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Bit-bang SPI communication with BK4829 3-wire interface: timing, C code, and pitfalls

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User question

bit bang SPI commands to BK4829

Artificial Intelligence Response

Direct answer to the question

  • The BK4829 uses a 3‑wire, bidirectional SPI interface: SCK (clock), SCN (chip select), SDATA (data). Transfers are 24 bits: 1 R/W bit, 7‑bit register address (A6..A0), then 16‑bit data, MSB first. The device latches inputs on SCK rising edges and drives outputs on SCK falling edges. Use a bit‑banged routine that:
    • Asserts SCN low, clocks out R/W and address on SDATA (master drives), then
    • For writes: clock out 16 data bits while continuing to drive SDATA.
    • For reads: release SDATA (input/Hi‑Z), wait the bus‑turnaround time, then clock in 16 bits, sampling on SCK rising edges. (device.report)
  • Maximum SCK is 8 MHz; typical control‑interface timing minima are on the order of a few tens of nanoseconds (ample margin if you bit‑bang with microsecond delays). (device.report)

Detailed problem analysis

  • Interface fundamentals
    • Pins: SCK=PIN25, SCN=PIN26, SDATA=PIN27; SCK/SCN are inputs, SDATA is bidirectional. (device.report)
    • Timing: “latch on rising, output on falling.” Your bit‑banged master must present valid SDATA before each rising edge (setup) and hold it briefly after the edge; on reads, sample SDATA on each rising edge since the BK4829 updated it on the previous falling edge. (device.report)
    • Frame format (24 bits, MSB first): R/W (1=read, 0=write), A6..A0, then D15..D0. (device.report)
  • Turnaround on reads
    • After shifting the 8 address bits (R/W + 7‑bit address), the master must stop driving SDATA and switch that GPIO to input before the data phase. The interface specifies a bus‑turnaround interval between the last address clock and the first data clock; allow ≥1 µs to be safe when bit‑banging. (device.report)
  • Practical timing margins
    • Max SCK frequency: 8 MHz. Representative minima from the control‑interface table include SCK high/low times ≈25 ns, SDATA setup/hold to SCK rising ≈20 ns/10 ns, and SCK↓ to SDATA valid ≈25 ns. If you bit‑bang with ~0.5–2 µs delays, you exceed all minima comfortably. (device.report)
  • Voltage levels
    • VDD is 3.0–3.6 V; I/O pins are not 5 V tolerant. Use 3.3 V logic or proper level shifting. (device.report)

Current information and trends

  • Latest BK4829 datasheet (DS‑BK4829‑E01, 2023‑05‑26) documents the 3‑wire SPI, timing (Figures 2‑5, 3‑1, 3‑2), and 8 MHz SCK limit. Community firmware/projects (e.g., UV‑K5 derivatives) routinely interact with BK48xx over this 3‑wire bus with a 24‑bit frame; avoid examples that assume 4‑wire MOSI/MISO. (device.report)

Supporting explanations and details

  • Why 3‑wire matters: Unlike standard 4‑wire SPI, SDATA is half‑duplex. Direction control and bus‑turnaround are the two “gotchas” that cause contention if mishandled.
  • Edge alignment: Because BK4829 changes SDATA on falling edges and the master samples on rising edges, your sampling is naturally centered in the valid window.

Practical guidelines

  • Minimal, portable C (bit‑bang) write/read
    • Replace GPIO macros with your MCU’s HAL or direct‑register operations. Delays of ~0.5–2 µs per edge are conservative and easy to hit on most MCUs.
// ---- GPIO hooks you must provide ----
#define BK_SCN_LOW() // drive SCN = 0
#define BK_SCN_HIGH() // drive SCN = 1
#define BK_SCK_LOW() // drive SCK = 0 (idle low)
#define BK_SCK_HIGH() // drive SCK = 1
#define BK_SDIO_LOW() // drive SDATA = 0
#define BK_SDIO_HIGH() // drive SDATA = 1
#define BK_SDIO_READ() // read SDATA pin (returns 0/1)
#define BK_SDIO_OUTPUT() // configure SDATA pin as push-pull output
#define BK_SDIO_INPUT() // configure SDATA pin as input (Hi-Z)
static inline void spi_delay(void) { /* ~1 us delay */ for(volatile int i=0;i<100;i++) __asm__ volatile(""); }
// Drive one output bit on SDATA, MSB-first, sampled by BK4829 on SCK rising edge
static inline void bk4829_send_bit(int bit)
{
BK_SCK_LOW();
if (bit) BK_SDIO_HIGH(); else BK_SDIO_LOW();
// setup time before SCK rising
spi_delay();
BK_SCK_HIGH(); // BK4829 latches here
// hold time after SCK rising
spi_delay();
BK_SCK_LOW();
}
// Read one input bit from SDATA; BK4829 changed SDATA on prior SCK falling edge
static inline int bk4829_read_bit(void)
{
BK_SCK_LOW(); // falling edge -> BK4829 updates SDATA
// allow SCK↓ -> SDATA valid (tCDV)
spi_delay();
BK_SCK_HIGH(); // sample on rising edge
spi_delay();
int b = BK_SDIO_READ();
BK_SCK_LOW();
return b;
}
// Write 16-bit register
void bk4829_write(uint8_t reg7, uint16_t data16)
{
// Safety: keep only 7 LSBs of address
reg7 &= 0x7F;
// Idle
BK_SCN_HIGH(); BK_SCK_LOW();
BK_SDIO_OUTPUT();
// Start
BK_SCN_LOW();
spi_delay();
// R/W = 0 (write)
bk4829_send_bit(0);
// Address A6..A0
for (int i = 6; i >= 0; --i)
bk4829_send_bit( (reg7 >> i) & 1 );
// Data D15..D0
for (int i = 15; i >= 0; --i)
bk4829_send_bit( (data16 >> i) & 1 );
// End
spi_delay();
BK_SCN_HIGH();
}
// Read 16-bit register
uint16_t bk4829_read(uint8_t reg7)
{
reg7 &= 0x7F;
BK_SCN_HIGH(); BK_SCK_LOW();
BK_SDIO_OUTPUT();
BK_SCN_LOW();
spi_delay();
// R/W = 1 (read)
bk4829_send_bit(1);
// Address A6..A0
for (int i = 6; i >= 0; --i)
bk4829_send_bit( (reg7 >> i) & 1 );
// Turnaround: release SDATA before data phase
BK_SCK_LOW();
BK_SDIO_INPUT();
// Datasheet shows a delay between address phase and first data bit; ≥1 us is safe
spi_delay();
// Read 16 data bits MSB-first
uint16_t val = 0;
for (int i = 0; i < 16; ++i)
{
val = (val << 1) | bk4829_read_bit();
}
BK_SCN_HIGH();
return val;
}

