Advance CMOS/I2C to a seemingly-valid read.

Components/I2C/I2C.cpp

@@ -24,7 +24,7 @@ void Bus::set_data(bool pulled) {
 bool Bus::data() {
 	bool result = data_;
 	if(peripheral_bits_) {
-		result |= !(peripheral_response_ & 0x80);
+		result |= !(peripheral_response_ & 0x200);
 	}
 	return result;
 }
@@ -41,35 +41,33 @@ void Bus::set_clock_data(bool clock_pulled, bool data_pulled) {
 		return;
 	}
 
-	auto info = logger.info();
-	info.append("C:%d D:%d; ", clock_, data_);
-
 	const bool prior_data = data_;
 	clock_ = clock_pulled;
 	data_ = data_pulled;
 
 	if(clock_) {
-		info.append("nothing");
 		return;
 	}
 
 	if(prior_data != data_) {
 		if(data_) {
-			info.append("start");
+			logger.info().append("S");
 			signal(Event::Start);
 		} else {
-			info.append("stop");
+			logger.info().append("P");
 			signal(Event::Stop);
 		}
 	} else {
-		--peripheral_bits_;
+		if(peripheral_bits_) {
-		peripheral_response_ <<= 1;
+			--peripheral_bits_;
+			peripheral_response_ <<= 1;
+		}
 
 		if(data_) {
-			info.append("zero");
+			logger.info().append("0");
 			signal(Event::Zero);
 		} else {
-			info.append("one");
+			logger.info().append("1");
 			signal(Event::One);
 		}
 	}
@@ -77,75 +75,95 @@ void Bus::set_clock_data(bool clock_pulled, bool data_pulled) {
 
 void Bus::signal(Event event) {
 	const auto capture_bit = [&]() {
-		if(event == Event::Zero || event == Event::One) {
+		input_ = uint16_t((input_ << 1) | (event == Event::Zero ? 0 : 1));
-			input_ = uint16_t((input_ << 1) | (event == Event::Zero ? 0 : 1));
+		++input_count_;
-			++input_count_;
+	};
+
+	const auto acknowledge = [&]() {
+		// Post an acknowledgement bit.
+		peripheral_response_ = 0;
+		peripheral_bits_ = 2;
+	};
+
+	const auto set_state = [&](State state) {
+		state_ = state;
+		input_count_ = 0;
+		input_ = 0;
+	};
+
+	const auto enqueue = [&](std::optional<uint8_t> next) {
+		if(next) {
+			peripheral_response_ = *next;
+			peripheral_bits_ = 9;
+			set_state(State::PostingByte);
+		} else {
+			acknowledge();
+			set_state(State::AwaitingAddress);
 		}
 	};
 
-	const auto await_byte = [&] {
+	// Allow start and stop conditions at any time.
-		peripheral_response_ = 0;
+	if(event == Event::Start) {
-		peripheral_bits_ = 2;
+		set_state(State::CollectingAddress);
-		phase_ = Phase::AwaitingByte;
+		active_peripheral_ = nullptr;
-		logger.info().append("Waiting for byte");
+		return;
-	};
-
-	// Check for stop condition at any time.
-	// "A LOW-to-HIGH transition of the data line
-	// while the clock is HIGH is defined as the STOP condition".
-	if(event == Event::Stop) {
-		logger.info().append("Stopped");
-		phase_ = Phase::AwaitingStart;
 	}
 
-	switch(phase_) {
+	if(event == Event::Stop) {
-		case Phase::AwaitingStart:
+		set_state(State::AwaitingAddress);
-			// "A HIGH-to-LOW transition of the data line while
+		if(active_peripheral_) {
-			// the clock is HIGH is defined as the START condition"
+			active_peripheral_->stop();
-			if(event == Event::Start) {
+		}
-				phase_ = Phase::CollectingAddress;
+		active_peripheral_ = nullptr;
-				input_count_ = 0;
+		return;
-				input_ = 0;
+	}
-				logger.info().append("Waiting for [remainder of] address");
-			}
-		break;
 
-		case Phase::CollectingAddress:
+	switch(state_) {
+		case State::AwaitingAddress:	break;
+
+		case State::CollectingAddress:
 			capture_bit();
-			if(input_count_ == 8) {
-				logger.info().append("Addressed %02x with %s?", uint8_t(input_) & 0xfe, input_ & 1 ? "read" : "write");
 
+			if(input_count_ == 8) {
 				auto pair = peripherals_.find(uint8_t(input_) & 0xfe);
 				if(pair != peripherals_.end()) {
 					active_peripheral_ = pair->second;
+					active_peripheral_->start(input_ & 1);
 
-					await_byte();
+					if(input_&1) {
+						enqueue(active_peripheral_->read());
+					} else {
+						acknowledge();
+						set_state(State::ReceivingByte);
+					}
 				} else {
-					phase_ = Phase::AwaitingStart;
+					state_ = State::AwaitingAddress;
-					logger.info().append("No device; not acknowledging");
 				}
 			}
 		break;
 
-		case Phase::AwaitingByte:
+		case State::ReceivingByte:
 			// Run down the clock on the acknowledge bit.
 			if(peripheral_bits_) {
 				return;
 			}
 
-			logger.info().append("Beginning byte");
-			phase_ = Phase::CollectingByte;
-			input_count_ = 0;
-		[[fallthrough]];
-
-		case Phase::CollectingByte:
 			capture_bit();
 			if(input_count_ == 8) {
-				logger.info().append("Got byte %02x?", uint8_t(input_));
+				active_peripheral_->write(static_cast<uint8_t>(input_));
-
+				acknowledge();
-				await_byte();
+				set_state(State::ReceivingByte);
 			}
 		break;
+
+		case State::PostingByte:
+			// Finish whatever is enqueued.
+			if(peripheral_bits_) {
+				return;
+			}
+
+			enqueue(active_peripheral_->read());
+		break;
 	}
 }
 

Components/I2C/I2C.hpp

@@ -9,14 +9,18 @@
 #pragma once
 
 #include <cstdint>
+#include <optional>
 #include <unordered_map>
 
 namespace I2C {
 
 /// Provides the virtual interface for an I2C peripheral; attaching this to a bus
 /// provides automatic protocol handling.
-class Peripheral {
+struct Peripheral {
-
+	virtual void start([[maybe_unused]] bool is_read) {}
+	virtual void stop() {}
+	virtual std::optional<uint8_t> read() { return std::nullopt; }
+	virtual void write(uint8_t) {}
 };
 
 class Bus {
@@ -48,12 +52,13 @@ private:
 	};
 	void signal(Event);
 
-	enum class Phase {
+	enum class State {
-		AwaitingStart,
+		AwaitingAddress,
 		CollectingAddress,
-		AwaitingByte,
+
-		CollectingByte,
+		PostingByte,
-	} phase_ = Phase::AwaitingStart;
+		ReceivingByte,
+	} state_ = State::AwaitingAddress;
 };
 
 }

Machines/Acorn/Archimedes/CMOSRAM.hpp

@@ -13,7 +13,25 @@
 namespace Archimedes {
 
 struct CMOSRAM: public I2C::Peripheral {
-	// All TODO.
+	void start(bool is_read) override {
+		expecting_address_ = !is_read;
+	}
+
+	std::optional<uint8_t> read() override {
+		return 0;
+	}
+
+	void write(uint8_t value) override {
+		if(expecting_address_) {
+			address_ = value;
+		} else {
+			// TODO: write to RAM.
+		}
+	}
+
+private:
+	bool expecting_address_ = false;
+	uint8_t address_;
 };
 
 }