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PCA9545D Datasheet(PDF) 5 Page - NXP Semiconductors |
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PCA9545D Datasheet(HTML) 5 Page - NXP Semiconductors |
5 / 14 page Philips Semiconductors Product data PCA9545 4-channel I2C switch with interrupt logic and reset 2002 Mar 28 5 853-2302 27311 INTERRUPT HANDLING The PCA9545 provides 4 interrupt inputs, one for each channel, and one open drain interrupt output. When an interrupt is generated by any device, it will be detected by the PCA9545 and the interrupt output will be driven LOW. The channel does not need to be active for detection of the interrupt. A bit is also set in the control register. Bits 4 – 7 of the control register correspond to channels 0 – 3 of the PCA9545, respectively. Therefore, if an interrupt is generated by any device connected to channel 1, the state of the interrupt inputs is loaded into the control register when a read is accomplished. Likewise, an interrupt on any device connected to channel 0 would cause bit 4 of the control register to be set on the read. The master can then address the PCA9545 and read the contents of the control register to determine which channel contains the device generating the interrupt. The master can then reconfigure the PCA9545 to select this channel, and locate the device generating the interrupt and clear it. It should be noted that more than one device can be providing an interrupt on a channel, so it is up to the master to ensure that all devices on a channel are interrogated for an interrupt. The interrupt inputs may be used as general purpose inputs if the interrupt function is not required. If unused, interrupt input(s) must be connected to VDD through a pull-up resistor. Table 2. Control Register Read — Interrupt INT3 INT2 INT1 INT0 B3 B2 B1 B0 COMMAND X X X 0 X X X X No interrupt on channel 0 X X X 1 X X X X Interrupt on channel 0 X X 0 X X X X X No interrupt on channel 1 X X 1 X X X X X Interrupt on channel 1 X 0 X X X X X X No interrupt on channel 2 X 1 X X X X X X Interrupt on channel 2 0 X X X X X X X No interrupt on channel 3 1 X X X X X X X Interrupt on channel 3 NOTE: Several interrupts can be active at the same time. Ex: INT3 = 0, INT2 = 1, INT1 = 1, INT0 = 0, means that there is no interrupt on channels 0 and 3, and there is interrupt on channels 1 and 2. RESET INPUT The RESET input is an active-LOW signal which may be used to recover from a bus fault condition. By asserting this signal LOW for a minimum of tWL, the PCA9545 will reset its registers and I2C state machine and will deselect all channels. The RESET input must be connected to VDD through a pull-up resistor. POWER-ON RESET When power is applied to VDD, an internal Power On Reset holds the PCA9545 in a reset state until VDD has reached VPOR. At this point, the reset condition is released and the PCA9545 registers and I2C state machine are initialized to their default states, all zeroes causing all the channels to be deselected. VOLTAGE TRANSLATION The pass gate transistors of the PCA9545 are constructed such that the VDD voltage can be used to limit the maximum voltage that will be passed from one I2C bus to another. 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 Vpass vs. VDD 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Vpass VDD MINIMUM TYPICAL MAXIMUM SW00820 2.0 Figure 5. Vpass voltage vs. VDD Figure 5 shows the voltage characteristics of the pass gate transistors (note that the graph was generated using the data specified in the DC Characteristics section of this datasheet). In order for the PCA9545 to act as a voltage translator, the Vpass voltage should be equal to, or lower than the lowest bus voltage. For example, if the main bus was running at 5 V, and the downstream buses were 3.3 V and 2.7 V, then Vpass should be equal to or below 2.7 V to effectively clamp the downstream bus voltages. Looking at Figure 5, we see that Vpass (max.) will be at 2.7 V when the PCA9545 supply voltage is 3.5 V or lower so the PCA9545 supply voltage could be set to 3.3 V. Pull-up resistors can then be used to bring the bus voltages to their appropriate levels (see Figure 12). More Information can be found in Application Note AN262 PCA954X family of I2C/SMBus multiplexers and switches. |
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