Proton Calorimetry/Equipment/FPGAs DDC232

Useful Material

Technical Documents

• DDC232 datasheet. This contains essential information from which the FPGA design was created.
• Zybo Z7 manual.

Presentations

• I/O signals and operation of TI DDC232CK based on datasheet information. Slides available here.
• Results of simulation of FPGA design discussed here.
• UART interface between FPGA and PC discussed in this presentation.
• First operational tests discussed in this presentation.
• DDC232 design features and demo given in this presentation.

Operation

DDC232

A Texas Instruments (Dallas, Texas, United States) DDC232CK (DDC) was chosen as the current-input analogue-to-digital converter for its speed, large dynamic range and low power requirements. It is capable of measuring the currents of up to 32 photodiodes with an adjustable integration time (160μs–1s) and full-scale range (FSR, 12pC–350pC). Each of the 32 inputs on the DDC has two integrators, allowing for zero-deadtime measurements: while one integrator digitises and transfers data, the other measures the input current. The DDC is housed on a compact custom circuit board manufactured by CosyLab (Ljubljana, Slovenia), where the charge collected by a photodiode is split across two DDC inputs to give 16 photodiodes per DDC. The role of each digital signal is described below:

• CLK (input): 10 MHz clocking signal that is used to time the internal operations of the DDC, including generation of DVALID.
• CONV (input): signal that controls integration, the time period of which is equal to the integration time. When this signal toggles, the integrator of each input switches.
• DIN_CFG (input): serial data stream of the 12-bit sequence used to set key parameters of the DDC, namely the FSR and measurement precision (16-bit or 20-bit).
• CLK_CFG (input): 20 MHz clocking signal used to time sending and reading of DIN_CFG.
• RESET (input): Asynchronous active-low reset signal for the DDC to revert it to its power-up state.
• DCLK (input): 20 MHz clocking signal used to time sending and reading of DOUT.
• DVALID (output): active-low signal used to indicate that data is ready to be read on DOUT.
• DOUT (output): serial data stream of the 640-bit sequence (when in 20-bit precision mode) containing measurements of the 32 inputs.
• DIN (input): serial data input to the DDC used to daisy-chain other DDCs.

Zybo Z7-10

The FPGA used to communicate with the DDC is a Xilinx (San Jose, California, United States) Zynq- 7000 on a Digilent (Pullman, Washington, United States) Zybo Z7-10 development board, which features a built-in ARM Cortex-A9 processor and a host of peripheral connections. The FPGA design was written in VHDL using Xilinx Vivado Design Suite 2020. Further details of the design-assigned purpose of the on-board buttons, switches and LEDs are provided below:

• Clocking Wizard Status LED: the 125 MHz FPGA master clock is converted to a 120 MHz clock using the Xilinx Clocking Wizard intellectual property (IP), to allow for easier generation of the 10 and 20 MHz signals required by the DDC. This LED is on when the Clocking Wizard component of the design is generating a stable clock. All FPGA operations are timed using this clock.
• Test-mode LED: the DDC can be configured into a test diagnostic mode for debugging, in which all the inputs give a zero signal (slightly above zero due to noise and a negative current offset). This LED is on when the user has enabled the test mode, which can be toggled using the Toggle Test-Mode button and becomes active after a Global Reset press.
• State LED 1: this LED is on whenever the DDC in the power-up, idle, or configuration state.
• State LED 2: this LED is on whenever the DDC is measuring or shifting out data.
• Acquisition mode: this switch toggles between the continuous and triggered acquisition modes. In continuous mode, data will be output to the PC as fast as possible, the rate of which is dependent on the chosen integration time and the speed of the data transfer. When in triggered mode, the next measurement after the Trigger Acquisition button is pressed is sent to the PC.
• FSR Switches: these 3 switches control the 3-bit FSR code, which allows the user to choose between the 8 different dynamic ranges.
• Global Reset: this button resets all aspects of the FPGA and DDC. Any changes since the last reset in the FSR, acquisition mode or test-mode settings are applied. The DDC returns to its power-up state and reconfigures before resuming operation.
• Pause Operation: this button idles the FPGA and DDC, all signals are held at their default value apart from the 120 MHz master clock. Pressing Global Reset restarts operation.
• RGB LED: this LED will be green if the DDC is configured correctly, red if configuration is incorrect or if too short an integration time is chosen, or blue if the first-in first-out (FIFO) interface is read when empty or is written to when full.

Configuration

The DDC must first be configured with a 12-bit sequence of data sent on DIN_CFG, where:

• Bits 11-9 correspond to the 3 FSR bits, allowing for 8 different dynamic ranges; 000 = 12.5 pC, 001 = 50 pC, 010 = 100 pC, 011 = 150 pC, 100 = 200 pC, 101 = 250 pC, 110 = 300 pC, 111 = 350 pC. These correspond to the maximum charge that can be integrated in the photodiodes. 350 pC is typically chosen to minimise risk of saturation of inputs.
• Bit 8 corresponds to the resolution of output data: 0 = 16-bits, 1 = 20-bits. 20-bit resolution is chosen for better precision, at the price of slower readout (due to more bits of data requiring shifting out).
• Bit 7 corresponds to the device version. For the DDC232CK, this bit is set to 1.
• Bit 6 corresponds to a DDC internal divider of the CLK signal. This is set to 0 for no division.
• Bits 5-1 are empty bits set to 0.
• Bit 0 corresponds to the diagnostic test mode setting: 1 = on, 0 = off.

