avr: Switch to input state prior to enabling pullup in gpio_in_reset()

If switching a pin from output low to input with pullup, there is an
intermediate state of either driven high or high impedance without a
pullup.  Similarly, when switching from output high to input without a
pullup, there is an intermediate state of either driven low or high
impedence with a pullup.  In both cases it is preferable for the
latter transition.

Also, calculate the final setting prior to making any changes to
reduce the time in that intermediate state.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/avr/gpio.c b/src/avr/gpio.c
index d10fc2a1..f5225177 100644
--- a/src/avr/gpio.c
+++ b/src/avr/gpio.c
@@ -70,8 +70,10 @@ void
 gpio_out_reset(struct gpio_out g, uint8_t val)
 {
     irqstatus_t flag = irq_save();
-    g.regs->out = val ? (g.regs->out | g.bit) : (g.regs->out & ~g.bit);
-    g.regs->mode |= g.bit;
+    uint8_t newmode = g.regs->mode | g.bit;
+    uint8_t newout = val ? (g.regs->out | g.bit) : (g.regs->out & ~g.bit);
+    g.regs->out = newout;
+    g.regs->mode = newmode;
     irq_restore(flag);
 }
 
@@ -114,8 +116,10 @@ void
 gpio_in_reset(struct gpio_in g, int8_t pull_up)
 {
     irqstatus_t flag = irq_save();
-    g.regs->out = pull_up > 0 ? (g.regs->out | g.bit) : (g.regs->out & ~g.bit);
-    g.regs->mode &= ~g.bit;
+    uint8_t newout = pull_up>0 ? (g.regs->out | g.bit) : (g.regs->out & ~g.bit);
+    uint8_t newmode = g.regs->mode & ~g.bit;
+    g.regs->mode = newmode;
+    g.regs->out = newout;
     irq_restore(flag);
 }