NAK I²C write when EDID protected

README.md

@@ -17,6 +17,11 @@ if the ERROR LED is on, the possible cause is one of the following:
 - tried reading the EDID from the monitor, but it is not connected
 - communication with monitor failed, due to damaged cable
 - monitor EDID is invalid
+- storing EDID in EEPROM failed
+
+the firewall only acts as an I²C EEPROM at address 0x50 toward the HDMI device to provide the EDID information.
+if the EDID switch is on the BLOCK position, the EEPROM is read only.
+if the EDID switch is on the ALLOW position, writing the EEPROM is possible over the HDMI connection using standard I²C write operations.
 
 limitations
 ===========

main.c

@@ -12,7 +12,7 @@
 #include "softi2c_master.h"
 
 // enable UART debug
-#define DEBUG 0
+#define DEBUG 1
 
 #define EEPROM_ADDR 0x4000 // EEPROM start address
 static bool eeprom_valid = true; // if the EDID can be read from EEPROM
@@ -48,8 +48,14 @@ static bool hpd_fwd = false; // if the I²C source line is connected to sink
 
 // if an I²C transaction started
 static volatile bool i2c_transaction_new = false;
-// if an I²C transaction with new data started
+// if an I²C transaction with incoming data started
 static volatile bool i2c_input_new = false;
+// if the byte should be programmed
+static volatile bool i2c_prog = false;
+// the address of the byte to be read or programmed
+static volatile uint8_t i2c_addr = 0;
+// the data to be programmed
+static volatile uint8_t i2c_data = 0;
 
 // blocking wait (in 10 us steps, up to UINT32_MAX / 10)
 static void wait_10us(uint32_t us10)
@@ -130,19 +136,21 @@ static uint16_t edid_length(const uint8_t* edid)
 
 	if (1 == edid[18]) { // EDID 1.3/1.4 128-byte structure
 		if (checksum_ok(&edid[0], 128)) { // ensure checksum of base EDID is ok
-			const uint16_t length = 128 + (1 + edid[126]) * 128; // get length with all extensions
-			puts("(v1.4 ");
+			const uint16_t length = 128 + edid[126] * 128; // get length with all extensions
+			puts("(v1.3/1.4, ");
 			putn(edid[126]  / 100);
 			putn((edid[126]  / 10) % 10);
 			putn(edid[126] % 10);
 			puts(" extension) ");
-			for (uint16_t i = 128; i < length && i < 256; i += 128) { // verify checksum of each extension (we actually only support one extension)
-				if (!checksum_ok(&edid[i], 128)) {
-					puts("extension CRC error\r\n");
-					return 0; // EDID is broken
+			if (edid[126]) { // extensions are present
+				for (uint16_t i = 128; i < length && i < 256; i += 128) { // verify checksum of each extension (we actually only support one extension)
+					if (!checksum_ok(&edid[i], 128)) {
+						puts("extension CRC error\r\n");
+						return 0; // EDID is broken
+					}
 				}
 			}
-			puts("CRC ok\r\n");
+			puts("CRC OK\r\n");
 			return length;
 		} else {
 			puts("base CRC error\r\n");
@@ -476,7 +484,7 @@ level pulse on the Hot Plug Detect pin. This pulse shall be at least 100 msec.
 
 	// verify stored EDID validity
 	if (!edid_length((uint8_t*)EEPROM_ADDR)) {
-		LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate I²C read fail
+		LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate EDID is invalid
 		puts("EEPROM EDID invalid\r\n");
 	}
 
