请问EK-TM4C123GXL的ButtonsPoll例程函数ui8SwitchClockA和ui8SwitchClockB的作用是什么？

在阅读EK-TM4C123GXL关于FreeRTOS的例程时，有个地方没搞懂。下面这个函数用来检测按键状态，其中ui8SwitchClockA和ui8SwitchClockB的作用是什么？
该函数是否能实现软件去抖动，如何实现的？谢谢指导。
uint8_t
ButtonsPoll(uint8_t *pui8Delta, uint8_t *pui8RawState)
[
uint32_t ui32Delta;
uint32_t ui32Data;
static uint8_t ui8SwitchClockA = 0;
static uint8_t ui8SwitchClockB = 0;
//
// Read the raw state of the push buttons. Save the raw state
// (inverting the bit sense) if the caller supplied storage for the
// raw value.
//
ui32Data = (ROM_GPIOPinRead(BUTTONS_GPIO_BASE, ALL_BUTTONS));
if(pui8RawState)
[
*pui8RawState = (uint8_t)~ui32Data;
]
//
// Determine the switches that are at a different state than the debounced
// state.
//
ui32Delta = ui32Data ^ g_ui8ButtonStates;
//
// Increment the clocks by one.
//
ui8SwitchClockA ^= ui8SwitchClockB;
ui8SwitchClockB = ~ui8SwitchClockB;
//
// Reset the clocks corresponding to switches that have not changed state.
//
ui8SwitchClockA &= ui32Delta;
ui8SwitchClockB &= ui32Delta;
//
// Get the new debounced switch state.
//
g_ui8ButtonStates &= ui8SwitchClockA | ui8SwitchClockB;
g_ui8ButtonStates |= (~(ui8SwitchClockA | ui8SwitchClockB)) & ui32Data;
//
// Determine the switches that just changed debounced state.
//
ui32Delta ^= (ui8SwitchClockA | ui8SwitchClockB);
//
// Store the bit mask for the buttons that have changed for return to
// caller.
//
if(pui8Delta)
[
*pui8Delta = (uint8_t)ui32Delta;
]
//
// Return the debounced buttons states to the caller. Invert the bit
// sense so that a '1' indicates the button is pressed, which is a
// sensible way to interpret the return value.
//
return(~g_ui8ButtonStates);
]