Gergely L?rincz wrote:
it took me a goog few weeks to realise that the LOAD
mode of the 1802
is in fact the reset mode, an the Cosmac Elf's LOAD mode is the DMA IN mode
of the microprocessor. I wish I knew that!
Not so fast. LOAD mode and RESET are two different things, and the LOAD
mode of the 1802 is NOT the same as RESET. LOAD mode is entered by
asserting WAIT (driving it low) during RESET, but just being in RESET is
not by itself LOAD mode. This is true whether it's an ELf or some other
1802 system.
In LOAD mode, whether an 1802 or otherwise, you are not in "DMA IN"
mode; there isn't even such a "mode". you have to be in actual LOAD
mode, as defined by the CLEAR and WAIT inputs both being low, then pulse
the DMA IN signal (pin 38) to strobe each byte.
If you try to just use DMA IN mode for loading, without being in LOAD
mode, you'll have problems.
From the datasheet (DIP pin numbers):
pin 3 pin 2
CLEAR WAIT Mode
----- ----- ----
low low LOAD
low high RESET
high low PAUSE
high high RUN
CLEAR, WAIT, and DMA IN are active low, shown with an overbar in the
datasheet. The "low" and "high" levels refer to the actual voltages,
low being near Vss (ground), and high being near Vdd (positive supply).