This gives a configuration input for normal operation of: 111110000000. Note that all vectors of data in this design are “little-endian”, in which the bits are labelled from the most-significant bit (MSB) to the least-significant bit (LSB) and are sent to and from devices MSB first. For the configuration input, this means bit 11 is sent first and bit 0 is sent last. After the configuration data is sent, a 640-bit (when in 20-bit mode, which is equal to the number of bits sent during a measurement readout cycle) read-back is sent to confirm configuration settings and test data output. The 640-bits are a 320-bit sequence sent twice, which contains the 12 configuration bits that the DDC received, a 4-bit revision ID (0001), 244 zeros and then a 70-bit test pattern, used as an extra check. In hexadecimal, the test pattern is: 30F066012480F69055.

After the DDC is powered up and power supplies have stabilised, a reset pulse of width t_RST = 1 μs must be sent and after t_WTRST = 2 μs, the configuration data is sent through DIN_CFG on the rising edges of CLK_CFG, to be read by the DDC on the falling edges of CLK_CFG. t_STCF and t_HDCF (both 10 ns) are the minimum times required for DIN_CFG to be valid before and after falling edges of CLK CFG respectively. t_WTWR = 2 μs later, configuration read-back begins on DOUT where the pattern is read on the rising edges of DCLK, after which CONV is strobed (i.e. high for one CLK cycle) to begin integration.

Integration

After CONV is strobed at the end of the configuration read-back, it begins alternating with a time period t_INT, which is the integration time set by the user. Toggles in CONV represent transitions in the dual-integrators, where one side completes integration and begins measurement, reset and auto-zeroing while the other side, currently idling after having finished these three tasks, begins integration. Measurement, reset and auto-zeroing take t_MRAZ = 1612 ± 2 CLK cycles and DVALID is asserted after t_CMDR = 1382 ± 2 CLK cycles, signalling that data ready to be shifted out on DOUT.

Data Transfer

Once DVALID is asserted, photodiode data can be shifted out to the FPGA on the rising edges of DCLK. Each of the 32 DDC inputs is represented by a 20-bit number, which can then be converted to a charge in pC. The simplest method of data retrieval is after DVALID goes low and before the next CONV toggle. A key requirement for the DDC to be operated in continuous mode with this method, i.e. with zero dead-time between measurements (not to be confused with the FPGA continuous/triggered acquisition settings), is that the integration time must be sufficient to complete measurement, reset, auto- zero and shifting out of data before the next CONV toggle. Otherwise, the DDC enters a non-continuous mode where integration stops until the aforementioned processes have finished on both input integrators.

Regardless of whether the FPGA is in continuous or triggered acquisition modes, each measurement sent by the DDC is read, though they are not always sent to the PC (see below). The FPGA has a 640-bit register that is overwritten every time new data arrives. Before being sent to the PC, a measurement is first sent to a FIFO interface on the FPGA. The FIFO was added to the design using the Xilinx FIFO Generator and has the following signals:

• WR_EN: FIFO write enable. When high, a byte is written to the FIFO on clock rising edges.
• RD_EN: FIFO read enable. When high, the oldest byte in the FIFO is read on clock rising edges.
• FULL: High when the FIFO is full.
• EMPTY: High when the FIFO is empty.
• WR ACK: High when a byte has been written successfully.
• VALID: High when a byte has been read successfully.
• OVERFLOW : High if FIFO is written to when full and causes the Zybo RGB LED to turn blue.
• UNDERFLOW: High if FIFO is read when empty and causes the Zybo RGB LED to turn blue.

A 640-bit measurement received by the FPGA is written to the FIFO in byte-sized pieces, such that the FPGA is quickly ready again to read new data. The measurement is then sent from the FIFO to the PC over the universal asynchronous receiver-transmitter (UART) protocol, which sends data a byte at a time (hence the storage of a measurement in bytes in the FIFO). The principle of the UART protocol is shown in Fig. 16 and the key user settings are:

• Baud rate: the number of bits transferred per second (including start and stop bits). This is set to 921600, which is often the maximum standard baud rate for serial ports.
• Data bits: the number of bits of data sent in a packet. This is set to the maximum number, 8.
• Stop bit: the number of bits at the end of the data bits to denote the end of data. This is set to the minimum, 1.

Key signals for the UART transmitter used to send data from the FPGA to the PC: • TX_DV: Data valid signal sent to the UART transmitter to state that data is ready for transmission.cFunctionally, this serves the same purpose as the FIFO VALID signal. • TX_ACTIVE: High when the UART transmitter is sending data to the PC. • TX_DONE: High for one clock cycle after a byte is sent. • TX_SERIAL: Actual data stream sent to the PC.

Note that a 32-bit custom end-of-line sequence is added to the end of each measurement, used to separate measurements when saved to a text file on a PC. Since each byte sent over UART contains a start and stop bit, there are actually 640 + 32 + 84(2) = 840 bits to be sent over UART for the (640+32)/8 = 84 bytes of data saved in the FIFO per measurement.