In the case of complicated wiring, inconvenience or no wiring, it is hoped that it can be solved by a short-range wireless communication scheme. Common applications include RFID, wireless meter reading, wireless catering ordering systems, wireless monitoring and more. With the maturity of radio frequency communication technology and related products and the continuous improvement of people's living standards and requirements, the information construction of the catering industry is on the rise. The a la carte in the hands of the restaurant waiter is the data collection part of the food and beverage ordering system, and the communication with the base station is wireless. This article introduces a popular wireless a la carte handheld device for small and medium-sized restaurants (which can be used in other fields with appropriate modifications). It uses C8051F340 MCU with high performance, low price and low development cost as MCU, and is newly launched by Chipcon. The integrated wireless transceiver chip CC1100 is an RF transceiver that operates at 433MHz in the license-free ISM (Industrial, Scientific, and Medical) band.
1 hardware designIt is mainly composed of MCU control unit (control each programmable chip, process input and output signals), RF transceiver unit (modulation and demodulation of RF signals, transmission and reception, and packing and unpacking of data, etc.), keyboard and LCD display. Unit (input and display of information), memory unit (stores Chinese character library and menu database), UART and USB interface unit (communication with PC, providing charging power), power management unit (providing various types of voltage for the system, Composition of charging, switching of external power supply, etc.
1.1 MCU unit
C8051F340 (hereinafter referred to as MCU) is a mixed-signal Flash microcontroller with full-speed USB function. It has built-in 256kB+4KB RAM and 64KB Flash memory. Accurately calibrated (0.25%) 12MHz internal oscillator (it is not necessary to work externally) Connected to the crystal oscillator), the processing speed can reach 48MIPS; based on the JTAG interface, non-intrusive in-system debugging interface, etc. [1]. Its function and performance fully meet the design requirements.
1.2 RF transceiver unit
CC1100 is a cost-effective "highest" RF monolithic transceiver chip from Chipcon/TI. It is small (20pins, QLP 4&TImes; 4mm), high receiving sensitivity (-110dBm/1.2kbps), and rate programmable ( 1.2kb/s to 500kb/s), low operating voltage (1.8V to 3.6V), low power consumption (15.4mA/433MHz/1.2kb/s, 1.8μA standby current, startup time in standby mode is only 0.3 Ms), output power up to + 10dBm in all frequency bands, efficient SPI interface, digital RSSI (received signal strength indication) output, wireless wake-up (WOR) function, etc. Its operating frequency is 300MHz ~ 1000MHz [2].
Taking into account the high frequency interference, the unit is made up of a PCB board and electromagnetically shielded by a metal cover, connected to the bottom plate through a 7-pin connector, as shown in Figure 2. GDO0 is connected to the P0.7 of the MCU to indicate whether a data packet has been completely received, and it can be used as an external interrupt source signal of the MCU. By default, GDO0 will output a square wave with a crystal frequency (192Hz), which can be used as a basis for judging whether the CC1100 is working properly during debugging. SCLK (serial clock), SO (serial output), SI (serial input), and CSn (chip select) are connected to P0.0, P0.1, P0.2, and P0.6 of the MCU to implement the MCU. Connected to the SPI interface of the CC1100. The configuration, control, and data transmission and reception of the CC1100 by the MCU are performed through the SPI interface. When designing the PCB, keep the crystal and chip pins as close as possible, and isolate the clock area with the ground wire. The crystal case is grounded and fixed, and the components use SMT (element) components to reduce the via and volume. The VCC is 3.3V. C4, C5, C6 and L3, L4 form a non-balance transformer for transforming between differential and single-ended RF (radio frequency) signals. C1, C2, C3 and L1, L2 form an LC filter. The balancing transformers are compatible with a 50 Ω antenna. Resistor R1 is used to set a precise reference current for the bias voltage of the CC1100. DS1 is used to indicate whether the power supply on the module is normal.
1.3 keyboard and LCD display unit
The keyboard includes numeric keys, launch keys, arrow keys, multi-function keys, power on/off keys, etc. It consists of a matrix of 8 rows & TImes; 3 columns and a separate key (power on/off). The row line is connected to the data line (P4 port) of the MCU, and the column lines are respectively connected to P2.0, P2.1 and P2.2. When the software determines that the lower 3 bits of the P2 port have changed, the content of the P4 port is immediately read, and the currently pressed key is obtained. The power on/off key is directly connected to the start/stop pin of the power management chip.
