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◆ riscv-1 x16-emulator / src / vera_spi.c
1.7 KiB · C · last changed main: while the CPU waits in WAI, step the machine to its next event at once 14 days ago source blame history raw
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// Commander X16 Emulator
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// Copyright (c) 2019 Michael Steil
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// All rights reserved. License: 2-clause BSD
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#include <stdio.h>
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#include <stdbool.h>
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#include "sdcard.h"
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#define SPI_CLOCK_RATE_MHZ 12.5f
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bool ss;
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bool busy;
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bool autotx;
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uint8_t sending_byte, received_byte;
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float outcounter;
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void
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vera_spi_init()
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{
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	ss = false;
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	busy = false;
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	autotx = false;
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	received_byte = 0xff;
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}
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void
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vera_spi_step(int mhz, int clocks)
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{
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	if (busy) {
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		outcounter += (float)clocks * SPI_CLOCK_RATE_MHZ / (float)mhz;
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		if (outcounter >= 10) { // A value of 9 here is closer to reality, but hardware
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		                        // can take slightly longer depending on clock-alignment.
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								// 10 cycles here should be safe and won't succeed in emulation
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								// while failing on hardware.
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			busy = false;
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			if (sdcard_attached) {
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				received_byte = sdcard_handle(sending_byte);
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			} else {
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				received_byte = 0xff;
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			}
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		}
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	}
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}
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// A byte is on its way: vera_spi_step() is counting clocks toward its end.
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bool
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vera_spi_busy(void)
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{
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	return busy;
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}
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uint8_t
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vera_spi_read(uint8_t reg)
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{
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	switch (reg) {
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		case 0:
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			if (autotx && ss && !busy) {
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				// autotx mode will automatically send $FF after each read
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				sending_byte = 0xff;
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				busy = true;
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				outcounter = 0;
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			}
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			return received_byte;
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		case 1:
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			return busy << 7 | autotx << 2 | ss;
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	}
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	return 0;
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}
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void
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vera_spi_write(uint8_t reg, uint8_t value)
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{
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	switch (reg) {
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		case 0:
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			if (ss && !busy) {
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				sending_byte = value;
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				busy = true;
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				outcounter = 0;
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			}
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			break;
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		case 1:
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			if (ss != (value & 1)) {
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				ss = value & 1;
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				if (ss) {
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					sdcard_select(ss);
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				}
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			}
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			autotx = !!(value & 4);
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			break;
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	}
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}