ascii art of hardware

  4 x 1N4148

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USI irq response time
---------------------

USI hardware does hold SCL line down  after detecting start condition or if
configured, SCL line is also held when counter overflow occur.  This feature
is know as clock stretching or clock extending.  More information about this
feature can be found in SMBus specification or TWI/i2c specifications.

Most popular raspberry pi have a serious bug in i2c hardware. Clock
stretching feature can break correct sampling of i2c data.

To get i2c run without clock stretching at 100kHz, maximal irq response time
must be below half of period of i2c clock.  At 100kHz this is 5uS.  For CPU
at 9.6MHz this is 48 ticks.

Because other interrupts (from timer or pin change) can block USI hardware,
these interrupts must reenable global interrupts.  This is known as nested
interrupts and can be enabled in ISR macro by ISR_NOBLOCK attribute (more
info in avr-libc documentation).  At worst case response time can be
calculated:

1. main code run RET instruction (4 ticks) and this is interrupted

2. timer irq starts, this generates (4 ticks) for jump to isr vector
3. isr vector does RJMP at isr code (2 ticks)
4. isr immediately run SEI (1 tick)
5. next instruction is usually PUSH, and can not be interrupted (2 ticks)

6. new irq starts from PCI (4 ticks)
7. isr vector does RJMP at isr code (2 ticks)
8. isr immediately run sei (1 tick)
9. next instruction is usually push, and can not be interrupted (2 ticks)

10. USI overflow irq (4 ticks) for jump to isr vector
11. isr vector does RJMP at isr code (2 ticks)

This all is 28 ticks. 

USI overflow must use switch to get  proper function:
        out     __GPIOR1__,r30	;1 tick
        in      r30,__SREG__	;1 tick	
        out     __GPIOR2__,r30	;1 tick
        out     __EEDR__,r31	;1 tick

//switch based om r14 value
        mov     r30,r14		;1 tick
        clr     r31		;1 tick
        ijmp			;2 ticks

This is 8 ticks, all together 36 ticks. 
ACK/NACK or data must be loaded and started by USI almost 12 ticks from code
addressed by ijmp. 


Some time (two ticks)  can be saved by moving USI overflow vector directly
at position 0x1c in flash. This can be done by linker script (check
file attiny25_i2c.x and i2c_fast.S).

Worst function ftom ijmp switch now can use 14 ticks, actual code (check
i2c_fast.S) uses maximum 9 ticks. 





Supported bus protocol:

quick command
 0 PEC reset
 1 no action

send byte
 no action, PEC update only

send byte with PEC 
 no action, PEC is not checked, is used like normal byte

receive byte
 return 1st byte from response buffer

receive byte with PEC
  return 1st byte from response buffer, PEC is replaced by 2nd byte from
  buffer (PEC is wrong or in some cases  valid..)






rapsberry pi support for i2c/SMBus
I2C                              yes
SMBus Quick Command              yes
SMBus Send Byte                  yes
SMBus Receive Byte               yes
SMBus Write Byte                 yes
SMBus Read Byte                  yes
SMBus Write Word                 yes
SMBus Read Word                  yes
SMBus Process Call               yes
SMBus Block Write                yes
SMBus Block Read                 no
SMBus Block Process Call         no
SMBus PEC                        yes
I2C Block Write                  yes
I2C Block Read                   yes