In the bus communication, the MCU in the bus device needs to connect to a bus transceiver to access the bus network. What happens when the MCU's supply voltage does not match the transceiver voltage? This article will be the importance of matching the level of the level as an example of the interface level as an example.
First, CMOS level
Most of the digital integrated circuits use a CMOS process, and the level of the interface is generally in line with the following definition:
VIL
Vol
Take the common 5V, 3.3V system as an example, the corresponding interface parameters are shown in Table 1.
Table 1 Leading requirements under different supply
Note: The data in the table is only the calculation reference value, and the actual parameters of the device need to refer to the corresponding data sheet.
Second, the level does not match
In order to ensure the reliable transmission of the signals of the two devices, it must be guaranteed:
The VOH (MIN) of the drive output must be higher than the VIH (min) input by the receiver;
The VOL (MAX) of the drive output must be lower than the VIL (MAX) input by the receiver;
The output voltage of the drive output must not exceed the I / O voltage tolerance input by the receiver;
When two CMOS devices are connected together, if the supply voltage is consistent, there is no problem with signal transmission. If the supply voltage of the two devices is inconsistent, there is a level of no matching.
Taking 3.3V devices and 5V device connections as an example, the following two issues occur:
5V device input pin may not be able to identify high levels of 3.3V devices output
As shown in Figure 1, 3.3V device output VOH maximum 3.3V can also reach the minimum of 5V device VIH 3.5V, and cannot guarantee that the high level of 3.3V device output is correctly identified. Since the device design has a certain amount, it may still work normally during the test, but there is a risk. If there is a voltage fluctuation, there will be problems.
Figure 1 3.3V device output, 5V signal input
The 5V device output high may damage the 3.3V device input interface.
As shown in Figure 2, 5V device output high level signal is much higher than 3.3 V, if the 3.3V device input pin does not support 5V level input, there is a current to enter 3.3V device, which is severely damaged.
Figure 2 5V signal output, 3.3V signal input
Third, isolation transceiver selection
Taking the CTM1051 (a) M series product as an example, the chip of the CMOS technology used inside, is in accordance with Figure 3, which conforms to the CMOS level standard. When selecting, the corresponding model should be selected for different MCUs to eliminate the problem of failure to match the level. If the MCU is 5V, CTM1051M should be selected; if the MCU is 3.3V, select CTM 1051AM.
Figure 3 CTM1051 (a) M pin level
Fourth, actual case
The customer is used by our Isolated CAN transceiver module, which has been shipped in large quantities, but individual exceptions occur in applications. Abnormal products are manifested as intermittent communication failures in CAN bus. When the product is in a high temperature environment (such as 65 ° C), it can be repeatedly powered up, and the communication failure can be reproduced.
Refill exception
The abnormal product is placed in an oven of 65 ° C, and the following signals are tested: MCU power supply, TXD, CAN differential, and CAN modules are powered. When there is an abnormality, the waveforms are shown in Figure 4. It can be seen that the MCU is 3.3V power supply, the voltage is stable in 3.2V, and the CAN module is stable at around 5.07V, and the CAN differential waveform corresponds to the TXD signal.
Figure 4 normal mode waveform
Repeat the abnormality card, the CAN bus has a large number of error frames, and the problem is recovered. When an abnormality, the waveforms of each point are fluctuated simultaneously when the MCU is supplied, and the CAN module is powered simultaneously. When the abnormal position appears, the MCU power is lowered to 3.08V, and the CAN module is powered up to 5.19V.
Figure 5 is abnormal mode waveform
Carefully observe the abnormal bit waveform, as shown in Figure 6, when the TXD becomes high, the CAN differential level is not followed, and when the TXD has a small noise spike again, the CAN differential level becomes a hidden level. . In connection with the case where the MCU supply voltage is lowered, the CAN module is powered by, and the problem is that the problem is caused by the supply voltage fluctuations, causing the TXD high level to be unrecognized.
Figure 6 abnormal bit waveform
1. Problem positioning
Due to the suspected TXD level, the TXD high-level valve voltage value of the CAN module is tested. Under the different input voltages, the test data is shown in Table 3.
Table 3 Abnormal Products TXD High Level Voltage Voltage
From the test data, the TXD high-level valve voltage variation is not large at different ambient temperatures. When 4.75V is powered, the valve value is about 2.91V; 5V is powered, the valve value is about 3.06V; 5.25V, the valve value is about 3.2V.
As shown in Figure 5, when the abnormal position occurs, the CAN module is powered by 5.19V. At this time, the TXD high level value should be about 3.17V, and the power supply of the MCU is only 3.08V, the IO output voltage cannot reach 3.17V, so it cannot be Identify high level. When the noise spike occurs in TXD, the TXD is higher than 3.17V, and the internal switching of the CAN module is triggered, and the bus differential signal changes.
Here, the above speculation is verified, and the cause of the fault is determined: When the high temperature is powered up, the power supply voltage exhibits fluctuations. The TXD pin of the CAN module cannot recognize the high level signal of the MCU, resulting in the error frame to continue. Resulting in CAN communication interruption.
2. Solution
After replacing the level of isolation module (replaced by a 5V isolation module), the TXD high-level valve voltage and the CAN bus communication level amplitude is shown in Figure 7, and it has returned to normal amplitude. Communication has no abnormalities.
Figure 7 Replacing the TXD high level valve value after the module and the CAN bus level
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