Industrial equipment of LED display and keyboard scan design

Industrial equipment, automotive electronics and instruments widely used in LED display, because of the low cost of LED display, the use of flexible, encapsulation is small in size.

the development of such devices, often caused by a lack of CPU resources and had to extend interface chip is used to meet the needs of the application system, and the case is one of the reasons why the keyboard display takes up too much system resources, keyboard display usually adopt the following two ways ( 1) The special parallel interface keyboard display chip 8279. But more than 8279 peripheral components needed for ( Display driver, decoding, etc. ) Large, occupies board area, comprehensive cost is higher; ( 2) Using general parallel I/O chip extension ( Such as 8155, etc. ) , but also need to drive, according to this solution and keyboard scan, which still need to take up a lot of CPU time. For this application, MAXIM company introduced MAX6955 with keyboard scanning LED display driver circuit, the device has a 400 KBPS, 2 line serial interface, I2C compatible and can greatly simplify the design.

1 MAX6955 description of the function block diagram and

MAX6955 is compact display drive, through the I2C compatible 2-wire serial interface connecting microprocessor to 7/14/16 segment LED display. Serial interface supports multiple MAX6955 cascade, can drive up to 16 seven, eight, eight 14 paragraphs 16 segments or 128 discrete leds, and the power supply voltage can be as low as 2. 7V。 The device also has 5 I/O extension line ( GPIO) P0 ~ P4, P0 ~ P3 may push switch input of some or all of the configuration, can automatically scan and to shake up to 32 matrix switch, P4 output can be configured to IRQ.

MAX6955 device contains all 14 and 16 104 ASCII characters font, 7 segment display using hexadecimal font, multiplexing scanning circuit and the anode and cathode drives and static RAM is used to store each show. Show a maximum current can be set through a single external resistance. Your display brightness available within the digital brightness control circuit level 16 independent regulation. MAX6955 also contains a low power off mode, limited scan register ( Allows the user to select 1 ~ 16 for display) , period of scintillation control ( If you need, can through the synchronization drive) And forcing all leds light up test mode. LED driver has limit slope output, in order to reduce EMI. Figure 1 is MAX6955 function block diagram.

the key features of 2 MAX6955 low 400 KBPS, I2C compatible 2-wire serial interface;

low working voltage 2. 7 v ~ 5。 5V;

low can control up to 16 seven, eight, eight 14 paragraphs 16 segments, 128 discrete leds, or a combination of the various types;

low can drive the common cathode single color and double color LED display;

low built-in 14 and 16, 104 ASCII characters font and 7 segment display using hexadecimal font;

low paragraphs independent automatic flash control;

　　●10μA( Typical values) Low power off mode ( Keeping data) ;

low level 16 each independent digital height control;

low electricity display during blanking;

low slope section of the current drive to reduce EMI limits;

5 GPIO port can be configured to low key switch input, automatically scan and to shake up to 32 key switch, and support multiple keys at the same time, press the

low key input to output after shaking IRQ;

low level work in a standard car temperature range.

3 USES MAX6955 note

MAX6955 using I2C compatible 2-wire serial interface to control and master send start bit + 7 bits from a + register address + address +, speaking, reading and writing one or more bytes of data + stop bit. Due to need to send back when the receiver in the ninth pulse response, so nine pulse is needed to complete a byte of communication.

MAX6955 has seven from address, eighth is to read and write, high level is read, the low level is to write. Seven from three of the address is 110, low four by the address input pin AD0 and connection decided to AD1, continuous GND, respectively, V +, CL and the SDA, and 16 from the address you need.

read and write operations to MAX6955 is done through the command register address, the first byte is the command byte ( Is actually register address) , it determines the second byte of data to which register or read data from the register, if there is no data, command byte MAX6955 is performing operations. If there are multiple bytes of data command byte, MAX6955 internal register address automatically accumulate, such MAX6955 can shorten the time to read and write. Figure 2 shows the MAX6955 read/write sequence.

4 LED display and keyboard scan design figure 3 shows the use MAX6955 design LED display and keyboard scanning circuit of a specific example. This scheme can realize eight key scanning, 32 independent LED light display and 8 7 segment LED display.

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