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Ramses Chacon
Joined: 15 Dec 2025
Posts: 4
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| tft st7796s problems |
 Posted: Mon Dec 15, 2025 5:53 pm |
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I am having significant trouble getting an ST7796S TFT display module (SPI interface) to work with a Microchip PIC18F45K22 microcontroller.
📝 The Problem
The code compiles successfully, but when I program the PIC, the display remains blank. The only thing that works is the LED backlight, which is powered directly from the source, indicating the display unit has power, but no data or commands are being processed.
| Code: | #include <18F45K22.h>
#FUSES NOWDT, NOPROTECT, NOLVP, PUT, BROWNOUT, XT
#use delay(clock=20000000)
#define TFT_CS PIN_A5
#define TFT_DC PIN_A4
#define TFT_RST PIN_A3
#use spi(MASTER, SPI1, MODE=0, BITS=8, BAUD=8000000, STREAM=TFTSPI)
#include "tft_st7796s.c" // el código de arriba
void main(){
tft_init();
while(TRUE){
fillScreen(Color565(255,0,0)); // rojo
delay_ms(800);
fillScreen(Color565(0,255,0)); // verde
delay_ms(800);
fillScreen(Color565(0,0,255)); // azul
delay_ms(800);
}
}
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| Code: |
#define _width 480
#define _height 320
// Comandos ST7796S
#define CMD_SWRESET 0x01
#define CMD_SLPOUT 0x11
#define CMD_DISPON 0x29
#define CMD_MADCTL 0x36
#define CMD_COLMOD 0x3A
#define CMD_CASET 0x2A
#define CMD_RASET 0x2B
#define CMD_RAMWR 0x2C
void write_command(int8 cmd){
output_low(TFT_CS);
output_low(TFT_DC);
spi_write(cmd);
output_high(TFT_CS);
}
void write_data(int8 data){
output_low(TFT_CS);
output_high(TFT_DC);
spi_write(data);
output_high(TFT_CS);
}
void tft_init(void){
output_high(TFT_RST); delay_ms(5);
output_low(TFT_RST); delay_ms(20);
output_high(TFT_RST); delay_ms(150);
write_command(CMD_SWRESET); delay_ms(150);
write_command(CMD_SLPOUT); delay_ms(150);
write_command(CMD_COLMOD); write_data(0x55); // RGB565
write_command(CMD_MADCTL); write_data(0x48); // rotación
write_command(CMD_DISPON); delay_ms(150);
}
void setAddrWindow(int16 x0, int16 y0, int16 x1, int16 y1){
write_command(CMD_CASET);
write_data(x0>>8); write_data(x0&0xFF);
write_data(x1>>8); write_data(x1&0xFF);
write_command(CMD_RASET);
write_data(y0>>8); write_data(y0&0xFF);
write_data(y1>>8); write_data(y1&0xFF);
write_command(CMD_RAMWR);
}
// --- Definición de fillRect (igual que en ST7735 pero adaptado a 480x320) ---
void fillRect(int16 x, int16 y, int16 w, int16 h, int16 color){
int16 x_end = x + w - 1;
int16 y_end = y + h - 1;
int32 n = (int32)w * (int32)h;
int8 hi = color >> 8;
int8 lo = color & 0xFF;
setAddrWindow(x, y, x_end, y_end);
output_low(TFT_CS);
output_high(TFT_DC);
while(n--){
spi_write(hi);
spi_write(lo);
}
output_high(TFT_CS);
}
// --- Definición de fillScreen (pantalla completa) ---
void fillScreen(int16 color){
fillRect(0, 0, 480, 320, color);
}
int16 Color565(int8 r,int8 g,int8 b){
return ((r&0xF8)<<8)|((g&0xFC)<<3)|(b>>3);
}
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temtronic
Joined: 01 Jul 2010
Posts: 9606
Location: Greensville,Ontario
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| Posted: Mon Dec 15, 2025 6:36 pm |
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hardware question is VDD 5 or 3 volts ??
The LCD is 3 volts, but the PIC can run at 5, so it is important to know what voltage the PIC and LCD are connected to. |
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Ramses Chacon
Joined: 15 Dec 2025
Posts: 4
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| Posted: Mon Dec 15, 2025 7:13 pm |
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| temtronic wrote: | hardware question is VDD 5 or 3 volts ??
The LCD is 3 volts, but the PIC can run at 5, so it is important to know what voltage the PIC and LCD are connected to. |
The VCC converts 5V to 3.3V |
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Ttelmah
Joined: 11 Mar 2010
Posts: 20001
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| Posted: Tue Dec 16, 2025 2:33 am |
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First thing, your clock setup is wrong. XT, is the medium power drive mode.
