Difference between revisions of "VGAX49"

From Land Boards Wiki
Jump to navigation Jump to search
(3 intermediate revisions by the same user not shown)
Line 45: Line 45:
 
* Next standard value from 330 ohms is 680 ohms
 
* Next standard value from 330 ohms is 680 ohms
 
** Voltage is 0.83V - better brightness
 
** Voltage is 0.83V - better brightness
 +
* Ideal resistor values (1%) are [https://www.mouser.com/ProductDetail/Yageo/RC0805FR-07348RL?qs=sGAEpiMZZMu61qfTUdNhG5eFuApKbqVdp8WUAQVCzWo%3D 348] ohms and [https://www.mouser.com/ProductDetail/Yageo/RC0805FR-07698RL?qs=sGAEpiMZZMu61qfTUdNhG4JucpvLNRyOROSAND4sApc%3D 698] ohms for 3.3V @ 8 mA drive
 +
**
  
 
[[File:VGA_Sim_2R_Current.PNG]]
 
[[File:VGA_Sim_2R_Current.PNG]]
Line 84: Line 86:
  
 
* Ideal case drive current
 
* Ideal case drive current
** 1V, 75 Ohms = 13.33 mA
+
** 1V into 75 Ohms = 13.33 mA
 
** 3.3V driver which can drive more than 10 mA
 
** 3.3V driver which can drive more than 10 mA
 
** [http://www.brannonelectronics.com/images/STANDARD%20VALUE.pdf 1% resistors]
 
** [http://www.brannonelectronics.com/images/STANDARD%20VALUE.pdf 1% resistors]
Line 90: Line 92:
 
*** R-2R values
 
*** R-2R values
 
** 2/3 of the drive comes from the smaller resistor
 
** 2/3 of the drive comes from the smaller resistor
** Ideal resistor values are 261, 523 ohms
+
** Ideal resistor values are [https://www.mouser.com/ProductDetail/Yageo/RC0805FR-07261RL?qs=%2Fha2pyFadugqH4HCO4leff4CZT3WBMKXJZMR3j%2FknJZa3N317Wcf%2Fw%3D%3D 261], [https://www.mouser.com/ProductDetail/Yageo/RC0805FR-07523RL?qs=sGAEpiMZZMu61qfTUdNhG6gKAQVNBKOo7pp9l1AJ1rM%3D 523] ohms
 
*** 0V, 0.331V, 0.661V, 0.992 V
 
*** 0V, 0.331V, 0.661V, 0.992 V
  

Revision as of 12:52, 13 February 2020

VGAX49 P901-cropped-720px.jpg

Features

  • Digital VGA adapter
  • 16-bit digital video
    • 5:6:5 (R:G:B) mapping (maximum)
    • 2:2:2 (R:G:B) mapping (option)
  • Uses summing resistors
  • DB-15F connector
  • 49x49mm ODAS form factor
  • Mounting holes

Connectors

VGAX49.PNG

J1 - Digital Connections

P1 - VGA connector

Simulation

  • Each color is independently driven and can be considered individually
  • Critical design criteria is output voltage and current capability of the driving part
  • Use common value, 5% resistors
  • Each resistor is about 2X the value of the previous resistor
  • 1V is drive level for VGA specification
    • VGA presents a 75 Ohm load
    • 1V at 75 Ohms is 13.3 mA
  • Series resistors present a voltage divider between the resistors on the card and the VGA load
  • The resistor with the smallest resistance value has larger current from the source device pin
  • Current switches from source to sink depending on the voltages on the other resistors
  • Each resistor is 2X the value of the previous resistor
  • Too much effort is put into using precision resistors - the human eye is not that picky

2:2:2 Resistor Simulation

  • Assumptions
    • 3.3V Drive out of FPGA
    • 8 mA max drive current (typical FPGA drive current)
  • Using common value, 5% resistors
  • 330 Ohm series resistor into a 75 Ohm load is 8.15 mA (close enough to max of FPGA)
    • Voltage with 330/75 ohm voltage divider is 0.61V - not full brightness
  • Next standard value from 330 ohms is 680 ohms
    • Voltage is 0.83V - better brightness
  • Ideal resistor values (1%) are 348 ohms and 698 ohms for 3.3V @ 8 mA drive

VGA Sim 2R Current.PNG

VGA Sim 2R Voltage.PNG

5:6:5 Resistor Simulation

  • Assumptions
    • 3.3V Drive out of FPGA
    • 8 mA max drive current (typical FPGA drive current)
  • Using common value, 5% resistors
  • 330 Ohm series resistor into a 75 Ohm load is 8.15 mA (close enough to max of FPGA)
    • Voltage with 330/75 ohm voltage divider is 0.61V - not full brightness
  • Next standard value from 330 ohms is 680 ohms
    • Voltage is 0.83V - better brightness
  • Voltage with all 6 resistors (Green case) is 1.07V (full brightness)
  • Simulation uses the same 4 value steps (real case would have 2^5 or 2^6 steps)

VGA Sim 2R Current 6 res.PNG

VGA Sim 2R Voltage 6 res.PNG

ESP32 Drive

  • The ESP32 drive current is significantly more than an FPGA
    • 40 mA
    • More than enough to drive to 1V (saturation level on VGA).
  • 1V at 75 Ohms is 13.3 mA
  • 2:2:2 270/470 Ohm simulation
  • -10 mA (max)
  • 0.96V (max)

VGA Sim 2R Current ESP32.PNG

VGA Sim 2R Voltage ESP32.PNG

Ideal Drive 2:2:2 Case

  • Ideal case drive current
    • 1V into 75 Ohms = 13.33 mA
    • 3.3V driver which can drive more than 10 mA
    • 1% resistors
      • Get closest values
      • R-2R values
    • 2/3 of the drive comes from the smaller resistor
    • Ideal resistor values are 261, 523 ohms
      • 0V, 0.331V, 0.661V, 0.992 V

VGA Sim 2R Voltage ESP32-IdealRes.PNG

Schematic

Assembly Sheet