Sundance SMT148 - Manual (Page 28)

Version 1.4 Page 28 of 35 SMT148 User Manual
DAC
The DAC interface is only an output node to the switch fabric and it interfaces to the
DAC device on the carrier card. All eight channels are controlled and updated by this
interface. The DAC interface is a slave and will disconnect itself from the input node
only if it receives the instruction to do so.
To initialize a transfer or disconnect itself from the input node the DAC interface
expects to receive the following 32-bit word (4 consecutive bytes, LSB first):
Bit 31: new transfer initialization if 1.
Bit 30: disconnect from input node if 1, will result in a DONE pulse sent to the switch
fabric.
Bit 15 down to 8: sampling ratio. The DAC devices are converting data from digital to
analogue at a fixed rate of 200 KHz. However the user can reduce this converting
rate by a factor determined by the sampling ratio value. Example: if sampling ratio =
8, the new conversion rate is 200KHz/8=25KHz. Sampling ratio=0 Ù Sampling
ratio=1.
Bit 7 down to 0: Channels enable, 1 on bit X indicates that channel X will be used, 0
that it will not.
If the DAC interface received a new transfer initialization command, the next words
will be interpreted as samples. One sample is coded on 4 bytes, LSB received first.
Bit 31: must be 0 (otherwise interpreted as initialization)
Bit 30: must be 0 (otherwise interpreted as disconnection)
Bit 23 down to 16: channel to which the sample belongs, 1 on bit X indicates that the
sample belongs to channel X-16. All the other bits must be set to 0.
Bit 15 down to 0: sample value
The DAC interface communicates to the DAC device on board using the following
signals. Please refer to the
DAC7742 datasheet for more details.
RnW output Read, not Write
nCS output 8-bit chip select, active low, one nCS per channel
nLDAC output Load DAC register to output, active low
RSTSEL output Tied to 1, output is half-scale code when DAC reset
nRST output Reset, active low
nREFEN output Enables internal +10V reference, active low
Data_to_dac output 16-bit data bus
Table 20 : DAC interface extra IO