SLASEC8B February 2017 – January 2019 PGA460-Q1
The run commands are used for device run-time operation and are most commonly used during the normal operation cycle of the PGA460-Q1 device. These device commands are specified by pulling the IO pull low for a specified period of time as defined in the Specifications section. The following are classified as run commands:
Figure 14 shows the process of the communication of the IO pin run command.
The status field of the PGA460-Q1 device is embedded in the run command and provided back to the master controller by extending the dead-time on the IO bus. The dead-time can be further extended for up to 3 × t(DT_TCI) which signifies three status bits, STAT[1:3]. Each status bit has an assigned diagnostic and a priority as listed in Table 1.
|STAT 1||1, low||Threshold settings uninitialized error|
|STAT 2||2||Frequency diagnostics error|
|Voltage diagnostic error|
|STAT 3||3, high||Power-up auto EEPROM CRC error|
|User triggered EEPROM download CRC error|
As listed in Table 1, the STAT3 bit has the highest priority. When a STAT3 error condition is present, then the dead-time is further extended by 3 × t(DT_TCI). In this case, if any STAT2 or STAT1 error conditions are also present, these conditions are overruled by the higher priority of STAT3 error conditions. In a similar way, a STAT1 condition is overruled by a STAT2 error condition in which case the dead-time is further extended by 2 × t(DT_TCI). When all STAT3 and STAT2 error conditions have cleared, a STAT1 condition further extends the dead-time by an additional t(DT_TCI).
The functions of the status bits can be explained as follows:
The user can write to any EEPROM-mapped register to clear the error.
The user must reprogram the EEPROM to prevent the error upon another automatically or manually triggered EEPROM download operation.
When the device receives a run command, the IO pin is actively driven by the PGA460-Q1 device depending on the final DSP output to indicate object detection. If, at any time, the processed echo signal exceeds the threshold at that time, the IO pin is pulled low (GND, strong pulldown) otherwise the IO pin is pulled up by the internal 10-kΩ (weak pullup) resistor. When the record time reaches the end of the record defined by the Px_REC parameters, the IO pin is released (pulled up as an input) and the device is ready for a next command. Figure 15 shows the object detection functionality of the IO pin. The device pulls the IO pin low during the burst and then releases it to provide a reference for the recording time-frame for the MCU. Knowing the time of reference, the duration of the programmed burst and following falling edges for each object detected, the master controller or MCU can calculate the object distance.
The PGA460-Q1 device forces IO pin to go low after the t(DT_TCI) time passes after receiving a run command for a minimum of 300 µs which indicates start of the record period. This process occurs to provide the master controller a reference edge to start the time of flight measurement and also for PGA460-Q1 device to separate the response of the status (STAT) bits from the record cycle information. In general, the duration of burst for lower frequency range followed by ringing causes the AFE to saturate and pull the IO pin low for more than 300 µs. For higher frequency burst or for listen-only command, or in situations where the saturation caused by the ultrasonic burst might not be a higher value than the assigned threshold, as shown in Figure 16, the minimum pulse width is 300 µs. With a certain filter and deglitch setting, a fake object can be detected directly after this 300-µs period.