28027的复位电路

（1）内部WDT在复位时是会让这个脚变低，但是外接了上拉电阻后这个脚同样会变低啊

（2）你说如果不拉高，外部干扰会让其触发复位，但是该引脚在片内是有一个弱上拉的，为什么还需要再外接上拉电阻？

Device Reset (in) and Watchdog Reset (out). Piccolo devices have a built-in poweron-reset (POR) and brown-out-reset (BOR) circuitry. As such, no external circuitry is
needed to generate a reset pulse. During a power-on or brown-out condition, this pin
is driven low by the device. See Section 6.3, Electrical Characteristics, for thresholds
of the POR/BOR block. This pin is also driven low by the MCU when a watchdog reset
occurs. During watchdog reset, the XRS pin is driven low for the watchdog reset
duration of 512 OSCCLK cycles. If need be, an external circuitry may also drive this
pin to assert a device reset. In this case, it is recommended that this pin be driven by
an open-drain device. An R-C circuit must be connected to this pin for noise immunity
reasons. Regardless of the source, a device reset causes the device to terminate
execution. The program counter points to the address contained at the location
0x3FFFC0. When reset is deactivated, execution begins at the location designated by
the program counter. The output buffer of this pin is an open-drain with an internal
pullup. (I/OD)

Piccolo devices have a built-in power-on-reset (POR) and brown-out-reset (BOR) circuitry. As such, no external circuitry is needed to generate a reset pulse.During a power-on or brown-out condition, this pin is driven low by the device.意思是它无需外接复位电路，因为片内集成了上电复位和欠压复位，对不对？

是的，内部有BOR和POR，BOR是电压掉到一个值时候就会触发，POR是上电时，电压上升到一个值时就会触发。

采用内部复位方式时，为防止外部干扰的误触发，该引脚在片外还必须接一个电阻上拉到+3.3V。我的理解对吗？

是啊，加个RC，抗干扰。

ERIC

你说的C是指图中的C21吧？看着没啥用啊，可以去掉吗？

不建议你去掉复位脚上的电容，复位脚的抗干扰能力会很差，容易导致MCU误复位

好吧，只是通常看惯了C放在R之后构成RC滤波，看不惯C放在R之前。