What better way to start the New Year than with a new Adam Taylor MicroZed Chronicles blog? – The Editor
Following on from the popularity of my final blog of last year where I presented several tips for better image-processing systems, I thought I would kick off the 2018 series of blogs by providing a number of tips for using the XADC and Sysmon in Zynq SoCs and Zynq UltraScale+ MPSoCs.
Whether our targeted device uses a XADC or Sysmon depends upon the device family. If we are targeting a 7 series FPGA or Zynq SoC device, we will be using the XADC. If the target is an UltraScale FPGA, UltraScale+ FPGA, or a Zynq UltraScale+ MPSoC, we’ll be using a Sysmon block. Behaviorally, the on-chip XADC and Sysmon blocks are very similar but there are some minor differences in architecture and maximum sampling rates between the two. Including the XADC or Sysmon adds a very interesting analog/mixed-signal capability to your design and helps reduce the number of external components. Because they can monitor internal device parameters along with external signals, you can also use the XADC/Sysmon blocks to implement a comprehensive system health and security monitoring solution critical for many applications.
Here are some of my favourite tips for using the Xilinx XADC/Sysmon blocks:
Remember Nyquist’s sampling theorem and configure sample clocks correctly
To prevent signal aliasing, you must set the XADC/Sysmon sampling rate to at least twice the frequency of the signal being quantized. When sampling external signals, the XADC and Sysmon have different maximum sampling frequencies of 1000Ksamples/sec and 200Ksamples/sec respectively. To set the appropriate sampling frequency, we need to consider the relationship between the clock provided to the XADC/Sysmon (called DClock) and the resultant internally derived clock used for sampling (called ADC Clock). Both the XADC and Sysmon take a minimum of 26 internal ADC Clock cycles to perform a conversion. To achieve the maximum conversion rate of 1000KSPS for the XADC, we therefore need to set the ADC Clock at 26 MHz. For the Sysmon block, we need to set the ADC Clock to 5.2MHz to achieve the full 200ksamples/sec sample rate. ADC clock frequencies below these will result in lower sampling rates. Correctly setting the sampling rate depends upon the device you are using and the access method:
Zynq PS APB Access – The XADC is clocked using the PCAP_2X clock which has a nominal frequency of 200MHz. This is divided further internally within the DevC to generate a DClock for the XADC, which has a maximum frequency of 50MHz using the XADCIF_CFG[TCKRATE] register settings. This clock is then further divided down (minimum division by 2) using the XDAC configuration register to select the desired conversion rate.
Zynq MPSoC APB Access – The PS and PL Sysmon are clocked by the AMS clock with a range 0 to 52 MHz. This is then divided further by the Sysmon to create the ADC Clock used for sampling. This further division is a minimum of 2 for the PS Sysmon or 8 for the PL Sysmon using the Sysmon configuration clock division registers PS/PL CONFIG_REG2.
AXI Access using FPGA, Zynq SoC, or Zynq UltraScale+ MPSoC (PL Sysmon) – Use the system management wizard or XADC wizard configuration to determine the AXI clock frequency and hence the sampling clock via the IP customization tab. The required sample clock frequency can then be supplied by either a fabric clock (Zynq SoC or Zynq UltraScale+ MPSoC) or clock wizard (FPGA, Zynq SoC, or Zynq UltraScale+ MPSoC).
Configure the Analog Inputs Correctly
The analog inputs are defined by IP Integrator or software to be either unipolar or bipolar and you can control the input configuration for each analog input individually. When a unipolar signal is quantized, the input signal can range between 0V and 1V. For a bipolar input, the differential voltage between the Vp and Vn inputs is ±0.5V. Selecting the right mode ensures the best performance and avoids damaging the analog inputs. For unipolar configurations, Vp cannot be negative with respect to Vn. For bipolar inputs, Vp and Vn can swing positive and negative with respect the common mode (reference) voltage. Bipolar mode provides better noise performance because any common-mode noise coupled onto the Vp and Vn signals will be removed thanks to differential sampling.
When it comes to providing better performance in electrically noisy environments, you can also turn on input-channel averaging to average out the noise.
Leverage the External Multiplexer Capabilities
Both the XADC and the Sysmon can accept as many as seventeen external differential analog signals using one dedicated Vp/Vn pair and sixteen Auxiliary Vp/Vn pins. Doing so of course uses several I/O signal pins—as many as 34 I/O pins if all analog inputs are used. This may present issues, especially on smaller devices where I/O-pin availability might be tightly constrained so the XADC/Sysmon can drive an external multiplexer that reduces the number of pins required and also allows you to use and external mux with added protection for harsh operating environments (e.g. ESD protection).
Consider the Anti-Aliasing Filter effect on Conversion Performance
Implementing an Anti-Aliasing filter on the front end of the XADC/Sysmon external inputs is critical to ensuring that only the signals we want are quantized.
The external resistor and capacitors in the AAF will increase the overall settling time. Therefore, we need to ensure the external AAF also does not adversely affect the total settling time and consequently the conversion performance. Failing to provide adequate system-level settling time can result in ADC measurement errors because the sampling capacitor will not charge to its final value.
Both the XADC and Sysmon can monitor internal power supply voltages and temperatures. This is a great feature when we initially commission the boards because we can verify that the power supplies are delivering the expected voltages. We can even use the temperature sensor to verify thermal calculations at the high and low end of qualification environments.
When it comes to creating the run-time application you should use the temperature and voltage alarms, which are based on defined thresholds for core voltages and device temperature. Should the measured parameter fall outside of these defined thresholds, an alarm allows further action to be taken. Configured correctly this alarm capability can be used to generate an interrupt which alerts the processing system to a problem. Depending upon which alarm which has been raised, the system can then act to either protect itself or undertake graceful degradation, thus preventing sudden failure.
Hopefully these tips will enable you to create smoother XADC/Sysmon solutions. If you experience any issues, I have a page on my website that links to all previous XADC / Sysmon examples in this series.