@@ -494,6 +502,40 @@ level pulse on the Hot Plug Detect pin. This pulse shall be at least 100 msec.
 			}
 		}
 */
+		if (i2c_prog) { // received data over i2c to be programmed
+			if (EDID_PORT->IDR.reg & EDID_PIN) { // EDID switched off
+				puts("I²C prog disabled\r\n");
+				I2C_CR2 &= I2C_CR2_ACK; // NACK next received byte to indicate programming it disabled
+			} else { // EDID programming allowed
+/*
+				puts("I²C prog ");
+				puth(i2c_data);
+				puts("@");
+				puth(i2c_addr);
+				puts("\r\n");
+*/
+				IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
+				// disable DATA (e.g. EEPROM) write protection
+				if (0 == (FLASH_IAPSR & FLASH_IAPSR_DUL)) {
+					FLASH_DUKR = FLASH_DUKR_KEY1;
+					FLASH_DUKR = FLASH_DUKR_KEY2;
+				}
+				// save need EDID
+				*(uint8_t*)(EEPROM_ADDR + i2c_addr) = i2c_data; // write byte
+				while (FLASH_CR2 & FLASH_CR2_WPRG); // wait for write to complete
+				// check if programming failed
+				// we don't check for WR_PG_DIS (while checking EOP) because EEPROM isn't (and can't be) write protected
+				if (!(FLASH_IAPSR & FLASH_IAPSR_EOP)) {
+					FLASH_IAPSR &= ~FLASH_IAPSR_DUL; // re-enable write protection
+					LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate programming failed
+					I2C_CR2 &= I2C_CR2_ACK; // NACK next received byte to indicate programming error
+					puts("EEPROM byte prog failed\r\n");
+				}
+				FLASH_IAPSR &= ~FLASH_IAPSR_DUL; // re-enable write protection
+			}
+			action = true; // re-run loop
+			i2c_prog = false; // clear flag
+		}
 		if (action) { // something has been performed, check if other flags have been set meanwhile
 			action = false; // clear flag
 		} else { // nothing down
@@ -510,25 +552,24 @@ void awu(void) __interrupt(IRQ_AWU) // auto wakeup
 
 void i2c(void) __interrupt(IRQ_I2C) // auto wakeup
 {
-	static uint8_t addr = 0; // EEPROM address to read
 	// make copies of status registers, since some bits might be cleared meanwhile
 	const uint8_t sr1 = I2C_SR1;
 	const uint8_t sr2 = I2C_SR2;
 	const uint8_t sr3 = I2C_SR3; // clears ADDR after reading SR1
 	if (sr1 & I2C_SR1_TXE) { // transmission buffer is empty
-		// transmit next byte
-		if (eeprom_valid) {
-			I2C_DR = *(uint8_t*)(EEPROM_ADDR + addr++);
-		} else {
-			I2C_DR = 0xff;
-		}
+		I2C_DR = *(uint8_t*)(EEPROM_ADDR + i2c_addr++); // transmit next byte (even if invalid)
 	}
 	if (sr1 & I2C_SR1_RXNE) { // receive buffer is full
-		const uint8_t data = I2C_DR; // read data (also clears flag)
-		if (I2C_CR2 & I2C_CR2_ACK) {
-			addr = data; // we only take the first address byte
+		i2c_data = I2C_DR; // read data (also clears flag)
+		if (i2c_input_new) { // we just received the first byte
+			i2c_addr = i2c_data; // we only take the first address byte
+			i2c_input_new = false; // next byte is not the first
+		} else { // received data byte
+			i2c_prog = true; // notify main loop data needs to be programmed
+		}
+		if (EDID_PORT->IDR.reg & EDID_PIN) { // EDID programming is not enabled
+			I2C_CR2 &= I2C_CR2_ACK; // NACK next received byte to indicate programming it disabled
 		}
-		I2C_CR2 &= I2C_CR2_ACK; // NACK next received byte
 	}
 	if (sr1 & I2C_SR1_STOPF) { // stop received
 		I2C_CR2 |= I2C_CR2_ACK; // this is just to clear the flag
@@ -536,14 +577,14 @@ void i2c(void) __interrupt(IRQ_I2C) // auto wakeup
 	if (sr1 & I2C_SR1_ADDR) { // our slave address has been selected
 		i2c_transaction_new = true; // notify main loop transaction started
 		if (sr3 & I2C_SR3_TRA) { // start data transmission
-			if (eeprom_valid) {
-				I2C_DR = *(uint8_t*)(EEPROM_ADDR + addr++); // transmit selected byte
-			} else {
-				I2C_DR = 0xff;
-			}
+			I2C_DR = *(uint8_t*)(EEPROM_ADDR + i2c_addr++); // transmit selected byte
 			i2c_input_new = false; // notify we send data
 		} else { // we will receive data
-			I2C_CR2 |= I2C_CR2_ACK; // ACK next received byte
+			if (EDID_PORT->IDR.reg & EDID_PIN) { // EDID switched off
+				I2C_CR2 &= I2C_CR2_ACK; // NACK next received byte to indicate programming it disabled
+			} else {
+				I2C_CR2 |= I2C_CR2_ACK; // ACK next received byte
+			}
 			i2c_input_new = true; // notify we get data
 		}
 	}