ben-blinkenlights/libubb/swuart.c

/*
 * swuart/swuart.c - Software-implemented UART for UBB
 *
 * Written 2012 by Werner Almesberger
 * Copyright 2012 Werner Almesberger
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 */


#include <stdbool.h>
#include <string.h>

#include <ubb/ubb.h>
#include <ubb/regs4740.h>
#include <ubb/swuart.h>


#define	TIMER	7


static uint32_t tx_mask, rx_mask;
static uint32_t ticks;


/* ----- Hardware timer ---------------------------------------------------- */


/*
 * We run the hardware timer at PCLK/16 = 7 MHz. This gives us 60 cycles at
 * 115200 bps and sets the lowest data rate to 106.8 bps.
 */

#define	PCLK	(112*1000*1000)		/* 112 MHz */
#define	TCLK	(PCLK/16)		/*   7 MHz */


static void get_timer(uint32_t cycle)
{
	TSCR = 1 << TIMER;		/* enable clock to timer */
	TDFR(TIMER) = cycle-1;		/* cycle = one bit time */
	TCSR(TIMER) = (2 << 3) | 1;	/* count at PCLK/16 */
}


static void reset_timer(void)
{
	TECR = 1 << TIMER;		/* disable timer */
	TCNT(TIMER) = 0;		/* reset timer */
	TFCR = 0x10001 << TIMER;	/* clear flags */
	TESR = 1 << TIMER;		/* enable timer */
}


static void set_half_cycle(uint32_t cycle)
{
	uint32_t t;

	t = (uint16_t) TCNT(TIMER)+(cycle >> 1);
					/* TCNT+cycle/2 mod cycle-1 */
	if (t >= cycle-1)
		t -= cycle-1;
	TDHR(TIMER) = t;
	TFCR = 0x10000 << TIMER;		/* clear half flags */
}


static void release_timer(void)
{
	TECR = 1 << TIMER;		/* disable timer */
	TFCR = 0x10001 << TIMER;	/* clear flags */
	TSSR = 1 << TIMER;		/* disable clock to timer */
}


static inline bool timer_full(void)
{
	if (!(TFR & (1 << TIMER)))
		return 0;
	TFCR = 1 << TIMER;
	return 1;
}


static inline bool timer_half(void)
{
	if (!(TFR & (0x10000 << TIMER)))
		return 0;
	TFCR = 0x10000 << TIMER;
	return 1;
}


/* ----- Enable/disable interrupts ----------------------------------------- */


static uint32_t old_icmr;


static void disable_interrupts(void)
{
	old_icmr = ICMR;
	ICMSR = 0xffffffff;
}


static void enable_interrupts(void)
{
	ICMCR = ~old_icmr;
}


/* ----- Transmit a block -------------------------------------------------- */


static struct swuart_err errors;


static int trx(uint8_t *out, int out_size, uint8_t *in, int in_size,
    unsigned wait_bits, unsigned idle_bits)
{
	unsigned tx, rx = 0, idle = 0;
	int tx_bits = 0, rx_bits = 0;
	int in_len = 0;

	reset_timer();
	while (1) {
full:
		/* timer has just crossed FULL */

		if (tx_bits) {
			if (tx & 1)
				SET(tx_mask);
			else
				CLR(tx_mask);
			tx >>= 1;
			tx_bits--;
		} else {
			if (out_size) {
				CLR(tx_mask);
				tx = *out++ | 0x100;
				out_size--;
				tx_bits = 9;
			} else  {
				if (idle++ == wait_bits)
					return in_len;
			}
		}

		/* wait for reception (if any) */

		if (rx_bits) {
			while (!timer_half());
		} else {
			while (!timer_full())
				if (!PIN(rx_mask)) {
					set_half_cycle(ticks);
					rx_bits = 10;
					break;
				}
			if (!rx_bits)
				continue;
			while (!timer_half())
				if (timer_full())
					goto full;
		}

		/* timer has just crossed HALF */

		rx = (rx >> 1) | (PIN(rx_mask) ? 0x200 : 0);
		if (!--rx_bits) {
			if (rx & 1) {
				errors.glitch++;
			} else if (!(rx & 0x200)) {
				errors.framing++;
			} else if (in_len == in_size) {
				errors.overflow++;
			} else {
				*in++ = rx >> 1;
				in_len++;
				idle = 0;
				wait_bits = idle_bits;
			}
		}

		while (!timer_full());
	}
}


int swuart_trx(void *out, int out_size, void *in, int in_size,
    int wait_bits, int idle_bits)
{
	int got;

	disable_interrupts();
	get_timer(ticks);

	got = trx(out, out_size, in, in_size, wait_bits, idle_bits);

	release_timer();
	enable_interrupts();

	return got;
}


/* ----- Access error counters --------------------------------------------- */


unsigned swuart_get_errors(struct swuart_err *res)
{
	unsigned sum = errors.glitch+errors.framing+errors.overflow;

	if (res)
		*res = errors;
	return sum;
}


void swuart_clear_errors(void)
{
	memset(&errors, 0, sizeof(errors));
}


/* ----- Open/close the software UART -------------------------------------- */


int swuart_open(uint32_t tx, uint32_t rx, int bps)
{
	ticks = TCLK/bps-1;
	tx_mask = tx;
	rx_mask = rx;

	ubb_power(1);
	OUT(tx);
	SET(tx);
	IN(rx);

	return 0;
}


void swuart_close(void)
{
}