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


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。

Figure 2: Addressable SENT Output Bus
Figure 2: Addressable SENT Output Bus




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

Number of Ticks
Number of Ticks
F_OUTPUT 15. 19. 触发了寄送
f_sample. 31. 39. Triggers sensors to
latch magnetic data,
but not send the SENT
F_DIAG. 56. 70 Triggers device self


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。



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.