6840.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "6840.h"

/*
 *	Motorola 6840 PTM
 */

struct ptm_timer {
    uint16_t timer;
    uint16_t wlatch;
    uint8_t ctrl;
    int output;
    int event;
};

struct m6840 {
    /* These are 1 based so we don't use entry 0. This makes the code
       easier to follow as the numbering matches the data sheet */
    struct ptm_timer timer[4];
    uint8_t sr;
    uint8_t msb;
    uint8_t lsb;
    uint8_t prescale;
    
    unsigned int trace;
    unsigned int lastout;
};

static void m6840_calc_irq(struct m6840 *ptm)
{
    int irq = 0;
    /* Turn our internal events for overflow into status bits */
    if (ptm->timer[1].event) {
        ptm->timer[1].event = 0;
        ptm->sr |= 1;
    }
    if (ptm->timer[2].event) {
        ptm->timer[2].event = 0;
        ptm->sr |= 2;
    }
    if (ptm->timer[3].event) {
        ptm->timer[3].event = 0;
        ptm->sr |= 4;
    }
    /* Check status versus masks and set the SR IRQ bit accordingly */
    if ((ptm->sr & 1) && (ptm->timer[1].ctrl & 0x40))
        irq = 0x80;
    if ((ptm->sr & 2) && (ptm->timer[2].ctrl & 0x40))
        irq = 0x80;
    if ((ptm->sr & 4) && (ptm->timer[3].ctrl & 0x40))
        irq = 0x80;
    ptm->sr &= 0x7F;
    ptm->sr |= irq;
}

int m6840_irq_pending(struct m6840 *ptm)
{
    return ptm->sr & 0x80;
}

/*
 *	Model the O1/O2/O3 pins
 */
static void m6840_calc_outputs(struct m6840 *ptm)
{
    unsigned int output = 0;
    int i;
    struct ptm_timer *p = &ptm->timer[1];
    for (i = 0; i <= 2; i++) { 
        if (p->output && (p->ctrl & 0x80))
            output |= (1 << i);
        p++;
    }
    if (output != ptm->lastout) {
        ptm->lastout = output;
        m6840_output_change(ptm, output);
    }
}

/* Count a timer in 16 or 8x8 bit mode */
static void m6840_timer_count(struct ptm_timer *p, int restart)
{
    if (!(p->ctrl & 0x04)) {
        /* The check occurs before the count down */
        if (p->timer == 0) {
            p->event = 1;
            p->output ^= 1;
            if (p->event && restart)
                p->timer = p->wlatch;
        }
        p->timer--;
    } else {
        if ((p->timer & 0xFF) != 0)
            p->timer--;
        else {
            p->timer &= 0xFF00;
            if (p->timer) {
                p->timer -= 0x0100;
                p->timer |= p->wlatch & 0xFF;
            } else {
                p->event = 1;
                if (restart)
                    p->timer = p->wlatch;
            }
        }
        if ((p->timer & 0xFF00) == 0)
            p->output = 1;
        else
            p->output = 0;
    }
    if (p->event && restart)
        p->timer = p->wlatch;
}

/*
 *	Handle a timer being clocked by something
 */
static void m6840_timer_clock(struct ptm_timer *p)
{
    switch((p->ctrl >> 3) & 7) {
        case 0:	/* Continuous */
            m6840_timer_count(p, 1);
            break;
        case 1:	/* Frequency compare (not supported yet) */
            break;
        case 2:	/* Continuous - not reset by write to latches */
            m6840_timer_count(p, 1);
            break;
        case 3:	/* Pulse width compare (not supported yet) */
            break;
        case 4:	/* One shot, reset by write to latches */
            m6840_timer_count(p, 0);
            break;
        case 5:	/* Frequency comparison (not supported yet) */
            break;
        case 6:	/* One shot, reset by gate/reset only */
            m6840_timer_count(p, 0);
        case 7:	/* Pulse width compare (not supported yet) */
            break;
    }
}

/* Perform an event tick on a timer */
static void m6840_event_tick(struct ptm_timer *p)
{
    /* Internal clock ? */
    if (p->ctrl & 2)
        return;
    m6840_timer_clock(p);
}

/* Perform internal tick on a timer */
static void m6840_timer_tick(struct ptm_timer *p)
{
    /* External clock */
    if (!(p->ctrl & 2))
        return;
    m6840_timer_clock(p);
}

