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SPI interface circuit
Work in progress...
This is an interface circuit for connecting a parallel interface LCD display with SPI.
The circuit is based on SpriteMods circuit.
74HC4094
+--------------+
| |
MOSI o-------------------------------------------------------| D QP0 |-------------o DB0
| |
SCLK o---------+---------------------------------------------| CP QP1 |-------------o DB1
| | |
CSn o------+ | Vcc o---| OE QP2 |-------------o DB2
| | | |
| | 74HC4040 | QP3 |-------------o DB3
| | +--------------+ | |
| | | | | QP4 |-------------o DB4
| | | Q0 |-- | |
| | | | | QP5 |-------------o DB5
| | | Q1 |-- | |
| | | | | QP6 |-------------o DB6
| | | Q2 |-------------+-----------| STR |
| | | | | | QP7 |-------------o DB7
| | | Q3 |-- | | |
| | | | | | |
| | | Q4 |-- | | QS1 |--
| | | | | | |
| | | Q5 |-- | | QS2 |--
| +----| CP | | | |
| | Q6 |-- | +--------------+
+-------| MR | |
| | Q7 |-- |
| | | | 74HC04
| | Q8 |-- +-----------------|>o--------------------o WR
| | |
| | Q9 |-- Vcc o--o RD
| | |
| | Q10 |--
| | |
| | Q11 |--
| | |
| +--------------+
|
|
+-----------------------------------------------------------------------------o CS
The circuit works at 32MHz on a breadboard.
I need to look into these timing constraints
- the time from the shift register data is stable to the time WR goes HIGH has to be long enough.
- clock signal delay through the 4040. It has to pull STR LOW before the clock rises again.
There is 5 inverters left in the 74HC04 Hex Inverter IC. These can be used to drive a LCD backlight.
- Backlight pulled to +3.3V. It can deliver 4x25mA = 100mA
- Backlight pulled to GND. Remove the first inverter and put it in parallel. It can sink 5x20mA = 100mA
led o-----|>o----+--|>o--+-----o LCD backlight
| |
+--|>o--+
| |
+--|>o--+
| |
+--|>o--+
The circuit is tested with this display. I get ~17fps at 32MHz (parallel ~25 fps).
I haven't updated the itdb28fb driver to support SPI yet, so I use flexfb for now
modprobe fbtft_device name=flexfb gpios=reset:25,dc:24,led:18
modprobe flexfb debug=3 rotate=0 width=240 height=320 regwidth=16 setaddrwin=1 init=-1,0x00E3,0x3008,-1,0x00E7,0x0012,-1,0x00EF,0x1231,-1,0x0001,0x0100,-1,0x0002,0x0700,-1,0x0003,0x1030,-1,0x0004,0x0000,-1,0x0008,0x0207,-1,0x0009,0x0000,-1,0x000A,0x0000,-1,0x000C,0x0000,-1,0x000D,0x0000,-1,0x000F,0x0000,-1,0x0010,0x0000,-1,0x0011,0x0007,-1,0x0012,0x0000,-1,0x0013,0x0000,-2,200,-1,0x0010,0x1690,-1,0x0011,0x0223,-2,50,-1,0x0012,0x000D,-2,50,-1,0x0013,0x1200,-1,0x0029,0x000A,-1,0x002B,0x000C,-2,50,-1,0x0020,0x0000,-1,0x0021,0x0000,-1,0x0030,0x0000,-1,0x0031,0x0506,-1,0x0032,0x0104,-1,0x0035,0x0207,-1,0x0036,0x000F,-1,0x0037,0x0306,-1,0x0038,0x0102,-1,0x0039,0x0707,-1,0x003C,0x0702,-1,0x003D,0x1604,-1,0x0050,0x0000,-1,0x0051,0x00EF,-1,0x0052,0x0000,-1,0x0053,0x013F,-1,0x0060,0xA700,-1,0x0061,0x0001,-1,0x006A,0x0000,-1,0x0080,0x0000,-1,0x0081,0x0000,-1,0x0082,0x0000,-1,0x0083,0x0000,-1,0x0084,0x0000,-1,0x0085,0x0000,-1,0x0090,0x0010,-1,0x0092,0x0600,-1,0x0007,0x0133,-3