Special Registers
VL-EPIC-25 Reference Manual
64
The 32-bit cascade mode is set in TM4MODE in the Timer Control Register. There are also
internal or external clock selections for the timers in this register using the external clocks ICTC3
and ICTC4 signals on the connector at J25. The internal clock is the PCI clock divided by 8
(33.33 MHz / 8 = 4.167 MHz). ICTC3 can only be used with Timer 3. ICTC4 can only be used
with Timer 4. The clock for Timer 5 is always the internal clock except in the 32-bit cascade
mode when the output from Timer 4 is the clock for Timer 5.
The timer outputs can generate interrupts. When a timer output transitions from a 0 to a 1 then an
interrupt status bit is set and can generate an interrupt. This bit sticks until cleared.
By default there are two external timer input clocks (ICTC3, ICTC4) and two timer outputs
(OCTC3, OCTC4) on connector J25. To use all three of the 16-bit timers, timers 4 and 5 are
configured in 32-bit mode by default. Custom options are available that can expand the external
controls to allow for three clock inputs and four, timer outputs as well as three timer gate inputs
for all three 16-bit timers by using some of the digital I/O signal pins on J25.
A/D and D/A Control/Status Register
This register is used to control A/D and D/A conversion.
ADCONSTAT (Read/Write) CAFh
D7
D6
D5
D4
D3
D2
D1
D0
Reserved
Reserved
Reserved
DACLDA0
Reserved
ADCBUSY0
Reserved
ADCONVST0
Table 32: A/D, D/A Control/Status Register Bit Assignments
Bit
Mnemonic
Description
D7-D6
Reserved
These bits are reserved. Only write 0 to these bits and ignore all read values.
D5
Reserved
This bit is reserved. Only write 0 to this bit and ignore read values.
D4
DACLDA0
This is a write-
only (pulsed) bit. When a ‘1’ is written it will strobe the LDAC signal
on the LTC2634 D/A Converter. Writing a ‘0’ is ignored.
D3
Reserved
This bit is reserved. Only write 0 to this bit and ignore read values.
D2
ADCBUSY0
This read-only status bit returns the A/D conversion status.
0
– A/D is not busy doing a conversion.
1
– A/D is busy doing a conversion.
D1
Reserved
This bit is reserved. Only write 0 to this bit and ignore read values.
D0
ADCONVST0
This is a write-
only (pulsed) bit. When a ‘1’ is written it will start a conversion on
the
LTC1857 A/D converter. Writing a ‘0’ is ignored.
