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Audio & Video

The VERA FPGA core, implemented on an iCE40 UltraPlus 5K FPGA, is used to generate all audio and video signals produced by Sentinel 65X. This core, developed for the Commander X16 project by Frank van den Hoef, is licensed under the MIT license. It has been modified by Brian Swetland for Sentinel 65X, including porting it to be built using the Yosys open-source FPGA toolchain.

Note: This page contains documentation specific to the VERA as it is used in Sentinel 65X, and cannot be taken as applicable to any other project.

Video

The VERA video generator outputs a fixed 640x480, 60Hz digital RGB signal, with a total of 4,096 possible colours, using a 12-bit R4G4B4 colour space.

The video generation is done using two independent tile/bitmap layers, and a third layer composed of 128 sprites. VERA supports a single palette of 256 colours, freely chosen from the 4,096 possible colours available. An approximation of this colour gamut of 4,096 colours can be seen below:

12-bit RGB image of a parrot 12-bit colour palette

Each of the tile/bitmap layers is fully indepentent for content, mode, scrolling, etc. Both layers support two special modes useful for text displays, in 16 and 256 colours.

There are enough sprites supported to act as a third bitmap layer, should the programmer wish to make use of sprites in that manner.

Audio

The VERA FPGA core produces audio output in I²S format, which is fed to the A/V port for conversion into an analog or digital format suitable for the target display device.

Audio is generated using a 16-voice stereo programmable sound generator, with each voice able to generate sounds independently, with the waveform for each chosen freely from among Pulse, Sawtooth, Triangle, and Noise.

VERA also generates sampled 48KHz 16-bit stereo audio from a 4KB on-chip memory buffer.

Interrupts

The VERA implementation in Sentinel 65X has a dedicated vectored interrupt line to the CPU, which can be triggered by any or all of four conditions: Sprite collision, line number, PCM buffer status, or VSYNC.

The Sentinel 65X A/V Port

Audio and video output in Sentinel 65X is directed to a pin header, which is connected to a small daughterboard which actually generates the final audio and video output signals, using electronics and connectors appropriate to the target display.

Pinout

The A/V port is a 24-pin female pin header, with a pin and row pitch of 2.54mm. The pinout of the port, as seen from above, is shown in the following table:

Pin Label Label Pin
1 +3.3V +5V 2
3 HSYNC VSYNC 4
5 RGB_R0 RGB_R1 6
7 RGB_R2 RGB_R3 8
9 RGB_G0 RGB_G1 10
11 RGB_G2 RGB_G3 12
13 RGB_B0 RGB_B1 14
15 RGB_B2 RGB_B3 16
17 LRCK BCK 18
19 ADATA SYSCLK 20
21 SCL SDA 22
23 GND GND 24

VERA Memory

VERA implements 128KB of internal memory, which exists in its own independent address space, connected to the CPU's address space through address and data registers. This RAM's address space is numbered from 0x00000 to 0x1FFFF, and is organized as in the following table:

Address Range Description
0x00000 - 0x1F9BF Video RAM
0x1F9C0 - 0x1F9FF Sound Registers
0x1FA00 - 0x1FBFF Colour Palette
0x1FC00 - 0x1FFFF Sprite Attributes

Hardware Registers

VERA is configured and controlled using a series of 32 memory-mapped I/O registers, located in the address space from 0x00DF00 to 0x00DF1F.

ADDRx_L

The ADDRx_L register is actually two registers, the ADDR0_L and ADDR1_L registers. Which of the two is accessed at memory address 0x00DF00 depends on the contents of ADDRSEL, which is bit 0 of the CTRL register, located at 0x00DF05.

The ADDR0_L and ADDR1_L registers control the low byte of the selected address in VERA memory for reading or writing with the DATA0 and DATA1 registers, located at 0x00DF03 and 0x00DF04 respectively.

ADDRx_M

The ADDRx_M register is actually two registers, the ADDR0_M and ADDR1_M registers. Which of the two is accessed at memory address 0x00DF01 depends on the contents of ADDRSEL, which is bit 0 of the CTRL register, located at 0x00DF05.

The ADDR0ML and ADDR1_M registers control the middle byte of the selected address in VERA memory for reading or writing with the DATA0 and DATA1 registers, located at 0x00DF03 and 0x00DF04 respectively.

