71M651X Hardware Supporting LCDs

　　The 71M651X is capable of driving LCDs directly in static or multiplexed mode. No driver or other interface circuitry is necessary. With its maximum of 32 pins useable to drive segments， the 71M6511 device is capable of driving 4x32 = 128 pixels at 25% duty cycle. At eight segments per digit， the LCD can be designed for a maximum of 16 digits. In the minimum configuration， i.e. using only the dedicated segment pins SEG0…SEG2 and SEG8…SEG19， 15 x 4 = 60 segments can be driven， resulting in seven digits with eight segments each. With its maximum of 42 pins useable to drive segments， the 71M6513 device is capable of driving 4x42 = 168 pixels at 25% duty cycle. At eight segments per digit， the LCD can be designed for a maximum of 21 digits. In the minimum configuration， i.e. using only the dedicated segment pins SEG0…SEG2 and SEG8…SEG13， 19 x 4 = 76 segments can be driven， resulting in nine digits with eight segments each. Table 1 shows the assignment of DIO pins and SEG pins for the 71M6511 and 71M6513 chips.

　　The LCD driver circuitry is grouped into 4 common outputs （COM0 to COM3） and up to 42 segment outputs. The COM0 through COM3 outputs are more than logical outputs， since they generate wave forms that， in conjunction with the wave forms generated by the SEG outputs， combine to true AC signals supplied to each individual segment. This means that the COM voltage is below the SEG voltage for some periods of time and above the SEG voltage for other periods of time. The voltage present at the VLCD input is used internally by the chip to generate the LCD drive signals. A charge pump suitable for driving VLCD is included on-chip. This circuit creates 5VDC from the 3.3VDC supply using an external capacitor and diode network connected to the VDRV output （see Figure 1）。 This feature can be enabled with the LCD_BSTEN register.