A9 private Timer

The timer is a peripheral that allows the user to measure real-time as a number of clock cycles. A user loads the timer with the number of clock cycles they'd like to wait, and then polls the "Timeout" bit or optionally enables an interrupt to indicate to the processor that the period has elapsed.

Device

Timer

Configuration

32-bit mapped registers

Input/Output

Both

Address Base

0xxFFFEC600

Address Map

tr> >Interrupt Status bits:

Address	R/W	Description
base	R/W	Load value - pre determined count value set by user
base+4	R/W	current value - at any time the counter is running
base+8	R/W	Control Register bits: [0] -Eable - write 1 to start timer [1] - Auto - write 1 - the timer will stop when it reaches 0. Write 1 the timer will reload timer value and start orver. [2] - Interrupt - write 1 timer will generate an interrupt when timer value goes to 0 [15 - 8] - Prescaler - used to slow down the counting rate [Prescaler + 1 clock cycle])
base+12	R/W	[0] - F - When the timer values goes to 0 this bit gets set. To clear the interrupt write a 1 to this bit

Initialization

None (though a period must be written before running the timer, or it will immediately timeout)

Interrupts

Triggered	On timeout
IRQ Line	A9 private Timer use IRQ Line 29
Enable	Bit 0 of Control Register
Acknowledge	Write 1 to status register to clear timeout bit (see address map above)

Hardware Setup

None

Reference

Notes

The timer counts downwards on a 200MHz clock, and runs in terms of clock cycles (a period of 500000000 will cause the timer to timeout in 1 second).

When a timeout occurs, the Timeout bit in the Control Register will stay as 1 until the user writes 1 to the status register.

When a timeout occurs, the timer's counter is reset to the pre determined period. The A bit determines whether the timer will then wait until 1 is written to the F bit or continue running immediately.

Assembly Example 1: Setting the timer to run for 2000 clock cycles, and continuous no interrupts

   ldr r0, =0xFFFEC600                   // r0 contains the base address for the timer 
   mov r1, #2000
   str r1, [r0]                          // Set the period to be 2000 clock cycles 

   mov r1, #3 
   str r1, [r0, #8]                     //  Start the timer continuing no interrupts