ADDRx_H

The ADDRx_H register is actually two registers, the ADDR0_H and ADDR1_H registers. Which of the two is accessed at memory address 0x00DF00 depends on the contents of ADDRSEL, which is bit 0 of the CTRL register, located at 0x00DF05.

The ADDR0_H and ADDR1_H registers control the low byte of the selected address in VERA memory for reading or writing with the DATA0 and DATA1 registers, located at 0x00DF03 and 0x00DF04 respectively.

Bit 0 of the ADDRx_H is bit 16 of the address of the data port selected by ADDRSEL. Bits 1 and 2 of ADDRx_H are unused. Bit 3 is called DECR0 or DECR1; when this bit is clear, the address of the selected data port will increment by the amount set with bits 4-7, called INCR0 or INCR1, according to the table below. When DECR0 or DECR1 is set, then the address will decrement by the same amount when the matching data register is read from or written to.

INCR value Address Increment
0 0
1 1
2 2
3 4
4 8
5 16
6 32
7 64
8 128
9 256
10 512
11 40
12 80
13 160
14 320
15 640

DATA0

The DATA0 register, located at 0x00DF03, is the first of two data registers available from VERA. It reads or writes the address in VERA's internal memory set in ADDR0_L, ADDR0_M, and ADDR0_H.

If the value in INCR0 is nonzero, then after reading or writing from or to DATA0, the address stored in ADDR0_L, ADDR0_M, and ADDR0_H will be incremented by the number of addresses in the table above. If DECR0 is set, then the address will decrement by that amount instead.

DATA1

DATA1, located at 0x00DF04, is identical in function to DATA0, but uses the address and increment settings stored in ADDR1_H instead of ADDR0_H.

CTRL

The CTRL register, located at 0x00DF05, contains three significant bits:

  • Bit 0, called ADDRSEL, determines which DATAx register the ADDRx registers refer to.
  • Bit 1, called DCSEL, controls which set of registers are accessed at addresses 0x00DF09 through 0x00DF0C.
  • Bit 7, called RESET, will reset VERA to the initial power-on state when a 1 value is written to it.

IEN

The IEN register, located at 0x00DF06, has six bits which relate to the generation of VERA interrupts:

  • Bit 0, called IEN_VSYNC, enables the VSYNC interrupt when set.
  • Bit 1, called IEN_LINE, enables the raster-line interrupt when set.
  • Bit 2, called IEN_SPRCOL, enables the sprite collision interrupt when set.
  • Bit 3, called IEN_AFLOW, enables the interrupt triggered when the PCM sample buffer is less than 1/4 full.
  • Bit 6 contains the high-order bit of the 9-bit value of the SCANLINE register, located at 0x00DF08. Both this bit and the SCANLINE register are read-only.
  • Bit 7 contains the high-order bit of the 9-bit value of the IRQLINE register, located at 0x00DF08. Both this bit and the IRQLINE register are write-only.

ISR

The ISR register, located at 0x00DF07, contains flag bits which identify the source of the active VERA interrupt:

  • Bit 0, called ISR_VSYNC, identifies a VSYNC interrupt condition when set.
  • Bit 1, called ISR_LINE, identifies a raster line interrupt condition when set.
  • Bit 2, called ISR_SPRCOL, identifies a sprite collision interrupt condition when set.
  • Bit 3, called ISR_AFLOW, identifies a PCM sample buffer low interrupt condition when set.
  • Bits 4-7 store the sprite collision data.

Writing a 1 value to bits 0, 1, or 2 will clear the interrupt state of the relevant interrupt source. Bit 3 can only be cleared by filling the PCM sample buffer to at least 1/4 of its capacity.

IRQLINE_L / SCANLINE_L

The low order 8 bits of the current scanline and the scanline which is set to trigger the scanline interrupt share the address 0x00DF08 -- IRQLINE_L is write-only, and SCANLINE_L is read-only, so there is no conflict.

DC_VIDEO / DC_HSTART

The DC_Video and DC_HSTART registers share the address 0x00DF09; which one is accessed at that address is determined by the DCSEL flag bit in the CTRL register -- a 0 value in DCSEL will enable the DC_VIDEO register, and a 1 value the DC_HSTART register.

