The principle, characteristics and typical application analysis of MXB7846
Время публикации: 2020-07-23 11:49:43
With the widespread application of household electronic equipment and smart instruments, people have higher and higher requirements for their man-machine interfaces. Humanized man-machine interfaces can greatly improve people's operation level and better perform device performance. Therefore, touch screens are widely used in the human-machine interface of household electronics, testing instruments, communication equipment and other devices.
MXB7846 is a 4-wire industry standard touch screen data converter integrated with ±15kV ESD protection from Maxim. It is widely used in resistive touch screen input systems. It integrates a 12-bit synchronous sampling analog-to-digital converter, which can use the internal integrated +2.5V reference power supply or an external reference power supply. It also integrates an on-chip temperature sensor, power monitoring channel and auxiliary AD converter. All analog input channels of the circuit are under ESD protection, so no additional electrostatic protection devices are required when in use.
2. Pin arrangement and pin function
MXB7846 is available in 16-pin QSOP and TSSOP packages. The pin layout is shown in Figure 1, and the functions of each pin are as follows.
Figure 1 Pinout of MXB7846
VDD: Power input, the voltage range is +2.375V～+5.25V. When using internal reference power supply, the size of the reference power supply is determined by this pin. 1μF capacitor is used to bypass the filter during operation.
X+, X-, Y+, Y-: horizontal and vertical coordinate input, where X+ and Y+ are the first and second channels of ADC input.
BAT: Power monitoring input terminal, the third channel of ADC.
AUX: auxiliary analog input terminal, which is the 4th channel of ADC.
REF: Reference voltage input/output channel, which provides reference voltage for AD conversion. When the internal reference power supply is used, this pin provides a 2.5V reference voltage output; when the external reference power supply is used, the reference voltage is input from this, and the voltage from 1V to VDD can be input. Use 0.1μF capacitor to bypass the filter during work.
PENIRQ: Touch the interrupt pin, which is pulled up by a 10kΩ～100kΩ resistor during operation. When the touch screen is touched, an interrupt is caused, and the ADC starts to convert.
DIN: Serial data input, data is read in on the rising edge of DCLK.
DOUT: Serial data output, the falling edge of DCLK outputs data. When CS is high, DOUT is in a high impedance state.
BUSY: Busy output flag, which lasts for one clock cycle high state during data conversion. When CS is high, BUSY is high impedance state.
CS: Chip select signal. Only when CS is low, serial data can be read in from DIN.
DCLK: Clock signal input, the input clock signal determines the conversion speed of the circuit, and its duty cycle must be 40% to 60%.
3. Working principle
MXB7846 uses successive approximation technology to realize the conversion from analog signal to 12-bit digital signal. Its internal structure is shown in Figure 2. When the circuit works in single-input mode, ADC uses REF as the reference input; when the circuit works in differential input mode, ADC's REF+ is connected to X+ or Y+, and REF- is connected to X- or Y-. The specific continuous mode is shown in Table 1.
The control word format of MXB7846 is listed in Table 1. START is the start flag bit of data transmission, and this bit must be "1". A2-A0 for channel selection (see Table 2). MODE is used to select the accuracy of AD conversion, "1" means 8 bits and "0" means 12 bits. SER/DFR selects the input mode of the reference voltage, "1" is the single input mode, and "0" is the differential input mode. PD1, PD0 select power saving mode: "00" means power saving mode is allowed, power is down between the second A/D conversion, and interrupt is allowed; "01" is the same as "00", but interrupt is not allowed; "10" is Reserved; "11" means that power saving mode is prohibited.
4. Typical application
MXB7846 is used to control the typical circuit principle of resistive touch screen as shown in Figure 3.
When the touch screen is working, the upper and lower conductor layers are equivalent to the resistance network, as shown in Figure 3. When a voltage is applied to a certain layer of electrodes, a voltage gradient is formed on the network. If there is an external force that makes the upper and lower layers contact at a certain point, the voltage at the contact point can be measured on the other layer where no voltage is applied to the electrode to know the coordinates of the contact. For example, if a voltage is applied to the top electrode (X+, Y-), a voltage gradient will be formed on the top conductor. When an external force makes the upper and lower layers contact at a certain point, the voltage at the contact point can be measured on the bottom layer , And then calculate the X coordinate of the place according to the distance relationship between the voltage and the electrode (X+). Then, switch the voltage to the bottom electrode (Y+, Y-), and measure the voltage at the contact point on the top layer to know the Y coordinate.
In order to complete an electrode voltage switch and A/D conversion, first send a control word to the MXB7846 through the serial port, and then read the voltage conversion value through the serial port after the conversion is completed. A standard conversion requires 24 clock cycles. Since the serial port supports two-way simultaneous transmission and can overlap between one reading and the next control word, the conversion rate can be increased to 16 clock cycles each time. If conditions permit, that is, the CPU can generate 15 clock cycles (such as FPGAs and ASICs), and the conversion rate can be increased to 15 clock cycles each time. The conversion sequence is shown in Figure 4.
This text adopts EPM7128LC84-6 CPLD of Altera Company to produce the control logic required by MXB7846, so that it can work at a higher conversion efficiency. The control program compiled in Verilog language is as follows:
module max(DATAIN, CLK, DOUT, BUSY, CS, DCLK, DIN, DATAOUT, FLAG);
input BUSY, CLK, DOUT;
output CS, DCLK, DIN, FLAG;
reg shizhong, CS, DCLK, DIN, FLAG;
always @(negedge CLK)
5‘b00000:begin DIN=DATAIN ;yiwei =DOUT;end
5’b00010:begin DIN=DATAIN ; yiwei =DOUT;end
5‘b00100:begin DIN=DATAIN ; yiwei =DOUT;end
5’b01000:begin DIN=DATAIN ;yiwei =DOUT;DATAOUT=yiwei;FLAG=1;end
This program is the control module of MXB7846 in the CPLD module. Its main function is to divide the CPLD clock (the number of divisions can be set) to produce the MXB7846 clock, and to give the MXB7846 control word through DATIN. Its CS, DOUT, DCLK, DIN, BUSY are respectively connected with the corresponding pin of MXB7846 to control it. When the MXB7846 completes a conversion, the CPLD reads the serial data and converts it into parallel data. After FLAG is high, the data can be read from DATAOUT.
MXB7846 is easy to interface with various commonly used CPUs at present, and its low power consumption characteristics make it very suitable for power supply systems, and has broad application prospects in various portable devices such as personal palmtop computers, mobile phones, medical instruments, and measuring instruments.Тег: MXB7846