Troubleshooting I2C Bus Failures with a Manual Output Circuit
In the course of experiments involving I2C circuits, the occurrence of bus failures without apparent reasons is not uncommon. Given the absence of a logic analyzer for everyone, the circuit introduced here can prove beneficial in various instances. Essentially, it is a manually operated I2C output where two switches control the clock and data lines, enabling users to set them high or low. Two LEDs, indicating the actual bus levels, enhance visibility. It’s important to note that if the I2C circuit has a built-in time-out (typically around 1 ms), this circuit becomes impractical due to the manual reaction speed limitations.
Dual-Functionality Design for Data and Clock Lines
The circuit is divided into two identical halves, with one dedicated to the data line (SDA) and the other to the clock (SCL). A change-over switch, S1 or S2, and a bistable, IC1a or IC1b, are responsible for generating logic 1 or 0. The bistable serves a crucial role in mitigating switch bounce, preventing its interpretation as a series of pulses. The switch position denotes whether a 1 or 0 is applied to the bus. However, the actual bus level indicated by D1 and D2 may differ from the level applied by this circuit due to the bus’s open collector structure. To account for this, the outputs of the bistables are not directly connected to the bus but rather via T2 and T1, respectively.
Efficient Utilization of Pull-Up Resistors in the Bus
Pull-up resistors, R4 and R11, are essential components on the bus and are required only once. Therefore, their inclusion in the present circuit may be omitted if they are already incorporated elsewhere in the system. Additionally, this circuit can serve as a straightforward I2C monitor. By positioning both switches in the ‘1’ position, the bus remains unaffected, and the LEDs function as indicators of bus activity. Given the need for substantial activity, a single transmission is insufficient for LED indication. The current is primarily determined by the LED’s and totals approximately 20 mA.