ESP32 direct port manipulation

Question

Dear StackOverflowers,

I am trying to use HX8357D 3.5" TFT from Adafruit (link) with an esp32. The TFT driver has two interfaces: SPI and 8-bit parallel. The provided library from Adafruit (link) only supports SPI on the esp32. I need to have higher display speeds, so I decided to try and add support for the esp32 myself. I'm not experienced at all with this kind of programming, but I liked the challenge.

I figured out how the 8-bit interface work by reverse engineering the Arduino Uno/Mega support. To add the esp32 support I need a way to directly manipulate the registers controlling the gpio ports of the esp32. I looked around on the internet, but there are very little examples of how to do this. The technical reference manual of Espressif (link) contains all the information needed, but I'm not skilled enough to figure out how to translate this into code.

To program the esp32 I use the esp32 Arduino core. This example (link) shows how to set gpio pins as output and make them HIGH and LOW directly using registers. The problem is that I need to be able to set 8 pins as output, write data to them, make them input and then read data from them, all using registers instead of using the pinMode, digitalRead and digitalWrite functions.

The way it works on the Arduino Uno/Mega is clear to me, there are three registers that control a port:

• DDR* to read/write
• PORT* to set gpio HIGH/LOW
• PIN* to read HIGH/LOW if the gpio is INPUT.

But how does this work on the esp32 and how can I make use of the registers to create this 8-bit parallel communication?

If there is anybody that has more know-how than me on this topic I would super be grateful for an explanation. Thanks in advance.

Answer 1

There are many ways to do this. I often do it pin-by-pin.

One simple way is to make your own 'register' by defining a variable. If the register is 8-bit wide, define byte variable:

unsigned char disp_register;

Then you write to this register like it would exist in display hardware. Of course, next you must output this register to the GPIO pins of ESP32. Since the pins are all over, you must do this pin-by-pin. Define your hardware pins for readability:

/* OUTPUTS (numbers mean GPIO port) */
#define REGISTER_BIT7_ON_PIN        9
#define REGISTER_BIT6_ON_PIN        10
#define REGISTER_BIT5_ON_PIN        5
// continue with all the pins you need

Somewhere at the beginning of your program, set these pins as output, and perhaps make their default value to '0':

io_conf.intr_type = GPIO_PIN_INTR_DISABLE;
io_conf.mode = GPIO_MODE_OUTPUT;
io_conf.pull_down_en = GPIO_PULLDOWN_DISABLE;
io_conf.pull_up_en = GPIO_PULLUP_DISABLE;
io_conf.pin_bit_mask =  ((1ULL<< REGISTER_BIT7_ON_PIN) | (1ULL<< REGISTER_BIT6_ON_PIN) | (1ULL<< REGISTER_BIT5_ON_PIN)); // of course, do like this all the pins
gpio_config(&io_conf);

gpio_set_level(REGISTER_BIT7_ON_PIN, 0); // do like this all the pins you need to set the boot-up value, pin-by-pin

Next you need your function to copy your register to outside world of GPIO pins:

/*
 * wrote this simply for ease of understanding, feel free to do this in a loop
 * or shifting bit by bit
 */
void copy_register_to_GPIO_pins(unsigned char disp_register)
{
    gpio_set_level(REGISTER_BIT7_ON_PIN, (disp_register & 0x80) >> 7);
    gpio_set_level(REGISTER_BIT6_ON_PIN, (disp_register & 0x40) >> 6);
    gpio_set_level(REGISTER_BIT5_ON_PIN, (disp_register & 0x20) >> 5);
    gpio_set_level(REGISTER_BIT4_ON_PIN, (disp_register & 0x10) >> 4);
    gpio_set_level(REGISTER_BIT3_ON_PIN, (disp_register & 0x08) >> 3);
    gpio_set_level(REGISTER_BIT2_ON_PIN, (disp_register & 0x04) >> 2);
    gpio_set_level(REGISTER_BIT1_ON_PIN, (disp_register & 0x02) >> 1);
    gpio_set_level(REGISTER_BIT0_ON_PIN, (disp_register & 0x01));
}

Then, after you wrote anything into your register, call your function to output it:

disp_register = 0x2A; // example value you want to send to display
copy_register_to_GPIO_pins(disp_register);

// or, output byte WITHOUT using any register:
copy_register_to_GPIO_pins(0x2A);

Hopefully, you can do the reverse by yourself, reading the pins is done by another function, where you copy each GPIO pin value and assemble it into byte variable. Of course, pins have to be set to inputs at this point. In principle:

/*
 * wrote this simply for ease of understanding
 */
unsigned char copy_GPIO_pins_to_register(void)
{
    unsigned char retval = 0;

    retval |= gpio_get_level(REGISTER_BIT7_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT6_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT5_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT4_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT3_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT2_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT1_ON_PIN);
    retval = retval << 1;
    retval |= gpio_get_level(REGISTER_BIT0_ON_PIN);

    return retval;
}

Answer 2

In order to minimize computational burden when operating the 8 pins, you will want these pins to correspond to consecutive GPIO numbers (e.g. GPIO12 to GPIO19). Below is an implementation that operates multiple input/output pins in parallel and works if the above requirement (consecutive GPIO numbers) is met and if GPIO numbers are all in the range 0-31; I used GPIO12 to GPIO19 (GPIO12 corresponds to bit 0 in the input/output 8-bit values), which are handy to use if you have an ESP32 dev board with an ESP-WROOM-32 or ESP32-WROVER module. So I defined the GPIO corresponding to bit 0 as below:

#define PARALLEL_0  12

At initialization, you need to configure all 8 pins a GPIOs, e.g. by setting them all as inputs:

void setup() {
  for (int i = 0; i < 8; i++) {
    pinMode(PARALLEL_0 + i, INPUT);
  }
}

After that, you can use the following functions to set the 8 pins as inputs or outputs, and to read the input values and write the output values:

void parallel_set_inputs(void) {
  REG_WRITE(GPIO_ENABLE_W1TC_REG, 0xFF << PARALLEL_0);
}

void parallel_set_outputs(void) {
  REG_WRITE(GPIO_ENABLE_W1TS_REG, 0xFF << PARALLEL_0);
}

uint8_t parallel_read(void) {
  uint32_t input = REG_READ(GPIO_IN_REG);

  return (input >> PARALLEL_0);
}

void parallel_write(uint8_t value) {
  uint32_t output =
    (REG_READ(GPIO_OUT_REG) & ~(0xFF << PARALLEL_0)) | (((uint32_t)value) << PARALLEL_0);

  REG_WRITE(GPIO_OUT_REG, output);
}

Answer 3

For high-bandwidth parallel data output, you might want to investigate the ESP32's I2S peripheral's LCD mode.

See section 12.5.1 in the ESP32 TRM and chapter 4 on mapping the peripheral to the desired pins. The nice thing about this approach is that you can map up to 24 bits of output from the peripheral to an output pin.

Section 12.4.4 states:

The ESP32 I2S module carries out a data-transmit operation [...] Clock out data serially, or in parallel, as configured by the user

Answer 4

Remember that you might need a 9th "strobe" pin for output to tell the receiving device that the other 8 pins are valid now.