DC_VIDEO stores three flag bits:

  • Bit 4 enables tile/bitmap layer 0 when set to 1.
  • Bit 5 enables tile/bitmap layer 1 when set to 1.
  • Bit 6 enables sprites when set to 1.

DC_HSTART controls the first active column of the screen, relative to the 640x480 display area. The value is the top 8 bits of a 10-bit number, the low order 2 bits of which are implied to be 0 -- that is, the value is an 8 bit number which will be multiplied by 4.

DC_HSCALE / DC_HSTOP

The DC_HSCALE and DC_HSTOP registers share the address 0x00DF0A; which one is accessed at that address is determined by the DCSEL flag bit in the CTRL register -- a 0 value in DCSEL will enable the DC_HSCALE register, and a 1 value the DC_HSTOP register.

DC_HSCALE sets a fractional horizontal scaling factor of the active part of the screen; a value of 128 will output one pixel for each input pixel, and a value of 64 will output two pixels for each input pixel.

DC_HSTOP controls the last active column of the screen, relative to the 640x480 display area. The value is the top 8 bits of a 10-bit number, the low order 2 bits of which are implied to be 0 -- that is, the value is an 8 bit number which will be multiplied by 4.

DC_VSCALE / DC_VSTART

The DC_VSCALE and DC_VSTART registers share the address 0x00DF0B; which one is accessed at that address is determined by the DCSEL flag bit in the CTRL register -- a 0 value in DCSEL will enable the DC_VSCALE register, and a 1 value the DC_VSTART register.

DC_VSCALE sets a fractional vertical scaling factor of the active part of the screen; a value of 128 will output one pixel for each input pixel, and a value of 64 will output two pixels for each input pixel.

DC_VSTART controls the first active row of the screen, relative to the 640x480 display area. The value is the top 8 bits of a 9-bit number, the low order bit of which is implied to be 0 -- that is, the value is an 8 bit number which will be multiplied by 2.

DC_BORDER / DC_VSTOP

The DC_BORDER and DC_VSTOP registers share the address 0x00DF0B; which one is accessed at that address is determined by the DCSEL flag bit in the CTRL register -- a 0 value in DCSEL will enable the DC_BORDER register, and a 1 value the DC_VSTOP register.

DC_BORDER stores the 8-bit pallette index of the colour used to fill the "inactive" region of the screen, as determined by the DC_HSTART, DC_HSTOP, DC_VSTART, and DC_VSTOP registers.

DC_VSTOP controls the last active row of the screen, relative to the 640x480 display area. The value is the top 8 bits of a 9-bit number, the low order bit of which is implied to be 0 -- that is, the value is an 8 bit number which will be multiplied by 2.

L0_CONFIG

The L0_CONFIG register, located at 0x00DF0D, controls the configuration of tile/bitmap layer 0.

Bits 0-1 store the colour depth of the layer; that is, the number of bits used per pixel in the tile or bitmap data used to render the layer:

Value Depth
0 1 bpp
1 2 bpp
2 4 bpp
3 8 bpp

Bit 2 sets the layer to use tiles mode when set to 0; a value of 1 will set the layer to use bitmap mode.

Bit 3, called T256C, is used with tiles modes to enable or disable 256-colour 1bpp mode. Other tile mode colour depths ignore this flag bit.

Bits 4-5 encode the tile map width, while bits 6-7 encode the tile map height, according to the values in the following table:

Value Map Width / Height
0 32 tiles
1 64 tiles
2 128 tiles
3 256 tiles

L0_MAPBASE

The L0_MAPBASE register, located at 0x00DF0DE, encodes the high 8 bits of the 17-bit address in VERA memory where the map data for layer 0 can be found; since the bottom 9 bits are implicitly 0, this means the map data must always begin on a 512-byte alignment boundary.

L0_TILEBASE

The L0_TILEBASE register, located at 0x00DF0F, encodes the following data:

  • Bit 0 controls the pixel width of tiles in layer 0; a 0 value means that a tile is 8 pixels wide, while a 1 value means that tile is 16 pixels wide.
  • Bit 1 controls the pixel height of tiles in layer 0; a 0 value means that a tile is 8 pixels tall, while a 1 value means that tile is 16 pixels tall.
  • Bits 2-7 hold the top six bits of the 17-bit address in VERA memory where the tile data can be found; as bits 0-10 are implicitly 0, this means that tile data is always aligned on a 2,048-byte boundary.

