machine/attiny85: fix USI SPI data corruption

Remove USICLK from Transfer() loop - setting both USICLK and USITC
simultaneously caused double-shifting of data. USICLK is a strobe bit
that shifts data when written, while USITC toggles the clock pin.
With both set, data was shifted twice per iteration.

Fix: Configure USICR once in Configure() with mode bits, then only
toggle USITC in Transfer() using SetBits() to preserve configuration.

Also removed incorrect pin validation that would fail for PB0 (value 0).

Co-authored-by: Ona <no-reply@ona.com>

Author: jespino

src/machine/machine_attiny85.go

@@ -61,11 +61,10 @@ var SPI0 = &SPI{
 
 // Configure sets up the USI for SPI communication
 func (s *SPI) Configure(config SPIConfig) error {
-	// Validate configuration
+	// Validate configuration - check that USI registers are set
 	if s.usicr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) ||
 		s.usisr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) ||
-		s.usidr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) ||
-		s.sck == 0 || s.sdi == 0 || s.sdo == 0 || s.cs == 0 {
+		s.usidr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) {
 		return errSPIInvalidMachineConfig
 	}
 
@@ -84,17 +83,23 @@ func (s *SPI) Configure(config SPIConfig) error {
 	s.cs.High()
 	s.cs.Configure(PinConfig{Mode: PinOutput})
 
-	// Reset USI registers
+	// Reset USI data register
 	s.usidr.Set(0)
 	s.usisr.Set(0)
 
+	// Configure USI for SPI mode:
+	// - USIWM0: Three-wire mode (SPI)
+	// - USICS1: External clock source (software controlled via USITC)
+	// - USICLK: Clock strobe - enables counter increment on USITC toggle
+	//
 	// Note: ATTiny85 USI doesn't have configurable frequency dividers like dedicated SPI hardware
 	// The SPI clock speed is determined by how fast the software toggles the clock
 	// For now, we'll ignore the Frequency parameter as it runs at maximum software speed
-
+	//
 	// Note: LSBFirst and Mode configurations are not directly supported by USI
 	// These would need to be implemented in software if required
 	// For now, we use the standard MSB-first, Mode 0 (CPOL=0, CPHA=0)
+	s.usicr.Set(avr.USICR_USIWM0 | avr.USICR_USICS1 | avr.USICR_USICLK)
 
 	return nil
 }
@@ -110,20 +115,18 @@ func (s *SPI) Transfer(b byte) (byte, error) {
 	s.usisr.Set(avr.USISR_USIOIF)
 
 	// Clock the data out/in
-	// We need 16 clock edges (8 bits × 2 edges per bit)
+	// We need 16 clock toggles (8 bits × 2 edges per bit)
 	// The USI counter counts each clock edge, so it overflows at 16
+	//
+	// IMPORTANT: Only toggle USITC here, not USICLK!
+	// - USITC toggles the clock pin
+	// - With USICS1 set (software clock strobe mode), the data shifts on clock edges
+	// - USICLK is a separate strobe that would cause extra shifts if set here
+	//
+	// The USICR register was configured in Configure() with USIWM0 | USICS1.
+	// We use OR-assign to preserve that configuration and only set USITC.
 	for !s.usisr.HasBits(avr.USISR_USIOIF) {
-		// Toggle the clock pin and strobe the clock
-		// This single write causes the USI hardware to:
-		// 1. Toggle the SCK pin (via USITC bit)
-		// 2. Shift one bit out on DO and read one bit in on DI (via USICLK bit)
-		// 3. Increment the 4-bit counter
-		//
-		// USIWM0 = 1: Three-wire mode (SPI mode)
-		// USICS1 = 1: Software clock strobe (we control the clock)
-		// USICLK = 1: Clock strobe (shift the data)
-		// USITC = 1: Toggle the clock pin
-		s.usicr.Set(avr.USICR_USIWM0 | avr.USICR_USICS1 | avr.USICR_USICLK | avr.USICR_USITC)
+		s.usicr.SetBits(avr.USICR_USITC)
 	}
 
 	// After 8 bits are transferred, return the received byte