[stella] AtariVox/SaveKey question

[Thomas Jentzsch]

Bob Montgomery wrote:
> I can't quite figure out how the overflow-flag should be set -
> can you help me out?  When do I need an acknowledge bit?  Is it
> ever necessary?

Here is what Alex wrote me (I had the very same questions :-):

"Normally, after reading a byte, we transmit a 0 bit for acknowledge.  This
tells the EEPROM to continue transmitting data on future pulses of SCL. 
However, once we have read all the bytes we want, a 1 is transmitted for 
"not-acknowledge" instead, which stops the chip from sending any more data. 
(It's not possible to issue a "stop" signal while the EEPROM is trying to 
transmit.  The I2C protocol is a little clumsy.)

When writing the driver code, I found it simpler to put the ACK part at the 
start of i2c_rxbyte (to acknowledge a *previous* byte read).  So we only 
transmit the 0 when we *know* that more data is going to be read.  (The V 
flag indicates whether a byte has been read previously - so we don't 
transmit ACK before reading the first byte.)  The Not-ACK is then done in 
i2c_stopread.

If you destroy the V flag during EEPROM reads (which I imagine will be the 
case if you're spreading reads across frames), and if you always read the 
same number of bytes, it might be simpler to have 2 different "rxbyte" 
routines.  A normal "rxbyte" which always transmits an ACK (0) at the end, 
and a "rxlastbyte" which transmits a NACK (1) which is only used when 
reading the last byte of data."

Hope that helps.

Have fun!
Thomas                            
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Thomas Jentzsch         | *** Every bit is sacred ! ***
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