L0_HSCROLL_L / L0_HSCROLL_H

The L0_HSCROLL_L and L0_HSCROLL_H registers, located at addresses 0x00DF10 and 0x00DF11, encode the low order byte and high order 4 bits, respectively, of the layer 0 horizontal scroll value; this value ranges from 0 to 4,095, and is defined in terms of pixels.

L0_VSCROLL_L / L0_VSCROLL_H

The L0_VSCROLL_L and L0_VSCROLL_H registers, located at addresses 0x00DF12 and 0x00DF13, encode the low order byte and high order 4 bits, respectively, of the layer 0 vertical scroll value; this value ranges from 0 to 4,095, and is defined in terms of pixels.

Layer 1 Registers

The layer 1 control registers function exactly the same as their equivalents for layer 0:

  • L1_CONFIG is located at 0x00DF14
  • L1_MAPBASE is located at 0x00DF15
  • L1_TILEBASE is located at 0x00DF16
  • L1_HSCROLL_L is located at 0x00DF17
  • L1_HSCROLL_H is located at 0x00DF18
  • L1_VSCROLL_L is located at 0x00DF19
  • L1_VSCROLL_H is located at 0x00DF1A

AUDIO_CTRL

The AUDIO_CTRL register at 0x00DF1B controls several aspects of the PCM audio subsystem:

  • Bits 0-3 control the volume of the PCM sample playback.
  • Bit 4 controls whether the PCM audio will be monaural (0 value) or stereo (1 value).
  • Bit 5 controls whether PCM audio sample data is 8-bit (0 value) or 16-bit (1 value).
  • Bit 6 is a read-only flag bit which indicates that the PCM sample buffer is empty when it is set to 1.
  • Bit 7 encodes two values; when read, this bit will be a 1 when the PCM sample buffer is full. In that case, writes to the buffer are silently ignored. When a 1 value is written to this bit, the PCM sample buffer will be cleared.

AUDIO_RATE

The AUDIO_RATE register at 0x00D1C controls the sample rate of the PCM audio playback:

Value Description
128 48828.125Hz
64 24414Hz
32 12207Hz
0 No playback

An intentional mismatch between the actual recorded sample rate and the sample rate of the playback can be used to speed up or slow down the PCM sample.

AUDIO_DATA

The AUDIO_DATA register at 0x00DF1D is a write-only register which accepts byte writes to the PCM audio buffer.

SPI_DATA

The SPI_DATA register at 0x00DF1E is used to read and write data from the selected SPI device a byte at a time.

SPI_CTRL

The SPI_CTRL register at 0x00DF1F is used to control the SPI subsystem:

  • Bit 0 is used to assert the SPI select line with a 1 value, and deassert it with a 0 value.
  • Bit 1 controls the clock rate of the SPI data clock; when set to 0 this will be approximately 12.5MHz, while when set to 1 the clock will run at approximately 390KHz, which can be useful with slow SPI devices or when an SD card requires a slower clock during initialization.
  • Bit 2 controls the "auto-transmit" function, which begins reading a new byte from SPI (sending the value 0xFF) automatically when the SPI_DATA register is read. This can be used to simplify and speed up SPI reads.
  • Bit 7 will read as a 1 value as long as a byte is being transferred over SPI.

Colour Palette

VERA supports a single, shared palette of 256 colours, each of which is freely chosen from among the 4,096 possible colours the hardware can generate.

The palette is stored in VERA memory beginning at VERA address 0x1FA00, and consists of 256 entries with the following format:

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 G3 G2 G1 G0 B3 B2 B1 B0
1 R3 R2 R1 R0

Note that the high order four bits in the second byte of each palette entry is unused.

Sprite Entries

Sprites are controlled using 128 eight-byte control entries, beginning at VERA address 0x1FC00. Each of these entries contains the following structure:

Offset Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 Bitmap (Address bits 5-12)
1 Mode - Bitmap (Address bits 13-16)
2 X Position (Low 8 bits)
3 - X Position (High 2 bits)
4 Y Position (Low 8 bits)
5 - Y Position (High 2 bits)
6 Collision Mask Layer Position Vertical Flip Horizontal Flip
7 Sprite Height Sprite Width Palette Offset

The bitmap address of a sprite is the top 12 bits of a 17-bit VERA memory address; the low order 5 bits are always 0, which means that sprite data is always aligned to a 32-byte boundary.

