BOR in software or when the BOR is automatically enabled after waking from Sleep. If the BOR is disabled, in software or by reentering Sleep before the FVR stabilizes, the BOR circuit will not sense a BOR condition. The FVRST bit of the VREFCON0 register can be used to determine FVR stability. Note: Even when BOR is under software control, the BOR Reset voltage level is still set by the BORV<1:0> Configuration bits. It cannot be changed by software. |
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