The UC1610 is a four-color gray-scale pseudo-color LCD controller and driver with 128 & TImes; 160 dot matrix. It is designed for ultra-low power handheld devices. The interface with MCU has parallel port, SPI and I2C. This system uses a 2-wire I2C bus to connect to the SMBus of the MCU. In this case, CD, WR0 and WR1 should be grounded, D0 and D3 should be connected to SCL and SDA of the MCU, and the other bits in the data line should be grounded. For its control, please refer to its data sheet [4].
1.4 memory unit
The on-chip memory of the MCU is enough to store the program code, but because of the Chinese character library and the library, an external data memory must be connected. This memory has both large capacity and small package size, so ATMEL's SPI Flash memory AT25F2048 was chosen. The memory capacity is 2Mbit, 8-pin chip package, connected to the MCU through a 4-wire SPI, and the data transfer rate is 12Mb/s. Related read and write and erase operations can be performed according to the command format in the data sheet [5].
1.5 UART and USB interface unit
The program can be downloaded via the JTAG interface, but the library and Chinese character library can only be downloaded via the UART/USB interface. The MCU provides two UART ports, which can be connected to the serial port of the PC through an external UART ↠→ RS232 conversion driver chip MAX3232. A USB2.0 (device) controller [1] is embedded in the MCU and can be directly connected to the USB interface of the PC. At the same time, the USB port also provides a power source for charging the device.
1.6 Power Management Unit
The LTC3455 is a complete single-chip power management solution for handheld applications with integrated USB power manager, Li-Ion battery charger and dual DC-DC in a 4mm x 4mm 24-pin QFN package. 5V to 3.3V and 5V to 1.8V) converters that combine several functions previously required to implement five or more chips. Seamless selection of power supply (3 out of 1), the process is completely automatic according to priority: 5.0V AC adapter → USB power → battery. When using an adapter or USB power supply, the battery charger is enabled and the power supply inside the device is fully powered by the selected external power supply [3]. The CON1 in the figure is connected to the USB port of the PC, the CON2 is connected to the power adapter, and the upper limit current of the USBHP for selecting the USB power source is 500 mA. P30 is used to enable SW2 and hot plug function, P31 is used to indicate the battery's power, P32 is used to indicate the status of the ON pin, P33 is used to enable the chip, and SW-VCC is used to turn the output power on/off.
2 software designThe software development environment is keil μVISION3, using the keil C51 language. The wireless communication part of the system adopts the master-slave structure, the slave station is composed of the handheld device, and the master station is composed of the base station located in the kitchen, and the working mode adopts the primary station polling and the slave station monitoring mode. In order to avoid collisions between multiple slave stations contending for the channel, it is stipulated that only the master station sends data to the master station when it is directed to a certain slave station name. The main process of the system software is shown in Figure 4.
2.1 Programming Points of CC1100
(1) Register access
The CC1100 has different configurations at different rates. For optimal performance, some configurations must be processed through complex calculations. Chipcon offers the SmartRF Studio software to optimally configure the relevant registers. For a certain rate, the software automatically provides a set of optimal register configuration parameters, and the programmer only needs to write these configurations to the corresponding registers during initialization. The configuration can only be performed when the CC1100 is in the IDLE state.
In addition to configuration operations, there are read status, issue commands, and read/write Rx/Tx FIFO operations. These operations have similar communication formats: head byte + data (except for read and send command operations) bytes. The header consists of R/W (bit7), Burst (bit6), and Addr (bit 5 to 0). R/W indicates whether the current operation is a read 1 or a write 0; Burst indicates whether the current operation is to access a single 0 or multiple 1 registers. However, there is an exception when reading the status register. Although only one status register can be read at a time, this bit must be 1, because the address of the status register overlaps with the address of the command register, so the burst bit is redefined; Bits 0 to 5 (Addr) are the addresses of the registers.
(2) state machine
Inside the CC1100, there is a state machine for managing the switching between states. The current state is available from the status register MARCSTATE. The state transition diagram is shown in Figure 5. Commands, internal events, and configuration information determine state transitions. TXOFF_MODE and RXOFF_MODE in the register MCSM1 determine which state will be entered after the transmission operation is completed and a valid packet is received. It should be noted that the wireless wake-up function WOR with power saving function can automatically turn the CC1100 to the IDLE state periodically in the SLEEP state, and then automatically transfer to the Rx state. However, if there is no data packet receivable in this state, it will return to the SLEEP state; otherwise, after receiving the data packet, the next state will be determined by RXOFF_MODE, but will not enter the SLEEP state, and must be entered by the MCU when entering the IDLE state. Send the SWOR command to enter again. In the SLEEP state, whether the crystal oscillator continues to operate is determined by XOSC_FORCE_ON in the register MCSM0 (0: continue working, 1: stop working). When the MCU accesses CC1100 (CSn = 0), the crystal will always work regardless of the value of XOSC_FORCE_ON.
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