The CN0577 provides an analog front-end and an FMC digital interface for LTC2387-18, its core. It is a low noise, high speed successive approximation register (SAR) ADC with a resolution of 18 bits and sampling rate up to 15MSPS.
CN0577 includes an on-board reference oscillator and a retiming circuit to minimize signal-to-noise ratio (SNR) degradation due to the FPGA additive jitter.
In order to support high speed operations while minimizing the number of data lines, a serial LVDS digital interface is used. It has a one-lane and two-lane output modes, allowing the user to optimize the interface data rate for each application, through setting a parameter.
More details about ADI reference designs architecture here.
The PD and TESTPAT parameters are tied to GPIOs (33 and 32 respectively) so they can be configured by the software.
The conversion starts when a rising edge is detected on the CNV signal. This signal is generated by the AXI_PWM_GEN core, alongside with a clk_gate signal.
The only parameter that can be changed so far, is
TWOLANES. It specifies whether the two-lane output mode is activated or not. When activated, the ADC outputs two bits at the same time, on DA+/DA- and DB+/DB-. When it is low, then DB+/DB- is disabled.
When developing the Linux software parts for an HDL project, the interrupts number to the PS have a different number in the software side. More details here.
|Interr. name||HDL interr.||Linux Zynq||Actual Zynq|
PS7 EMIO offset = 54
|GPIO HDL name||GPIO nb.||HDL GPIO nb.|
| || ||LVDS clock input|
|LVDS ADC interface|
| || ||LVDS output clock going to the chip, to serially shift out the conversion result|
| || ||LVDS data clock input|
| || ||Serial LVDS data input|
| || ||Serial LVDS data input|
| || ||Conversion start LVDS|
| || ||Conversion enable, used as clear signal for the retiming flop from the chip (see schematic)|
| || ||Powerdown control signal|
| || ||Enable for testing with a predefined pattern|
| || ||Selection between one-lane mode and two-lane mode. Default it is 1|
The clock architecture of the CN0577 is designed with careful consideration to ensure low jitter and low phase noise.
An on-board 120 MHz voltage controlled crystal oscillator (VCXO) is used to provide the clock for the CN0577 board and the FPGA. It is further named as
reference clock. This clock is gated and fed back to the device as the
sampling clock, on which the data was sampled.
The DMA runs on the ZynqPS clock FCLK_CLK0 which has a frequency of 100MHz.
The digital interface consists of the following signals:
ref_clk_*- LVDS clock input, used as reference clock for the system, after it is converted to single-ended signal by ad_data_clk module
clk_*- LVDS clock coming from the FPGA, that serially shifts out the conversion result
dco_*- LVDS clock input
da_*- serial LVDS data input
db_*- serial LVDS data input
cnv_*- conversion start LVDS; a rising edge starts the conversion cycle
cnv_en- signal that is used as CLR_N for the retiming flop from inside the CN0577 board
pd_cntrl- exposed on GPIO 33
testpat_cntrl- exposed on GPIO 32
twolanes_cntrl- hardcoded to 1 to always be used in two-lane mode
The timing considerations specified in the datasheet of the LTC2387-18 were taken in consideration, and are implemented in the constraints file of the project.
What must be noted, is that the
cnv_en signal must be constrained in order to meet timing requirements for tCYC, tCONV, based on tFIRSTCLK and tLASTCLK.
cnv_en being a CLR_N of the retiming flop, forces us to simulate the clock of the retiming flop.
In other words, a virtual clock is created, being a shadow of
ref_clk_* (LVDS_CLK in schematic), and because it comes from the output of the local oscillator, it needs to be shifted with the propagation delay that is given from passing the level translator. Thus, it is shifted with 0.225ns compared to the ref_clk_* signal.
Depending on what configuration of pins is chosen on the jumpers P1, P2 and P3, the device can act in different modes, as described below.
Of course, the PD jumper overrides the PD signal from the FPGA. It is controlled by a one-bit-adc-dac, in software.
The FMC connector connects to the LPC connector of the carrier board.