What is the exact meaning of the Initial Power-Up Section of the MAX7219 data sheet to be found on Page 7?
It says:
On initial power-up, all control registers are reset, the display is blanked, and the MAX7219/MAX7221 enter shutdown mode. Program the display driver prior to display use. Otherwise, it will initially be set to scan one digit, it will not decode data in the data registers, and the intensity register will be set to its minimum value.
Most of this is clear. But specifically, what does the "the display is blanked" mean? Does it mean that the MAX7219 internal SRAM is cleared as part of the power-up sequence or does it mean that because the display is set to shutdown mode by power-up the display will be blank.
I have sought widely and cannot find a definitive statement. Which of these interpretations is correct, or perhaps is there another interpretation?
Why does it matter? Idle curiosity really. My read of the datasheet was that the RAM was cleared and hence did not expect to have to manually clear it.
So I didn't, and was wondering where the flash of random data at power up in my bank of displays was coming from, prior to the first display data write. And I was a bit too slow in accepting I got reading the datasheet wrong, or the datasheet didn't apply to the clone I was using.
I was wondering if someone had a definitive answer from some deep investigation or perhaps insider information. It seems not, and it may not be consistent anyway.
For the clone MAX7219s in my collection, the SRAM is not cleared on power-up. I don't know if that is the case for genuine MAX7219 or for different clones for that matter. In future, I will just assume not and zero out the SRAM manually.
That point about a pull-up resistor on the chip select(CS) line to mitigate erratic behaviour is an excellent one. Random noise here can easily generate unexpected results.
It should be noted though that if the CS line is not the default Slave Select (SS) of the SPI pin group, it is not enough to use a pull-up resistor alone.
The SPI begin function takes care of the case that the default SS pin will glitch low as it is made an output if the corresponding bit in the output register is not set to 1 prior to setting the pin to output mode.
For the default SS pin, begin() does a digitalWrite HIGH first, setting the internal pullup resistor on (and the output register bit to 1) then a pinMode to OUTPUT, thus if the SS pin is already pulled high by a resistor there are no glitches.
However, if the CS pin is a pin other than the designated SS, then the same process must be undertaken by user code. If forgotten, even a pulled up pin will glitch low as the output register, with the bit set to 0, is transferred through to the pin when it is configured as output.
It is easily fixed using the same method as the SPI library uses, digitalWrite a HIGH to the CS pin, then set that pin to OUTPUT. And pull it up, of course.