/* Runs for every E clock */
void m6840_tick(struct m6840 *ptm, int tstates)
{
    while(tstates--) {
        m6840_timer_tick(&ptm->timer[1]);
        m6840_timer_tick(&ptm->timer[2]);
        m6840_timer_tick(&ptm->timer[3]);
    }
    m6840_calc_irq(ptm);
    m6840_calc_outputs(ptm);
}

/* External clock event */
void m6840_external_clock(struct m6840 *ptm, int timer)
{
    /* Timer 3 has an external pre-scaler option */
    if (timer == 3 && (ptm->timer[3].ctrl & 0x01)) {
        ptm->prescale++;
        ptm->prescale &= 7;
        if (ptm->prescale)
            return;
    }
    m6840_event_tick(&ptm->timer[timer]);
    m6840_calc_irq(ptm);
    m6840_calc_outputs(ptm);
}

/* High to low transition on gate */    
void m6840_external_gate(struct m6840 *ptm, int gate)
{
    if (ptm->timer[gate].ctrl & 8) {
        ptm->timer[gate].timer = ptm->timer[gate].wlatch;
        /* IRQ clear ? */
    }
}

static void m6840_soft_reset(struct m6840 *ptm)
{
    ptm->timer[0].timer = ptm->timer[0].wlatch;
    ptm->timer[1].timer = ptm->timer[0].wlatch;
    ptm->timer[2].timer = ptm->timer[0].wlatch;
    m6840_calc_irq(ptm);
    if (ptm->trace)
        fprintf(stderr, "[PTM] Reset.\n");
}

void m6840_reset(struct m6840 *ptm)
{
    ptm->timer[0].wlatch = 0xFFFF;
    ptm->timer[1].wlatch = 0xFFFF;
    ptm->timer[2].wlatch = 0xFFFF;
    m6840_soft_reset(ptm);
    ptm->lastout = 0x100;	/* Impossible value to force update */
}

uint8_t m6840_read(struct m6840 *ptm, uint8_t addr)
{
    struct ptm_timer *p;
    addr &= 7;
    if (addr == 0)
        return 0xFF;		/* Probably tri-stated */
    if (addr == 1) {
        if (ptm->trace)
            fprintf(stderr, "[PTM]: Read status register %02X\n", ptm->sr);
        return ptm->sr;
    }
    if (addr & 1)
        return ptm->lsb;
    addr >>= 1;
    p = &ptm->timer[addr];
    ptm->lsb = p->timer;
    ptm->sr &= ~(1 << (addr - 1));	/* And clear the interrupt */
    m6840_calc_irq(ptm);
    if (ptm->trace)
        fprintf(stderr, "[PTM] Read timer %d IRQ now %02X\n", addr, ptm->sr);
    return p->timer >> 8;
}

void m6840_write(struct m6840 *ptm, uint8_t addr, uint8_t val)
{
    struct ptm_timer *p;
    addr &= 7;
    if (addr > 1) {
        if ((addr & 1) == 0)
            ptm->msb = val;
        else {
            addr >>= 1;
            p = &ptm->timer[addr];
            p->wlatch = (ptm->msb << 8) | val;
            /* Writing the timer also clears the interrupt if CR3/4 are 0 */
            if (ptm->trace)
                fprintf(stderr, "[PTM] Timer %d set to %d\n", addr, p->wlatch);
            if ((p->ctrl & 0x18) == 0x00) {
                p->timer = p->wlatch;
                p->output = 0;
                ptm->sr &= ~(1 << addr);
                m6840_calc_irq(ptm);
                m6840_calc_outputs(ptm);
            }
        }
        return;
    }
    if (addr == 0) {
        if (ptm->timer[2].ctrl & 0x1)
            addr = 1;
        else
            addr = 3;
    } else
        addr = 2;
    if (ptm->trace)
        fprintf(stderr, "[PTM] Control %d set to %02X\n", addr, val);
    ptm->timer[addr].ctrl = val;
    /* Effects of control changes */
    if (addr == 1 && (val & 1))
        m6840_soft_reset(ptm);
    m6840_calc_irq(ptm);
}

void m6840_trace(struct m6840 *ptm, int onoff)
{
    ptm->trace = onoff;
}

struct m6840 *m6840_create(void)
{
    struct m6840 *ptm = malloc(sizeof(struct m6840));
    memset(ptm, 0x00, sizeof(struct m6840));
    m6840_reset(ptm);
    return ptm;
}

void m6840_free(struct m6840 *ptm)
{
    free(ptm);
}