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User’s Manual
PPC440x5 CPU Core Preliminary
Page 210 of 589
timers.fm.
September 12, 2002
Software access to TBU and TBL is non-privileged for read but privileged for write, and hence different SPR
numbers are used for reading than for writing. TBU and TBL are written using mtspr and read using mfspr.
The period of the 64-bit time base is approximately 1462 years for a 400 MHz clock source. The time base
value itself does not generate any exceptions, even when it wraps. For most applications, the time base is set
once at system reset and only read thereafter. Note that Fixed Interval Timer and Watchdog Timer excep-
tions (discussed below) are caused by 0→1 transitions of selected bits from the time base. Transitions of
these bits caused by software alteration of the time base have the same effect as transitions caused by
normal incrementing of the time base.
Figure 7-2 illustrates the TBL.
Figure 7-3 illustrates the TBU.
7.1.1 Reading the Time Base
The following code provides an example of reading the time base.
loop:
mfspr Rx,TBU # read TBU into GPR Rx
mfspr Ry,TBL # read TBL into GPR Ry
mfspr Rz,TBU # read TBU again, this time into GPR Rz
cmpw Rz, Rx # see if old = new
bne loop # loop/reread if rollover occurred
The comparison and loop ensure that a consistent pair of values is obtained.
7.1.2 Writing the Time Base
The following code provides an example of writing the time base.
lwz Rx, upper # load 64-bit time base value into GPRs Rx and Ry
Figure 7-2. Time Base Lower (TBL)
0:31 Time Base Lower Low-order 32 bits of time base.
Figure 7-3. Time Base Upper (TBU)
0:31 Time Base Upper High-order 32 bits of time base.
0 31
0 31