Rev 1.00

March 22, 2010

7

HT45B0K

SPI to USB Bridge

USB Module

To communicate with an external USB host, the internal USB module has the external pins known

as UDP and UDN along with the 3.3V regulator output V33O. All data transmissions and receptions

between MCU and USB module including USB commands are conducted along the interconnected

SPI interface. The USB module has 6 endpoints and 160 bytes FIFO for the endpoints respectively.

A Serial Interface Engine (SIE) decodes the incoming USB data stream and transfers it to the

correct endpoint buffer memory (FIFO). The USB function control is implemented using a series

of registers accessed by the MCU using its SPI Master serial interface. A series of status registers

provide the user with the USB data transfer situation as well as any error conditions. The USB

contains its own independent interrupt which can be used to indicate when the USB FIFOs are

accessed by the host device or a change of the USB operating conditions including the USB

suspend mode, resume event or USB reset occurs.

There are both read and write commands for accessing the USB Module. For reading and writing

to registers, both command and address information is contained within a single byte. A series of

registers which are addressed by 5 address bits A4~A0 are available to control the USB module for

Read/Write Registers commands. The format for reading and writing commands is shown in the

following table.

Command Type

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Read Registers

0

x

x

A4

A3

A2

A1

A0

Write Registers

1

x

x

A4

A3

A2

A1

A0

Note: “x” here stands for “don’t care”.

USB Accessing Commands

There are several registers associated with the USB function. Some of the registers control the

overall function of the USB module as well as the interrupts, while some of the registers contain

the status bits which indicate the USB data transfer situations and error condition. Also there are

FIFOs for the USB endpoints to store the data received from or to be transmitted to the USB host.

The USB module has 6 endpoints (EP0~EP5) with different FIFO size for each one. The FIFO size

is 8 bytes for EP0~EP2 and EP4 which support “Interrupt transfer”, while the FIFO size for EP3

and EP5 is 64 bytes which can support “Bulk transfer”.

USB Status and Control Registers