Here is an example of using the PRUSS cycle counter to time a pulse.
Files at: github.org/jstampfl/Prutimer
here is the PRUSS code:
`
/ pt.p
// To get the echo pulse on pin P8 15.
// assemble: pasm -v3 -b pt.p (could be pasm_2 if you follow directions
.setcallreg r29.w0 // Going to use r31 for interrupt
.origin 0
.entrypoint T811
#define CTRL 0x22000 //Start of control registers.
T811:
mov r8,CTRL //set r8 for the lbbo
ldi r12, 0 // Local memory address for pru0 data memory
ldi r2, 0 // zero to zero the cycle counter
TB1:
qbbs TB1,r31,15 // wait here for echo line to go low
// WAIT FOR THE ECHO PULSE TB2:
qbbc TB2,r31,15 // wait here for echo line to go high - start of pulse
// START THE CYCLE COUNTER
lbbo r9,r8,0,4 // Get the control register
set r9,3 // Set the cycle counter enable
sbbo r9,r8,0,4 // Put back to the register to start
// WAIT FOR THE PULSE TO END
TB3:
qbbs TB3,r31,15 // wait here until echo line goes low.
// STOP THE CYCLE COUNTER
lbbo r9,r8,0,4 // get control register
clr r9,3 // clear counter to disable
sbbo r9,r8,0,4 // put back
ldi r10,1 // not neccessary, was going to use as signal
// GET THE CYCLE COUNT AND PUT IN PRU0 LOCAL DATA MEMORY
lbbo r11,r8,12,4 // get the CYCLE count
sbbo r10,r12,0,8 // put r10 & r11 (the unused signal & cycle count) in
// pru0 data memory
sbbo r2,r8,12,4 // Zero the cycle count
// SEND INTERRUPT SIGNAL TO LINUX SIDE
mov r31.b0,35 //Send interrupt to Linux side
lbbo r11,r12,8,4 // Check for end flag from Linux side
qbne TB1,r11,2 // go back for more
TB4:
halt
`