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	// initialize
	spi_bus_config_t buscfg = {
		.miso_io_num=PIN_NUM_SPI_MISO, .mosi_io_num=PIN_NUM_SPI_MOSI, .sclk_io_num=PIN_NUM_SPI_CLK,
		.quadwp_io_num=-1, .quadhd_io_num=-1, .max_transfer_sz=1024
	};
	spi_device_interface_config_t devcfg = {
		.spics_io_num=PIN_NUM_SPI_CS,
		.clock_speed_hz=4000000, // 4MHz max
		.mode=1, // SPI MODE 1
		.command_bits=8, // 8-bit command length
		.queue_size=1,
		.cs_ena_pretrans = 1,
		.flags = SPI_DEVICE_HALFDUPLEX
	};

	spi_bus_initialize(VSPI_HOST, &buscfg, 1 /*dma_chan - 0 no DMA*/);
	ESP_ERROR_CHECK(spi_bus_add_device(VSPI_HOST, &devcfg, &spi));
	
	// transmit
	uint8_t N = 1; // ****** set N to 0, 1, 2, ...
	if (N > 0) { // skip if N == 0
	spi_transaction_t transaction;
	memset(&transaction, 0, sizeof(transaction));
	transaction.cmd = 0x81;
	transaction.length = N*8;
	transaction.flags = SPI_TRANS_USE_TXDATA;
	transaction.tx_data[0] = 0x00;
	transaction.tx_data[1] = 0x00;
	transaction.tx_data[2] = 0x00;
	transaction.tx_data[3] = 0x00;
	assert(spi_device_polling_transmit(spi, &transaction) == ESP_OK);
	ESP_LOGI(TAG, "Transmitted: %d bytes [%02x %02x %02x %02x]", N, transaction.tx_data[0], transaction.tx_data[1], transaction.tx_data[2], transaction.tx_data[3]);
	}
	
	// receive
	char * buffer = heap_caps_malloc(8, MALLOC_CAP_DMA);
	spi_transaction_t transaction;
	memset(&transaction, 0, sizeof(transaction));
	transaction.cmd = 0x01;
	transaction.rx_buffer = buffer;
	transaction.rxlength = 8 * 8;
	transaction.length = transaction.rxlength;
	assert(spi_device_polling_transmit(spi, &transaction) == ESP_OK);
	ESP_LOGI(TAG, "Received: %02x %02x %02x %02x %02x %02x %02x %02x", buffer[0], buffer[1], buffer[2], buffer[3], buffer[4], buffer[5], buffer[6], buffer[7]);