Achieving Fast SENT Message Response with A1346 Linear Hall-Effect Sensor ICs on Shared Bus
Achieving Fast SENT Message Response with A1346 Linear Hall-Effect Sensor ICs on Shared Bus
By Braden Blanchette和Nevenka Kozomora，
Allegro MicroSystems, LLC
Introduction
在安全关键磁感测应用中，例如电子动力转向（EPS）模块中的扭矩传感，冗余亚博尊贵会员传感器通常是用于在保持严格的安全要求的同时实现所需性能的方法。双芯线性磁传感器IC，如Allegro A1346, package two sensors in a single, low-profile surface-mount device to save room on PCB designs. Along with the need for redundancy in the magnetic system and the desire for simple wiring harnesses with few wires is the requirement of a busable output protocol.
建立在明确的和广泛使用的Single Edge Nibble Transmission (SENT) protocol, Allegro has developed the proprietary Sequential SENT (SSENT) and Addressable SENT (ASENT) options. Each of these options allow up to four devices to share the output line. This can be any four devices that also use the SSENT and/or ASENT protocols, such as Allegro angle sensor ICs, set to the same configuration. The option to bus the outputs together has the limitation of how quickly magnetic information and diagnostic information critical to achieve required safety ratings can be received. In this application note, the extended data nibbles are not considered, as these add the same duration to every SENT protocol and therefore do not affect the determination of the fastest SENT protocol.
Sequential SENT (SSENT)
The SSENT protocol used in the A1346 is split into two options, short trigger mode (SSENT Short) and long trigger mode (SSENT Long). Both SSENT options require polling the sensors in sequential order, as seen in Figure 1. The difference between these two options is how it handles function pulses, or triggers, of certain lengths. With both protocols, the host controller must pull the output line low for a set length of time to tell the sensor IC to either send the magnetic data, perform a diagnostic test, latch magnetic data, or resynchronize its counter. SSENT Long mode allows for a larger range of F_OUTPUT pulse (SENT trigger) lengths to allow for SPC compatibility. This allows a shorter trigger to be used to request the SENT message, resulting in a faster total message time.
Table 1: SSENT Function Pulses in Short Trigger Mode, outmsg_mode = 4
| 功能脉冲 | 最小滴答数 | 最大刻度数 | Function |
| F_OUTPUT | 15. | 19. | 触发了寄送在设备上的消息输出。 |
| f_sample. | 31. | 39. | 触发传感器以锁存磁数据，但不会发送已发送的消息。 |
| F_DIAG. | 56. | 70 | 触发设备自我诊断。 |
| f_sync. | 93. | 115. | Resets the Sensor counter to count 0. |
表2：SSENT功能脉冲长触发模式，OUTMSG_MODE = 3
| 功能脉冲 | 最小滴答数 | 最大刻度数 | Function |
| F_OUTPUT | 9. | 81 | 触发了寄送在设备上的消息输出。 |
| .f_sync. | 105 | 171 | Resets the Sensor counter to count 0. |
| F_DIAG. | 216 | 264. | 触发设备自我诊断。 |
SSENT选项高度可配置，以适应各种系统要求。一个可用的选项是插槽标记。此选项在发送同步脉冲开始之前添加特定的长度高周延迟，其长度由响应的传感器的传感器ID确定。这通常用于通过允许传感器基于所看到的插槽标记脉冲重新同步其计数器来向协议添加鲁棒性，允许传感器在没有主控制器干预的情况下在线返回。看A1346数据表想要查询更多的信息。
虽然插槽标记选项可以提供输出线的稳健性，但在传感器输出断开的情况下，Allegro不建议使用此选项的折叠时间小于或等于1.22μs，因为与传感器ID相关的延迟时间对传感器来说可能太短，以处理并提供新的样本。当希望在系统允许时快速生成已发送的输出时，插槽标记会导致刻度记时期限制。因此，在此应用程序注意中不考虑使用槽标记的选项。
可寻址（即）
Unlike the SSENT protocol, where the sensors must be polled in sequential order, the ASENT protocol in the A1346 allows for random polling of the sensors. This makes the protocol more robust for when the sensors handle going offline and allows for more flexibility in sensor sampling. This robust nature of the protocol comes at a cost of additional message length. For it to be clear which sensor is being polled, the function pulse is accompanied by an addressing pulse consisting of a 7 tick high period followed by the 5 tick low period. This is incremented for each sensor address, with sensor 1 (address 0) having none of these pulses, sensor 2 (address 1) having one of these pulses, etc., as shown in Figure 2. This can add up to 36 additional ticks to the message length for the sensor with address 3, resulting in a tradeoff between the robust data bus and possible message polling rate.
Table 3: ASENT Function Pulses, outmsg_mode = 5-7
| Function Pulse |
最低限度 Number of Ticks |
最大 Number of Ticks |
Function |
| F_OUTPUT | 15. | 19. | 触发了寄送 在设备上的消息 输出 |
| f_sample. | 31. | 39. | Triggers sensors to latch magnetic data, but not send the SENT message |
| F_DIAG. | 56. | 70 | Triggers device self 诊断 |
最快消息速率的输出模式
从图3和图4可以看出，输出协议具有最短的总消息传输时间是SSEN长选项。使用OutMSG_Mode = 3（地址0x14位[2：0]）实现此选项。此模式的最短可能的触发长度为9个刻度，允许它达到比SSENT短模式更快的数据速率，这需要15个刻度最小触发。该模式也与可能需要使用更长触发器的控制器兼容，使其成为磁数据传输的理想选择。
结论
In safety-critical applications, such as torque sensing in EPS applications, redundancy of devices is often not only desirable, but required. To save space and cost in the harness connecting the sensors to the host controller, having the different sensors connected to a single output bus is becoming increasingly popular. To maximize performance, the magnetic data should be polled as fast as possible to allow for external processing. The three proprietary, busable output modes on the A1346, SSENT Short, SSENT Long, and ASENT, each have their strengths and weaknesses in balancing speed, complexity, and sensor communication robustness.
当主要问题是如何快速接收磁数据时，A1346输出协议都是专有的快速发送刻度线，这大大减少了标准发送协议的总消息时间。由于不需要寻址脉冲，在长触发或短触发模式下的SUSENT将始终导致这些专有输出模式之间的最短时间的磁数据。在这两种模式中，SUSENT长期产生最短的最小触发时间，为每个传感器节省6个刻度。使用A1346中可用的专有快速发送选项使传感器能够在单个输出线上以高频率向电子控制单元提供从多个传感器的精确磁数据，从而实现高性能，以及低系统成本和复杂性。