diff options
Diffstat (limited to 'bbc_micro_de1.vhd')
| -rw-r--r-- | bbc_micro_de1.vhd | 163 |
1 files changed, 113 insertions, 50 deletions
diff --git a/bbc_micro_de1.vhd b/bbc_micro_de1.vhd index f48507f..0d29726 100644 --- a/bbc_micro_de1.vhd +++ b/bbc_micro_de1.vhd @@ -337,10 +337,16 @@ signal reset_n : std_logic; -- Clock enable counter
-- CPU and video cycles are interleaved. The CPU runs at 2 MHz (every 16th
-- cycle) and the video subsystem is enabled on every odd cycle.
-signal clken_counter : unsigned(3 downto 0);
+signal clken_counter : unsigned(4 downto 0);
signal slomo_counter : unsigned(17 downto 0);
-signal cpu_clken : std_logic;
-signal vid_clken : std_logic;
+signal cpu_cycle : std_logic; -- Qualifies all 2 MHz cycles
+signal cpu_cycle_mask : std_logic; -- Set to mask CPU cycles until 1 MHz cycle is complete
+signal cpu_clken : std_logic; -- 2 MHz cycles in which the CPU is enabled
+-- IO cycles are out of phase with the CPU
+signal vid_clken : std_logic; -- 16 MHz video cycles
+signal mhz4_clken : std_logic; -- Used by 6522
+signal mhz2_clken : std_logic; -- Used for latching CPU address for clock stretch
+signal mhz1_clken : std_logic; -- 1 MHz bus and associated peripherals, 6522 phase 2
-- Testing
signal test_uart_do : std_logic_vector(7 downto 0);
@@ -376,6 +382,10 @@ signal crtc_lpstb : std_logic := '0'; signal crtc_ma : std_logic_vector(13 downto 0);
signal crtc_ra : std_logic_vector(4 downto 0);
+-- Decoded display address after address translation for hardware
+-- scrolling
+signal display_a : std_logic_vector(14 downto 0);
+
-- "VIDPROC" signals
signal vidproc_invert_n : std_logic;
signal r_in : std_logic;
@@ -430,10 +440,6 @@ signal user_via_pb_in : std_logic_vector(7 downto 0); signal user_via_pb_out : std_logic_vector(7 downto 0);
signal user_via_pb_oe_n : std_logic_vector(7 downto 0);
--- Common VIA signals
-signal via_clken : std_logic;
-signal via_phase2 : std_logic;
-
-- IC32 latch on System VIA
signal ic32 : std_logic_vector(7 downto 0);
signal sound_enable_n : std_logic;
@@ -465,6 +471,8 @@ signal adlc_enable : std_logic; -- 0xFEA0-FEBF (Econet) signal adc_enable : std_logic; -- 0xFEC0-FEDF
signal tube_enable : std_logic; -- 0xFEE0-FEFF
+signal mhz1_enable : std_logic; -- Set for access to any 1 MHz peripheral
+
begin
-------------------------
-- COMPONENT INSTANCES
@@ -555,13 +563,13 @@ begin );
-- MOS ROM
--- mos : os12 port map (
--- cpu_a(13 downto 0),
--- clock,
--- mos_d );
- test_rom : ehbasic port map (
+ mos : os12 port map (
cpu_a(13 downto 0),
- clock, mos_d );
+ clock,
+ mos_d );
+-- test_rom : ehbasic port map (
+-- cpu_a(13 downto 0),
+-- clock, mos_d );
-- System VIA
system_via : m6522 port map (
@@ -589,9 +597,9 @@ begin sys_via_pb_in,
sys_via_pb_out,
sys_via_pb_oe_n,
- via_phase2,
+ mhz1_clken,
reset_n,
- via_clken,
+ mhz4_clken,
clock
);
@@ -621,9 +629,9 @@ begin user_via_pb_in,
user_via_pb_out,
user_via_pb_oe_n,
- via_phase2,
+ mhz1_clken,
reset_n,
- via_clken,
+ mhz4_clken,
clock
);
@@ -631,16 +639,20 @@ begin pll_reset <= not SW(9);
reset_n <= not (pll_reset or not pll_locked);
- -- Clock enable generation
- -- CPU is on cycle 0 of 16 (2 MHz)
- -- Video is on all odd cycles
- cpu_clken <= '1' when clken_counter = 0 and (SW(8) = '1' or slomo_counter = 0) else '0';
- vid_clken <= clken_counter(0);
- -- FIXME: VIAs - this is wrong. They should actually run at 1MHz (so 4 MHz clken in this
- -- case) and the CPU should be stalled until the 1 MHz bus cycle is complete
- via_clken <= clken_counter(0) and clken_counter(1);
- via_phase2 <= clken_counter(3);
- process(clock,reset_n)
+ -- Clock enable generation - 32 MHz clock split into 32 cycles
+ -- CPU is on 0 and 16 (but can be masked by 1 MHz bus accesses)
+ -- Video is on all odd cycles (16 MHz)
+ -- 1 MHz cycles are on cycle 31 (1 MHz)
+ vid_clken <= clken_counter(0); -- 1,3,5...
