- fixed mistake tying half the tri-state buffer bits to always on

[romdev] / fw / src / main.c

diff --git a/fw/src/main.c b/fw/src/main.c
index c07b34e..1d248f2 100644 (file)
--- a/fw/src/main.c
+++ b/fw/src/main.c
@@ -34,6 +34,7 @@ static void end_prog(void);
 static void write_ram(uint16_t addr, uint8_t val);
 static uint8_t read_ram(uint16_t addr);
 static void set_data_bus(unsigned char val);
+static void release_data_bus(void);
 static void set_addr_bus(uint16_t addr);
 static void sys_reset(void);
 static inline void iodelay(void);
@@ -48,12 +49,12 @@ static unsigned char inp_cidx;
 
 int main(void)
 {
-       /* SPI (SS/MOSI/SCK) are outputs, as are the low 2 data bus bits */
-       DDRB = PB_SS | PB_MOSI | PB_SCK | 3;
+       /* SPI (SS/MOSI/SCK) are outputs */
+       DDRB = PB_SS | PB_MOSI | PB_SCK;
        PORTB = PB_SS;  /* drive SS high when not in programming mode */
        DDRC = 0xff;    /* control signals are all outputs */
        PORTC = PC_CS | PC_WE | PC_OE | PC_SYSRST;
-       DDRD = 0xff;    /* always drive the (high bits of the) data bus */
+       DDRD = 0;       /* tri-state the data bus by default (high 6 bits, low 2 are in DDRB) */
        PORTD = 0;
 
        /* disable all pullups */
@@ -184,6 +185,7 @@ static void write_ram(uint16_t addr, uint8_t val)
        iodelay();
        PORTC |= PC_CS;
        PORTC |= PC_WE;
+       release_data_bus();
 }
 
 static uint8_t read_ram(uint16_t addr)
@@ -193,15 +195,24 @@ static uint8_t read_ram(uint16_t addr)
        PORTC |= PC_WE;
        PORTC &= ~(PC_CS | PC_OE);
        iodelay();
-       val = (PORTD & 0xfe) | (PORTB & 3);
+       val = (PIND & 0xfc) | (PINB & 3);
        PORTC |= PC_CS | PC_OE;
        return val;
 }
 
 static void set_data_bus(unsigned char val)
 {
-       PORTB = (PORTB & 0xfe) | (val & 3);
-       PORTC = val;
+       /* drive the data bus */
+       DDRD = 0xff;
+       DDRB |= 3;
+       PORTB = (PORTB & 0xfc) | (val & 3);
+       PORTD = val;
+}
+
+static void release_data_bus(void)
+{
+       DDRD = 0;
+       DDRB &= 0xfc;
 }
 
 static void set_addr_bus(uint16_t addr)