It is suitable for medium drive resonator, or a crystal below perhaps 8MHz.
Not 20MHz.
For a 20MHz crystal, HS needs to be selected.
Safer in the long run to just get rid of all the oscillator fuses, and tell the
clock setting what your are doing. So (for example):
#use delay(crystal=20000000)
Will automatically set the correct fuses.
Get your oscillator running and tested before anything else. Flash an LED
and check this flashes at the correct rate.
Now your post about Vcc, makes no sense. Vcc is just the supply volt6age
it does not 'convert' anything. If you mean that the display has a voltage
regulator on it that converts 5v to 3.3v, then this is your problem. You need
to understand that there are two 'parts' to mixing supplies. The supply
voltage itself, and the _signalling_ voltage. Now the ST7796S display
generates SPI supporting 3.3v signalling. The PIC if running at 5v, will not
accept the signals from this for it's inputs. Either the PIC needs to be
running at 3.3v as well, or there needs to be signal level translators on
the data connections.
Read the sticky at the top of the forum about 5v to 3.3v interfacing. |
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Ramses Chacon
Joined: 15 Dec 2025
Posts: 4
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| Posted: Tue Dec 16, 2025 9:39 am |
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| Ttelmah wrote: | First thing, your clock setup is wrong. XT, is the medium power drive mode.
It is suitable for medium drive resonator, or a crystal below perhaps 8MHz.
Not 20MHz.
For a 20MHz crystal, HS needs to be selected.
Safer in the long run to just get rid of all the oscillator fuses, and tell the
clock setting what your are doing. So (for example):
#use delay(crystal=20000000)
Will automatically set the correct fuses.
Get your oscillator running and tested before anything else. Flash an LED
and check this flashes at the correct rate.
Now your post about Vcc, makes no sense. Vcc is just the supply volt6age
it does not 'convert' anything. If you mean that the display has a voltage
regulator on it that converts 5v to 3.3v, then this is your problem. You need
to understand that there are two 'parts' to mixing supplies. The supply
voltage itself, and the _signalling_ voltage. Now the ST7796S display
generates SPI supporting 3.3v signalling. The PIC if running at 5v, will not
accept the signals from this for it's inputs. Either the PIC needs to be
running at 3.3v as well, or there needs to be signal level translators on
the data connections.
Read the sticky at the top of the forum about 5v to 3.3v interfacing. |
But I have a problem because when I put hs the ccs c gives an error |
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Ttelmah
Joined: 11 Mar 2010
Posts: 20001
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| Posted: Tue Dec 16, 2025 10:30 am |
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Double check how you are spelling crystal/
What compiler version do you have?????.
| Code: |
#include <18F45K22.h>
#FUSES NOWDT, NOPROTECT, NOLVP, PUT, BROWNOUT
#use delay(crystal=20MHz)
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Compiled completely happily for me on any of the last 20 compilers. |
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Ramses Chacon
Joined: 15 Dec 2025
Posts: 4
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| Posted: Tue Dec 16, 2025 10:39 am |
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| Ttelmah wrote: | Double check how you are spelling crystal/
What compiler version do you have?????.
| Code: |
#include <18F45K22.h>
#FUSES NOWDT, NOPROTECT, NOLVP, PUT, BROWNOUT
#use delay(crystal=20MHz)
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Compiled completely happily for me on any of the last 20 compilers. |
Yes, it compiles that way, but if I use HS as you said, it doesn't work because the HS for this PIC is HSH. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 20001
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| Posted: Tue Dec 16, 2025 11:22 pm |
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I did _not_ say to use HS. I said to _remove all the clock fuses_
Do not use XT, HS, HSH etc. Let the compiler set the fuses. Otherwise
you risk getting them wrong.
HS is just the codename for the high speed crystal fuse. The actual
fuse needed depends on the chip. |
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temtronic
Joined: 01 Jul 2010
Posts: 9606
Location: Greensville,Ontario
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| Posted: Wed Dec 17, 2025 6:31 am |
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| my simple clock 'cheat', is to use the INTERNAL one. they are easy to setup, saves 2 pins, stable over room temperature. Unless you need precise timing they'll work 99.44% of the time. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 20001
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| Posted: Wed Dec 17, 2025 8:24 am |
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However his big problem is likely to be level translation, if he thinks:
The VCC converts 5V to 3.3V".....  |
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temtronic
Joined: 01 Jul 2010
Posts: 9606
Location: Greensville,Ontario
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| Posted: Wed Dec 17, 2025 10:23 am |
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have to wonder why not run on 3
Volts...seems to be the 'standard' for 99.44% of all peripherals and that PIC is good on 3 volts. |
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