Hardware Timers

The PSVita system embeds 8 Word timers (SceWT0 to SceWT7) and 6 Longrange timers (SceLT0 to SceLT5). The timers consist of a counter that gets incremented at a configurable time interval (multiple of a base frequency?), and can generate an interrupt when a certain value is reached.

Most timers are managed by SceSystimer.

Available timers

The Address column indicates the physical address at which the interface for a timer is located.

Each timer interface takes 0x1000 bytes.

List of timers

Name	Address	Interrupt ID	Usage
SceLT0 / SceSystimerLongrangeTimer0	0xE20B1000	0x88	Available for SceSystimer
SceLT1 / SceSystimerLongrangeTimer1	0xE20B2000	0x89	Available for SceSystimer
SceLT2 / SceSystimerLongrangeTimer2	0xE20B3000	0x8A	Available for SceSystimer
SceLT3 / SceSystimerLongrangeTimer3	0xE20B4000	0x8B	Available for SceSystimer
SceLT4 / SceSystimerLongrangeTimer4	0xE20B5000	0x8C	Available for SceSystimer
SceLT5	0xE20B6000	0x8D	Used by SceKernelThreadMgr as CPU timer
SceWT0 / SceSystimerWordTimer0	0xE20B7000	0x80	Available for SceSystimer
SceWT1 / SceSystimerWordTimer1	0xE20B8000	0x81	Available for SceSystimer
SceWT2 / SceSystimerWordTimer2	0xE20B9000	0x82	Available for SceSystimer
SceWT3 / SceSystimerWordTimer3	0xE20BA000	0x83	Available for SceSystimer
SceWT4 / SceSystimerWordTimer4	0xE20BB000	0x84	Available for SceSystimer
SceWT5 / SceSystimerWordTimer5	0xE20BC000	0x85	Available for SceSystimer
SceWT6 / SceSystimerWordTimer6	0xE20BD000	0x86	Available for SceSystimer
SceWT7 / SceTimerForUsleep	0xE20BE000	0x87	Used by Tzs SceKernelIntrMgr for usleep

Misc Reg

Miscellaneous features related to all timers can be accessed from an interface at physical address 0xE20BF000.

Timer Misc Reg interface

Offset	Usage
0x0	Bus Error Address
0x4	Bus Error Attributes
0x8	Secure Bus Error Address
0xC	Secure Bus Error Attributes

Configuration register

The configuration register is identical for all timers. The period of a Systimer tick is calculated using the following formulas:

• t = (prescale_factor + 1) / src_clk in seconds (multiply by 10^9 for ns)
• f = 1 / t = src_clk / (prescale_factor+1) in Hz (divide by 10^6 for MHz)

src_clk is the frequency of the timer's input clock - see below for more information.

Configuration register bit mappings

Mask	Name	Effect / Notes
0x00000001	enable	0: timer stopped - 1: timer counting
0x0000000E	?
0x00000070	?	?Mode select?
0x00000080	?
0x00000700	clk2_ctrl	Changes clock if clk_select == 2
0x00000800	RAZ/WI	Read As Zero / Write Ignore (always 0)
0x0000F000	?
0x000F0000	?
0x00F00000	clk_select	Controls which clock is used as input to prescaler (see below)
0xFF000000	prescale_factor	Input clock division factor (the higher this is, the slower the timer will tick)

Available input clocks

clk_select	Input clock
0	ScePower SysClock (190/222MHz)
1	~37MHz
2	Varies based on clk2_ctrl clk2_ctrl 0 27MHz 2-3 ~24.57MHz 4 ~37.38MHz 5 ~74MHz 1,6,7 No clock (timer doesn't tick)
3	48MHz
4	60Hz
5	~35.6kHz
8	60Hz
9	~45kHz
6-7,A-F	No clock (timer doesn't tick)

Known configurations

Timer	Value	Explaination
SceWT7 (SceTimerForUsleep)	0xDD00000D	SysClock source / (prescale_factor = 221) -> 0.85/1MHz frequency
SceLT5	0x2F34500D	48MHz source / (prescale_factor = 47) -> 1MHz frequency

Word Timers

Timers with 32-bit counters.

typedef volatile _SceWordTimer {
    SceUInt32 unk0; // Value at which an interrupt is triggered?
    SceUInt32 current; // Current value of the timer's counter
    SceUInt32 cfg; // Configuration register
    SceUInt32 unkC; // Another counter?
    SceUInt32 unk10; // Unused?
    SceUInt32 unk14; // Interrupt status? Write 0x3 to clear pending IRQ?
} SceWordTimer;

Long Timers

Timers with 64-bit counters.

SceLT5 timer is configured to increment every microsecond (1MHz frequency).

typedef volatile _SceLongTimer {
    SceKernelSysClock current;  // Current value of the timer counter
    SceKernelSysClock unk8;  // Value at which an interrupt is triggered?
    SceKernelSysClock unk10; // Another counter?
    SceUInt32 unk18; // Interrupt status? Write 0x3 to clear pending IRQ?
    SceUInt32 cfg; // Configuration register
} SceLongTimer;

NOTE: On ARMv7 processors that do not support the Large Physical Address Extension, such as the PS Vita's CPU, 64-bit accesses are not guaranteed to be atomic.

This can lead to issues when reading the timer if the low word of a counter is about to overflow.

To ensure the readings from a timer are accurate, use code similar to the following code:

//Return value should hopefully be optimized into registers
SceKernelSysClock readCounter(volatile SceKernelSysClock* pTimer) {
   SceKernelSysClock ctr;
   SceUInt32 ctrHi;
   do {
       ctr.u.hi = pTimer->u.hi; //Read high word first
       ctr.u.lo = pTimer->u.lo; //Read low word next
       ctrHi = pTimer->u.hi;  //Read high word again to make sure no overflow happened
    } while (ctr.hi != ctrHi); //Try again if high word changed while reading low word

    return ctr;
}