The X and Y positions are 10-bit numbers representing the offset from the screen origin of the sprite's origin.

The four-bit collision mask is used in detecting collisions between sprites in hardware. When two sprites' defined areas on screen overlap, their collision masks are used as operands in a binary AND operation; if the result is nonzero, then the collision will trigger a sprite collision interrupt if that interrupt source is enabled. Thus, a sprite with a collision mask of 0b0101 can trigger interrupts by colliding with sprites whose interrupt masks have bit 0, bit 2, or both set, regardless of the status of the other two bits. This makes sprite collision more efficient, by allowing groups of sprites which can and cannot detect collision with rach other to be defined, saving time resolving needless collisions.

The layer position value determines the drawing order of the sprite relative to the two tile/bitmap layers:

Value Description
0 Disabled
1 Between background and layer 0
2 Between layer 0 and 1
3 Above layer 1

The horizontal and vertical flip bits, as you would expect, flip the sprite's bitmap along the horizontal and vertical axes, respectively.

The palette offset is used to translate colour values from the sprite bitmap data, where those values are between 1 and 15. Such values are modified by adding the product of 16 and the palette offset. This can be used to enable more colourful sprites when the sprite is set to 4bpp mode, with proper organization of the VERA palette.

Sprite Multiplexing

While it should rarely be necessary, the presence of the raster line interrupt does allow for the re-use of sprites during the rendering of a single frame; the maximum number of effective sprites possible on screen in a single frame has not been experimentally demonstrated.

Tile Data

The individual tile bitmaps are 8x8, 8x16, 16x8, or 16x16 pixel bitmaps, which can be in a 1, 2, 4, or 8bpp format, with each pixel stored as a colour palette index.

Map Data

Tilemaps are stored as sequences of two-byte structures. The interpretation of those bytes depends on the settings of the layer.

1-Bit Tile Mode

In 1bpp tile mode, with the T256C flag cleared, the first byte of the map data is an index into the tile data, and the second byte is split into two nybbles, the low-order nybble holding the foreground colour, and the high-order nybble holding the background colour for that tile.

In 1bpp tile mode, with the T256C flag set, instead the second byte is the foreground colour of the tile, with the background colour being transparent.

2-, 4-, and 8-Bit Tile Modes

In other bit depths, tile mode interprets the first byte of the map data is the low-order eight bits of the tile index. The second byte is interpreted as follows:

  • The low two bits of the second byte are the high-order bits of the tile index.
  • Bits 2 and 4 control horizontal and vertical flipping of the bimap used for that position in the map.
  • Bits 4-7 contain a 4-bit palette offset.

Palette offsets modify the colour indices in the tile data as follows:

  • Color 0 (transparent) and 16-255 are used as-is
  • Colours 1-15 are modified by adding the product of the palette offset and 16 -- that is, if the palette offset is 2, then colours 1-15 will have 32 added to the palette index.

16-Bit Reads/Writes

With appropriate configuration of registers, it is possible to perform sequential 16-bit reads and writes to VERA address space:

  • Set ADDR0 to the lowest address to read or write in VERA memory, and clear DECR0.
  • Set ADDR1 to a value exactly one address higher than ADDR0, and clear DECR1.
  • Set both INCR0 and INCR1 to a value of 0 or 2.
  • Set the CPU register to be used to read from or write to VERA memory to be 16 bits wide.
  • Read or write to DATA0 using a 16-bit load or store instruction. This will implicitly read DATA1.
  • If INCR0 and INCR1 are both set to 0, then the address will not increment at all; if they are set to 2, then the address of both DATA0 and DATA1 will be incremented by 2 addresses.

Such 16-bit reads and writes are almost twice as cycle-efficient as reading or writing in 8-bit increments, as the extra byte takes only a single extra CPU cycle.

The SDK will provide a C function pair vera_memcpy_read() and vera_memcpy_write() handle bulk data transfers; it will use this mode of reading and writing without futher intervention by the programmer.