+ mhz4_clken <= clken_counter(0) and clken_counter(1) and clken_counter(2); -- 7/15/23/31
+ mhz2_clken <= mhz4_clken and clken_counter(3); -- 15/31
+ mhz1_clken <= mhz2_clken and clken_counter(4); -- 31
+ cpu_cycle <=
+ not (clken_counter(0) or clken_counter(1) or clken_counter(2) or clken_counter(3))
+ when (SW(8) = '1' or slomo_counter = 0) else '0'; -- 0/16
+ cpu_clken <= cpu_cycle and not cpu_cycle_mask;
+
+ clk_gen: process(clock,reset_n)
begin
if reset_n = '0' then
clken_counter <= (others => '0');
@@ -652,6 +664,24 @@ begin end if;
end if;
end process;
+
+ cycle_stretch: process(clock,reset_n)
+ begin
+ if reset_n = '0' then
+ cpu_cycle_mask <= '0';
+ elsif rising_edge(clock) and mhz2_clken = '1' then
+ if mhz1_enable = '1' and cpu_cycle_mask = '0' then
+ -- Block CPU cycles until 1 MHz cycle has completed
+ cpu_cycle_mask <= '1';
+ end if;
+ if mhz1_clken = '1' then
+ -- CPU can run again
+ -- FIXME: This may not be correct in terms of CPU cycles, but it
+ -- should work
+ cpu_cycle_mask <= '0';
+ end if;
+ end if;
+ end process;
-- CPU configuration and fixed signals
cpu_mode <= "00"; -- 6502
@@ -675,6 +705,11 @@ begin io_fred <= '1' when cpu_a(15 downto 8) = "11111100" else '0';
io_jim <= '1' when cpu_a(15 downto 8) = "11111101" else '0';
io_sheila <= '1' when cpu_a(15 downto 8) = "11111110" else '0';
+ -- The following IO regions are accessed at 1 MHz and hence will stall the
+ -- CPU accordingly
+ mhz1_enable <= io_fred or io_jim or
+ adc_enable or sys_via_enable or user_via_enable or
+ serproc_enable or acia_enable or crtc_enable;
-- SHEILA address demux
-- All the system peripherals are mapped into this page as follows:
@@ -750,7 +785,8 @@ begin user_via_do when user_via_enable = '1' else
test_uart_do when io_fred = '1' else
SRAM_DQ(7 downto 0);
- cpu_irq_n <= sys_via_irq_n and user_via_irq_n;
+ --cpu_irq_n <= sys_via_irq_n; -- and user_via_irq_n;
+ cpu_irq_n <= '1';
-- SRAM bus
SRAM_UB_N <= '1';
@@ -783,21 +819,58 @@ begin else
-- Fetch data from previous display cycle
SRAM_WE_N <= '1';
- SRAM_ADDR <= "000" & crtc_ma(11 downto 0) & crtc_ra(2 downto 0);
+ SRAM_ADDR <= "000" & display_a;
end if;
end if;
end process;
+ -- Address translation logic for calculation of display address
+ process(crtc_ma,crtc_ra,disp_addr_offs)
+ variable aa : unsigned(3 downto 0);
+ begin
+ if crtc_ma(12) = '0' then
+ -- No adjustment
+ aa := unsigned(crtc_ma(11 downto 8));
+ else
+ -- Address adjusted according to screen mode to compensate for
+ -- wrap at 0x8000.
