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TI I/O expanders provide parallel bus monitor solutions

Posted: 13 Oct 2004 ?? ?Print Version ?Bookmark and Share

Keywords:microcontroller? microprocessor? sn74lv8153? output expander? texas instruments?

Designed to provide an alternative to monitoring and controlling the inputs and outputs of a system's microcontroller or microprocessor, the SN74LV8153 8bit output expander from Texas Instruments Inc. (TI) complements its PCF8574 and PCF8574A I/O expansion devices.

According to TI, these expanders offer a viable method of taping a parallel bus and a practical means of interfacing with different peripheral devices.

Since virtually all modern processors feature dedicated Universal Asynchronous Receivers/Transmitters (UARTs) and I?C interfaces separate from the pool of general-purpose pins, they can utilize UART and I?C controlled expanders to add system functionality without sacrificing GPIOs. The LV8153 and PCF8574/PCF8574A provide this expansion through connection to an existing UART or I?C bus, respectively. By using expanders such as the LV8153, PCF8574 or PCF8574A, system designers can incorporate additional processor-controlled outputs (LV8153) or inputs/outputs (PCF8574/PCF8574A) in their system without using general-purpose input/output pins.

The SN74LV8153 UART-controlled 8bit output expander has a 3V to 5.5V voltage supply rail for interfacing with the microprocessor and a wider voltage (3V to 12V) supply rail for driving its outputs, making it suitable for interfacing between low voltage processors and higher voltage peripherals.

Additionally, the LV8153 outputs can be configured as either open-collector or push-pull for easy connection to actively-driven and open-drain buses. In the open-drain mode, the outputs are capable of sinking 40mA. Three user selectable address pins allow for up to eight SN74LV8153s to be individually controlled from the same UART.

The PCF8574 and PCF8574A are I?C controlled 8bit general-purpose remote input/output expanders which operate from supply voltages between 2.5V to 6V. These devices have low standby current consumption (10?A max) and feature latched outputs with 25mA current sinking capability suitable for driving LEDs. Both the PCF8574 and PCF8574A use the standard 8bit (7-address followed by 1-read/write bit) I?C protocol.

The press release stated that the four most significant address bits are preset internally (0100 for the PCF8474 and 0111 for the PCF8574A), and the three least significant address bits are user selectable via the hardware address pins. This combination of predetermined and user-selectable addressing allows a total of 16 remote I/O expanders to be connected to the same I?C bus without address conflicts while providing a total of 128 additional I/O's for the processor.

Offered in the industry standard TSSOP (PW) package, the SN74LV8153 is already available for samples. Pricing for this device is set at $1.21 in quantities of 1,000. The PCF8574 and the PCF8574A are offered in the space low profile 20-pin QFN (RGY) package, along with the TVSOP (DGV) and TSSOP (PW) packaging. Available for samples, both devices are priced at $1.76 in quantities of 1,000.

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