Logic block

The PLB, or programmable logic block, consists of:

3 shared control inputs
- CLK, the clock
- CE, the clock enable
- RST, the reset signal
8 LCs (logic cells), each consisting of:
- a four-input LUT
- carry logic
- a flip-flop
a carry chain
a LUT cascade chain (iCE40 only) or hard logic input (iCE40T0* only)

LUTs and LUT inputs

Each LC contains a 4-input LUT. The 4 inputs to the LUT are called LC[0-7].I[0-3]. The output of the LUT is called LC[0-7].LTOUT.

The LUT inputs usually come directly from general interconnect. However, two of the inputs are special:

I3 can be driven by the LC’s carry input, LC[0-7].CIN. This is encoded in the bitstream as a special CI value for the relevant general interconnect MUX field.
on iCE40 only, I2 can be driven from the special LC[0-7].LTIN input. This is encoded in the bitstream as a separate LC{i}:MUX.I2 field, selecting between general interconnect and LTIN.

For most PLBs, the LTIN input is used to implement LUT cascading: LTIN is connected directly to LTOUT of the previous LC in the PLB, or to LC7.LTOUT of the PLB to the south for LC0.LTIN.

However, on iCE40T0*, some PLB tiles are designated as “IP connect” tiles, and their LTIN is instead connected to the output of some hard logic block. To route the hard logic output to the fabric, the LC should be configured as combinational, with 0xf0f0 as the LUT table, and with LTIN input selected. The LUT cascading cannot be used in those tiles.

The PLB tiles designed as “IP connect” are:

iCE40T04 and iCE40T05: all PLBs within the westernmost and easternmost columns
iCE40T01: the southernmost three and northernmost three PLBs within the westernmost and easternmost columns

Carry chain and carry logic

In addition to the LUT, each LC also has:

a carry input, LC[0-7].CIN
a carry output, LC[0-7].COUT

a carry logic primitive, implementing a fixed majority function over the LC’s CIN, I1, I2 inputs:

LC[i].COUT = MAJ(LC[i].I1, LC[i].I2, LC[i].CIN) = (
  (LC[i].CIN & LC[i].I1) |
  (LC[i].CIN & LC[i].I2) |
  (LC[i].I1 & LC[i].I2)
)

The carry logic primitive is only functional when the LC{i}:CARRY_ENABLE bit is set in the bitstream. Without that, the carry output is indeterminate.

The CIN inputs of LCs 1-7 are connected directly to COUT outputs of LCs 0-6 of the same PLB.

The CIN input of LC 0 can be selected (via LC0:MUX.CI) from one of:

const 0
const 1
CHAIN: LC7.COUT of the PLB to the south

Flip-flops

Every LC includes a flip-flop that may be optionally enabled. If the flip-flop is enabled, the combinational output of the LUT is not available outside the LC, except for the LUT cascade function.

The flip-flop always has an initial value of 0. The reset signal can be synchronous or asynchronous, and the reset value can be configured via the bitstream. The clock enable has priority over the synchronous reset.

The flip-flop implements the following logic:

if (!LC[i].FF_ENABLE) begin

    assign LC[i].OUT = LC[i].LTOUT;

end else if (LC[i].FF_SR_ASYNC) begin

    initial LC[i].OUT = 0;

    always @(posedge CLK, posedge RST)
        if (RST)
            LC[i].OUT <= LC[i].FF_SR_VALUE;
        else if (CE)
            LC[i].OUT <= LC[i].LTOUT;

end else begin

    initial LC[i].OUT = 0;

    always @(posedge CLK)
        if (CE) begin
            if (RST)
                LC[i].OUT <= LC[i].FF_SR_VALUE;
            else
                LC[i].OUT <= LC[i].LTOUT;
        end

end

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Logic block

LUTs and LUT inputs

Carry chain and carry logic

Flip-flops