+ case disp_addr_offs is
+ when "00" =>
+ -- Mode 3 - restart at 0x4000
+ aa := unsigned(crtc_ma(11 downto 8)) + 8;
+ when "01" =>
+ -- Mode 6 - restart at 0x6000
+ aa := unsigned(crtc_ma(11 downto 8)) + 12;
+ when "10" =>
+ -- Mode 0,1,2 - restart at 0x3000
+ aa := unsigned(crtc_ma(11 downto 8)) + 6;
+ when "11" =>
+ -- Mode 4,5 - restart at 0x5800
+ aa := unsigned(crtc_ma(11 downto 8)) + 11;
+ when others =>
+ null;
+ end case;
+ end if;
+
+ if crtc_ma(13) = '0' then
+ -- HI RES
+ display_a <= std_logic_vector(aa(3 downto 0)) & crtc_ma(7 downto 0) & crtc_ra(2 downto 0);
+ else
+ -- TTX VDU
+ display_a <= std_logic(aa(3)) & "1111" & crtc_ma(9 downto 0);
+ end if;
+ end process;
+
-- VIDPROC
vidproc_invert_n <= '1';
r_in <= '0';
g_in <= '0';
b_in <= '0';
- GPIO_0(0) <= crtc_clken;
- GPIO_0(1) <= crtc_hsync;
- GPIO_0(2) <= crtc_vsync;
- GPIO_0(3) <= not (crtc_hsync xor crtc_vsync);
+ GPIO_0(0) <= not (crtc_hsync xor crtc_vsync);
+ GPIO_0(1) <= cpu_clken;
+ GPIO_0(2) <= cpu_cycle_mask;
+ GPIO_0(3) <= mhz1_enable;
-- CRTC drives video out (CSYNC on HSYNC output, VSYNC high)
VGA_HS <= not (crtc_hsync xor crtc_vsync);
@@ -808,10 +881,12 @@ begin -- Connections to System VIA
sys_via_ca1_in <= crtc_vsync;
- sys_via_cb1_in <= crtc_lpstb;
+ sys_via_ca2_in <= '0'; -- Keyboard interrupt
+ sys_via_cb1_in <= '0'; -- /EOC
+ sys_via_cb2_in <= crtc_lpstb;
-- Connections to User VIA (user port is output on green LEDs)
- --LEDG <= user_via_pb_out;
+ LEDG <= user_via_pb_out;
-- IC32 latch
sound_enable_n <= ic32(0);
@@ -834,8 +909,6 @@ begin end if;
end process;
- LEDG <= ic32;
-
-- Keyboard LEDs
LEDR(0) <= not caps_lock_led_n;
LEDR(1) <= not shift_lock_led_n;
@@ -855,22 +928,12 @@ begin when 7 => sys_via_pa_in(7) <= SW(2);
when 8 => sys_via_pa_in(7) <= SW(1);
when 9 => sys_via_pa_in(7) <= SW(0);
- when others => sys_via_pa_in(7) <= '1';
+ when others => sys_via_pa_in(7) <= '0';
end case;
else
- sys_via_pa_in(7) <= '1';
+ -- 'W' output of keyboard IC2 is inverse data, so 0 is no key pressed
+ sys_via_pa_in(7) <= '0';
end if;
end process;
-
--- process(clock,reset_n)
--- begin
--- if reset_n = '0' then
--- LEDG <= "00000000";
--- elsif rising_edge(clock) then
--- if cpu_a(15 downto 0) = "0000001010001111" and cpu_r_nw = '0' then
--- LEDG <= cpu_do;
--- end if;
--- end if;
--- end process;
end architecture;
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