50 V7768/V7769* Intel® Core™ Duo Processor VME Single Board Computer
Publication No: 500-9300007768-000 Rev. H.0
The “Read Latch Select” bit is used to select the latching mode of the
programmable timers. If this bit is set to “0”, then each timer output is latched
upon a read of any one of its address. For example, a read to the TMRCCR12
register latches the count of timers 1 and 2. A read to the TMRCCR3 register
latches the count of timer 3. This continues for every read to any one of these
registers. As a result, it is not possible to capture the values of all four timers at a
given instance in time. However, by setting this bit to “1”, all four timer outputs
will be latched only on reads to the Timer 1 & 2 Current Count Register
(TMRCCR12). Therefore, to capture the current count of all four timers at the
same time, perform a read to the TMRCCR12 first (with a 32-bit read), followed
by a read to TMRCCR3 and TMRCCR4. The first read (to the TMRCCR12 register)
causes all four timer values to be latched at the same time. The subsequent reads
to the TMRCCR3 and TMRCCR4 registers do not latch new count values,
allowing the count of all timers at the same instance in time to be obtained.
Table 3-3 Selectable Clock Source for Timers
Clock Ratio
MSb
LSb
2 MHz
0
0
1 MHz
0
1
500 kHz
1
0
250 kHz
1
1
Each timer can be independently enabled by writing a “1” to the appropriate
“Timer x Enable” field. Similarly, the generation of interrupts by each timer can be
independently enabled by writing a “1” to the appropriate “Timer x IRQ Enable”
field.
If an interrupt is generated by a timer, the source of the interrupt may be
determined by reading the “Timer x Caused IRQ” fields. If the field is set to “1”,
then the respective timer caused the interrupt. Note that multiple timers can
cause a single interrupt. Therefore, the status of all timers must be read to ensure
that all interrupt sources are recognized.
A particular timer interrupt can be cleared by writing a “0” to the appropriate
“Timer x Caused IRQ” field. Alternately, a write to the appropriate Timer x IRQ
Clear (TxIC) register will also clear the interrupt. When clearing the interrupt
using the “Timer x Caused IRQ” fields, note that it is very important to ensure
that a proper bit mask is used so that other register settings are not affected. The
preferred method for clearing interrupts is to use the “
Timer x IRQ Clear”
3.3.2 Timer Control Status Register 2 (TCSR2)
The timers are also controlled by bits in the Timer Control Status Register 2
(TCSR2) located at offset 0x04 from the address in BAR2. The mapping of the bits
in this register is shown in
Table 3-4 TCSR2 Bit Mapping
Field
Bits
Read or Write
Read Latch Select
TCSR2[0]
R/W
Reserved
All Other Bits
R/W
All of these bits default to “0” after system reset.
