Rainbow Electronics AT76C551 - Manual (Page 72)

72
AT76C551
1612D–08/01
• Bit 1 – TM1_PR
Enables FIQ priority for the Timer 1 interrupt line
• Bit 0 – TM2_PR
Enables FIQ priority for the Timer 2 interrupt line
Note: Default Value: 00 hex
Timer Device
Registers AT76C551 incorporates two identical and completely independent system timer devices,
Timer Device 1 and Timer Device 2.
Each Timer Device is implemented as a 32-bit down counter which advances at a programma-
ble rate driven by a prescale circuit. The counter is loaded with a programmable 32-bit value
(preload) when the Timer Device is enabled. When the counter reaches zero an interrupt to
the ARM core is generated. Timer Device behavior after the counter reaches zero depends on
the selected operating mode. In one-shot mode the counter stops and no interrupt is gener-
ated to the ARM until the Timer Device is reenabled. In periodic mode the counter is reloaded
with the preload value and countdown is continued, effectively generating a periodic interrupt
to the ARM core.
AT76C551 firmware can program and control each Timer Device via registers mapped into
AMBA memory space and thus accessible by the ARM core. (See Table 13).
Timer Preload Low Register
R/W 16 bits
Timer Preload High Register
R/W 16 bits
Timer Preload Low Register and Timer Preload High Register contain the 32-bit preload value.
Timer Value Low Register
R 16 bits
Timer Value High Register
R 16 bits
Timer Value Low Register and Timer Value High Register reflect the 32-bit counter current
value. Note that full 32-bit counter value can be acquired by reading both Timer Value Low
Register and Timer Value High Register, but result consistency cannot be guaranteed by the
Timer Device and must be ensured by software means.
Table 13. Timer Device Registers
Register Name
AMBA Address for
Timer Device 1 AMBA Address for
Timer Device 2
Timer Preload Low Register F200000h F200020h
Timer Preload High Register F200004h F200024h
Timer Value Low Register F200008h F200028h
Timer Value High Register F20000Ch F20002Ch
Timer Prescale Register F200010h F200030h
Timer Control Register F200014h F200034h
Timer Interrupt Clear Register F200018h F200038h