42 MicroBlaze Development Kit Spartan-3E 1600 Edition User Guidewww.xilinx.com UG257 (v1.1) December 5, 2007Chapter 5: Character LCD Screen ROnce mastered, the LCD is a practical way to display a variety of information usingstandard ASCII and custom characters. However, these displays are not fast. Scrolling thedisplay at half-second intervals tests the practical limit for clarity. Compared with the50 MHz clock available on the board, the display is slow. A PicoBlaze processor efficientlycontrols display timing plus the actual content of the display.Character LCD Interface SignalsTable 5-1 shows the interface character LCD interface signals.Voltage CompatibilityThe character LCD is power by +5V. The FPGA I/O signals are powered by 3.3V. However,the FPGA’s output levels are recognized as valid Low or High logic levels by the LCD. TheLCD controller accepts 5V TTL signal levels and the 3.3V LVCMOS outputs provided bythe FPGA meet the 5V TTL voltage level requirements.The 390: series resistors on the data lines prevent overstressing on the FPGA andStrataFlash I/O pins when the character LCD drives a High logic value. The character LCDdrives the data lines when LCD_RW is High. Most applications treat the LCD as a write-only peripheral and never read from from the display.Table 5-1: Character LCD InterfaceSignal Name FPGA Pin FunctionSF_D<15> T8 Data bit DB7Shared with StrataFlash pinsSF_D<15:8>SF_D<14> R8 Data bit DB6SF_D<13> P6 Data bit DB5SF_D<12> M16 Data bit DB4SF_D<11> M15 Data bit DB3SF_D<10> P17 Data bit DB2SF_D<9> R16 Data bit DB1SF_D<8> R15 Data bit DB0LCD_E M18 Read/Write Enable Pulse0: Disabled1: Read/Write operation enabledLCD_RS L18 Register Select0: Instruction register during write operations. BusyFlash during read operations1: Data for read or write operationsLCD_RW L17 Read/Write Control0: WRITE, LCD accepts data1: READ, LCD presents dataLCD_RET E3LCD_CS1 P3LCD_CS2 P4
