USB/IrDA bridge control chip STIr4200S

    Abstract: A new ASIC developed by SigmaTel specifically for bridge control between USB and IrDA: STIr4200S is introduced in detail from the functional block diagram, pin description, frame format, register and its control instructions.

    Keywords: Infrared wireless data communication USB IrDA STIr4200S

STIr4200S is a new ASIC developed by SigmaTel specifically for bridge control between USB and IrDA. It adopts a low-power CMOS design and is fed directly from the USB port. It integrates a 4K-byte FIFO buffer on-chip and is packaged in a 28-pin SSOP. The IrDA data transfer rate ranges from 2.4 Kbps to 4 Mbps and is mainly used to achieve infrared wireless data communication through the USB port.

1 Functional block diagram

　　 The functional block diagram is shown in Figure 1. Generally speaking, STIr4200 consists of two parts: USB controller and digital infrared transmitter. The USB controller provides a control endpoint, a batch data input endpoint and a batch data output endpoint to the USB host. The digital infrared transmitter consists of a sending interface and a receiving interface, which are connected to the analog infrared transceiver.

2 pin description

　　 The STIr4200S pin arrangement is shown in Figure 2.

　　 The STIr4200S pin description is listed in Table 1.

Table 1 STIr4200S pin description

3 Instructions

　　 The STIr4200S provides users with an instruction set that allows reading and writing the registers of the digital IR transmitter as well as the ROM of the USB controller through the USB driver. Its instruction set is detailed below.

　　 (1) Write multiple register instructions

　　 The write multiple registers instruction allows the user to write multiple consecutive registers of the digital IR transmitter. The instruction format is listed in Table 2. Each register is 1 byte long, and the command gives the first register to be read and written, the number of consecutive registers to be read and written, and the data cycle in which the data is written.

Table 2 Description of instructions for writing multiple registers

Table 3 Write 1 register instruction description

Table 4 Description of instructions for reading multiple registers

Table 5 Read ROM instruction description

　　 (2) Write a register instruction description, as listed in Table 3.

　　 (3) Instructions for reading multiple registers are listed in Table 4.

　　 (4) Read ROM instruction instructions, as listed in Table 5.

　　 The read ROM command allows the DRIVER to read the ROM in USB controller endpoint 0. This is mainly used to verify the contents of the ROM during debugging. Up to 64 bytes can be read at a time.

4 Digital infrared transmitter

　　 Digital IR transmitter that receives digital input from an analog IR transceiver. It is mainly composed of transmitting modulator, receiving demodulator, FIFO, analog transmission part and register group, as shown in Figure 3. You can operate the device by setting the register value, such as modulation mode, baud rate, frame size in FIFO, and RX input mode. The size of the FIFO is 4 K bytes.

　　 The data sent to the TX modulator by the USB controller must form an IrLAP frame. The format of an IrLAP frame consists of the following parts. Among them, BOF is the frame start flag, A is the address field, C is the control field, I is the information field, FCS is the CRC check code of the frame, and EOF is the end of frame flag.

　　 In addition, before the content in the FIFO is sent to the USB controller, a 2-byte ID code and a 2-byte frame size must be added to the message.

5 frame format

　　 In order to identify the boundaries of infrared transmission frames, some special characters are used in the frame format, as listed in Table 6. When these characters appear in the data area, they must be escaped. At the same time, because during the transmission process, the boundary of the USB bulk data input/output buffer may not be aligned with the boundary of the infrared transmission frame, therefore, a character 0x7D is also defined in the frame format for actual frame boundary description. Additionally, 0x7F is introduced as a pre-sync character in FIR mode.

5.1 Frame format in SIR mode

　　 The transmission rate in SIR mode can be 2.4, 9.6, 19.2, 38.4, 57.6 and 115.2 Kbps. The frame format is shown in Figure 4 and Figure 5.

5.2 Frame format in MIR mode

　　 The transmission rate in MIR mode can be 576 Kbps and 1.152 Mbps. The frame format is shown in Figure 6 and Figure 7.

5.3 Frame format in FIR mode

　　 The transmission rate in FIR mode is 4 Mbps, and the frame format is shown in Figure 8 and Figure 9.

6 Register settings

　　 There are 16 registers in the digital infrared transmitter, as listed in Table 7. The working mode, baud rate, etc. of the digital infrared transmitter can be set, and its working status can also be obtained by accessing the status register. The following describes the settings of each register except the test register and DPLL register in detail. The test register and DPLL register are only used for manufacturer debugging.

Table 6 Description of escape characters

Table 7 Description of registers in digital IR transmitter

Table 8 Mode register setting instructions

Table 9 Baud rate register setting instructions

Table 10 Sensitivity register setting instructions

Table 11 Status register setting instructions

Table 12 IRDIG initial register setting description

　　 (1) FIFO data register

　　 The default setting is 0x00, which is used for data entering the FIFO from the USB interface, but this register is rarely used. Because under normal circumstances, USB can operate FIFO through Bulk read/write instructions.

　　 (2) Mode register

　　 The default status is 0x20. Setup instructions are listed in Table 8.

　　 (3) Baud rate register

　　 This 8-bit register is full of PDCLK bits, with a default value of 0x77. Table 9 Set the IrDA transmission mode by setting the mode register and baud rate register.

　　 (4) Control register

　　 The default value is 0x00. Bit7 is the SDMODE bit. This bit is only used when the STIr4200S is connected to a TEMIC type infrared transmitter to set the infrared transmitter to a power-saving state. Bit6 is the RXSLOW bit. When set to 1, RXSLOW is used to receive the input signal; when set to 0, RXFAST is used to receive the input signal. Bit5 is the DLOOP1 bit. When set to 1, it means connecting the infrared transmitter to the infrared receiver. This bit cannot be used for the STIr4200S integrated solution. Bit4 is the TXPWD bit. When set to 1, the infrared modulator power supply will be stopped. Bit3 is the RXPWD bit. When set to 1, it means to stop the infrared demodulator power supply. Bit2~1 is the TXPWR(1:0) bit, which is used to set the internal pull-down resistor to control the current of the transmission diode: 00 means setting it to the maximum current, 01 means setting it to medium-high current, 10 will set it to medium-low current, and 11 means setting it to the minimum current. Bit0 is the SRESET bit. When set to 1, the infrared modem will be reset.

　　 (5) Sensitivity register

　　 The default value is 0x26, and the specific setting rules are listed in Table 10.

　　 (6) Status register

　　 The default value is 0x14, and the setting instructions are listed in Table 11.

　　 (7) IRDIG initial register

　　 IRDIG is mainly used to set the pin polarity connecting the infrared transmitter and the front-end infrared analog transceiver. The setting instructions are listed in Table 12.

Conclusion

　　 This article introduces the principle and structure of a USB/IrDA bridge control chip STIr4200S. The chip integrates the protocol conversion function between USB and IrDA. The communication protocol fully complies with USB1.1 spec and IrDA spec, and the communication speed can reach 4 Mbps.