Microchip Technology AT43USB326-AC

Overview

AT43USB326-AC from Microchip Technology (formerly Atmel) is an 8-bit AVR RISC microcontroller with integrated USB hub and keyboard controller, delivering 12 MIPS throughput at 12 MHz, 16 KB masked ROM program memory, 512 bytes SRAM, and native support for 18×8 keyboard matrix scanning with four LED drivers. It operates at 5 V with on-chip 3.3 V power supply and targets embedded USB peripheral controllers in industrial keyboards and human interface devices.

For engineers reviewing the AT43USB326-AC datasheet, AT43USB326-AC pinout, AT43USB326-AC application, or AT43USB326-AC equivalent, this page provides verified technical context, validated pin functions, confirmed USB hub topology (1 attached + 2 external ports), exact oscillator requirements (6 MHz crystal + PLL), and real-world design implications of its three programmable USB endpoints and keyboard scan architecture.

Technical Context

The AT43USB326-AC implements a compound USB device architecture: a 3-port hub (Port 0 = upstream, Port 1 = permanently attached keyboard function, Ports 2–3 = configurable external downstream ports) with independent device addresses and control endpoints. Its USB hardware includes dedicated interrupt endpoint for hub and two additional 8-byte FIFO programmable endpoints (EP1/EP2) for keyboard data transfer.

Internally, it uses a Harvard architecture with 32 general-purpose registers directly connected to the ALU for single-cycle execution, a 13-bit program counter addressing 8K × 16-bit ROM, and USB registers mapped to SRAM space ($1F00–$1FFF). Clocking derives exclusively from a 6 MHz crystal with on-chip PLL generating the precise 12 MHz USB timing (±20.8 ns cycle deviation during phase adjustment).

Key Specifications

Pinout & Package

AT43USB326-AC is housed in a 48-lead LQFP package (7 mm × 7 mm, 0.5 mm pitch) with exposed thermal pad. Pin functions are validated per Atmel document 3313D–USB–04/06.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar USB keyboard controller applications.

Compared with AT43USB326-AC, ATMEGA32U4-AU offers reprogrammability but lacks native keyboard scan and hub functionality, while USB2514B-AEZG provides higher hub performance but demands external MCU coordination - making AT43USB326-AC uniquely optimized for cost-sensitive, fixed-function USB keyboard hubs.

Availability

AT43USB326-AC is available at Aetrix Electronics and suitable for industrial keypads, medical device keyboards, gaming peripherals, point-of-sale terminals, and ruggedized human interface devices requiring stable component supply and long-lifecycle availability.

Supply support for AT43USB326-AC includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Microchip Technology acquired Atmel in 2016 and maintains full support for legacy AVR USB microcontrollers, providing documentation, tools, and long-term product assurance.

The AT43USB326-AC belongs to Atmel's specialized USB HID controller product line, designed specifically for embedded keyboard and keypad applications requiring integrated hub functionality, matrix scan acceleration, and minimal external components.

FAQ

What is the maximum keyboard matrix size supported by the AT43USB326-AC?

The AT43USB326-AC supports up to 18 columns (via PA0–PA7, PB0–PB7, PE0–PE1) and 8 rows (PC0–PC7), enabling a full 18×8 matrix for 144-key keyboards. Column strobes include internal slew-rate control and configurable drive strength, while row inputs feature programmable internal pull-ups - all managed in hardware without CPU polling overhead. This configuration is fixed in the AT43USB326-AC silicon and cannot be extended beyond 18×8.

Does the AT43USB326-AC require an external crystal, and what are the tolerance requirements?

Yes, the AT43USB326-AC requires a 6 MHz parallel-resonant crystal with ≤10 pF load capacitance and ±100 ppm frequency stability to meet USB hub timing specifications. Ceramic resonators are explicitly discouraged due to insufficient accuracy. The crystal connects between XTAL1 and XTAL2; if using an external clock source, drive XTAL1 only and leave XTAL2 floating. The LFT pin must be terminated with a precision RC filter (0.01 µF || [100 Ω + 0.1 µF]) to ground for stable PLL operation.

Can the AT43USB326-AC operate without the on-chip 3.3 V regulators?

Yes - the AT43USB326-AC can be powered solely by an external 3.3 V supply applied to CEXT1 and CEXT2, with VCC left unconnected. This mode disables the internal regulators and is intended for applications where GPIO current demands exceed the 30 mA per regulator limit or where tighter voltage regulation is required. In this configuration, the 1.5 kΩ upstream pull-up resistor must be driven externally, and all USB transceiver biasing must be supplied externally.

How does USB reset separation work on the AT43USB326-AC?

USB reset separation is enabled by setting BUS INT EN (bit 3) in the SPRSIE register. When enabled, a USB bus reset (SE0 ≥4 slow-speed cycles on Port 0) resets only the USB hardware - the microcontroller continues executing firmware and receives a suspend/resume interrupt. When disabled, the same bus reset triggers a full MCU reset. This feature allows firmware to maintain state across USB reconnect events, which is essential for robust HID operation in AT43USB326-AC-based keyboards.

Are the PE4–PE7 pins truly LED-driver capable without external components?

Yes - PE4 through PE7 integrate calibrated series current-limiting resistors (typical 45 Ω) enabling direct connection to standard 20 mA LEDs at 3.3 V. Each pin supports ~15 mA sink current with <1 V saturation voltage, eliminating the need for external current-limiting resistors. This capability is validated in the AT43USB326-AC datasheet and applies only to these four pins; other I/Os require external drivers or resistors for LED interfacing.