Question

Table : CPLD USER_REG Register Bit Name R/W Description 7 USER_SW3 User DIP Switch 311 = Off(UP), 0 = On(DOWN)) 6 USER_SW2 User DIP Switch 211 = Off(UP), 0 = On(DOWN)) 5 USER_SW1 User DIP Switch 1(1 = Off(UP), O = On(DOWN)) 4 USER_SWO User DIP Switch 011 = Off(UP), 0 = On(DOWN)) 3 USER_LED3 R/W User-defined LED 3 Control (0 = Off, 1 = On) 2 USER_LED2 R/W User-defined LED 2 Control (0 = Off, 1 = On) 1 USER_LEDI R/W User-defined LED 1 Control (0 = Off, 1 = On) O USER_LEDO R/W User-defined LED O Control (0 = Off, 1 = On) if (USER_REG >> 4) & (0x8)) /* Copy input to output. */ for (i=0; i<BUFFER_SAMPLES; i++) Task 2: Try to find out what happens when we press Down or Up of Switch 3? SW3 SW2 SW1 SWO 0 0 outl[i] = inL[i]; outR[i] = inR[i]; In dsp_top.c clse /* Call the user's process routine. */ dsp_process (inl, inn, outl, outR); In dsp_ap.c

Try to find out what happens when we press down or up of switch 3? (Please refer to the picture given)

Answer 1

From the given image we can understand that USER_REG is an 8-bit register whose upper 4 bits are the state of four dip switches. We will begin by understanding the condition present in the if statement.

The condition given to the if statement is (USER_REG >> 4) & (0x8). This checks if the bit in position 7 is 1 or 0. It does this as follows:

USER_REG consists of the following bits in the following positions:

7th USER_SW3
6th USER_SW2
5th USER_SW1
4th USER_SW0
3rd USER_LED3
2nd USER_LED2
1st USER_LED1
0th USER_LED0

USER_REG >> 4 shifts the 8-bit number to the right by four positions.

(USER_REG >> 4) is a new number that consists of the following bits (the four new freed up positions are filled up with 0):

7th 0
6th 0
5th 0
4th 0
3rd USER_SW3
2nd USER_SW2
1st USER_SW1
0th USER_SW0

(USER_REG >> 4) & (0x8) checks the 3rd bit position in (USER_REG >> 4) if its 0 or 1. It does this as follows:

(USER_REG >> 4) & (0x8) = (USER_REG >> 4) & (0b00001000) = USER_SW3 & 1

Which is true only when USER_SW3 is 1.

Based on this understanding we can see that when the switch is down (USER_SW3 == 1), the if clause is executed and the input is copied to the output. Similarly, when the switch is up (USER_SW3 == 0), the else clause is executed and the function dsp_process is called.

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