SIS Documentation
SIS3820
VME Scaler
Page 65 of 79
15.2 Read on the fly
The SIS3820 supports read on the fly (i.e. readout in parallel to the acquisition of new counts)
with an accuracy down to the lowest bit at the full counting rate of 200 MHz. While the
uncertainty on a read on the fly is one count no counts are lost. A read from a counter register
of the SIS3820 initiates a clock shadow transaction and the actual counter value that is read
from VME is taken from the shadow register. The counter values of all 32 scaler channels are
latched to the shadow registers simultaneously when the read on the fly is done in BLT32.
Minimum difference in time (i.e. less than 5 ns variation) of the read values can be achieved
with this mechanism.
15.3 Latching scaler
Counter data can be copied to the shadow registers in three ways
•
Key VME LNE/clock shadow
•
external next pulse LNE/clock shadow with input modes 1, 2 and 3
•
read from the counter registers
Shadow register data are not altered/updated in the normal counting process, i.e. the latched
values can be re/read until they are overwritten by the next clock shadow cycle.
15.4 Preset Scaling
In preset mode the counting mode is started by an enable command and stopped by a channel
reaching the preset value of counts. The preset reached state can be used as interrupt source.
As an alternative you can poll on the preset reached bit in the preset enable and hit register to
check on the occurrence of the preset reached condition.. Preset scaling can be used as LNE
source in multi channel scaling (channel N mode) also.
It will take in the order of 100 ns after the preset condition was detected before the counters
will stop counting. This implies, that the actual stored counter value will be greater than the
preset value for frequencies in excess of some 10 MHz.