  • Bring‑up checklist

    • Confirm logic levels (3.3 V), solid decoupling on VCC, clean 26 MHz reference per application notes.
    • On a logic analyzer, each transaction must be exactly 24 clocks with SDATA idle Hi‑Z during the read‑data phase when the master is not driving.
    • Start with a slow virtual SCK (~100 kHz via delays), then reduce delays to speed up once stable.

  • Common pitfalls and fixes

    • Bus contention during read turnaround: ensure SDATA is switched to input immediately after the address phase, before the first data clock.
    • Wrong frame length or address order: BK4829 is 24‑bit (not 32‑bit), with 1+7+16 format, MSB first. Some online examples mistakenly use 4‑wire MOSI/MISO or 32‑bit frames—avoid them. (device.report)
    • CPOL/CPHA confusion: treat it as “Mode 0–like,” idle SCK low; master changes SDATA while SCK is low, BK4829 latches on the subsequent rising edge. (device.report)

Ethical and legal aspects

  • Configuring a transceiver can enable transmission outside authorized bands or power levels. In the United States, ensure all operation complies with FCC rules (e.g., Part 97 for amateur service or the applicable Part for your use case). Use the SPI interface only to configure legal modes/frequencies.

Potential challenges and how to overcome them

  • MCU pin muxing: Some MCUs cannot easily switch a hardware SPI MOSI pin to input quickly. If using hardware SPI in half‑duplex mode, prefer an MCU that supports single‑wire or 1‑line bidirectional SPI. Otherwise, bit‑bang as shown.
  • Electrical integrity: If you must “merge” MOSI/MISO to one SDATA line with hardware SPI, use series resistors and ensure the MCU output is Hi‑Z during reads; however, pure bit‑bang is simpler and safer.

Possible disclaimers or additional notes

  • The public BK4829 brief datasheet documents the bus timing and frame format but may not include a complete register map. Consult the latest vendor documentation or reference firmware for register definitions. (device.report)

Suggestions for further research

  • Examine Figures 2‑5, 3‑1, and 3‑2 in the BK4829 datasheet for timing waveforms and characteristic tables; they are your ground truth for edge placement and delays. (device.report)
  • If you work with BK4819 as well, note it uses the same 3‑wire concept and timing (useful for cross‑checking behavior). (alldatasheet.com)

Brief summary

  • Bit‑banging the BK4829 is straightforward once you implement a 24‑bit transaction with proper edge timing and a clean read‑turnaround. Keep SCN low for the entire 24‑bit frame, drive SDATA during the command/address and write‑data phases, release SDATA and sample on SCK rising edges during reads, and leave generous microsecond‑scale delays. The datasheet confirms the 3‑wire pins, edge directions, and 8 MHz ceiling; following these guarantees robust communication. (device.report)

If you share your MCU/SDK, I can adapt the GPIO macros to it and provide a logic‑analyzer checklist tailored to your board.

User question

Is there any initial commands to set up a new beken BK4829 before attempting to write to the chip?

Artificial Intelligence Response

Direct answer to the question

  • There is no hidden “unlock” or special preamble required to talk to a new BK4829. You can issue 3‑wire SPI transactions as soon as power and the 26 MHz reference are stable.
  • Practical bring‑up still benefits from a short, standard initialization: verify the clock and supply, sanity‑check SPI by reading a known register, optionally perform a software reset (if your register map exposes one), then proceed to configure operating registers. (device.report)

Key points

  • BK4829 uses a 3‑wire SPI (SCK, SCN, SDATA) with defined write/read framing; no mode‑select pin or password is needed. (device.report)
  • Ensure VDD within 3.0–3.6 V and a properly loaded/stable 26 MHz crystal before your first SPI access. (device.report)
  • Community firmware for radios using BK4829 shows that early, deterministic register writes (e.g., to a low‑numbered control register) are part of normal init; there is no extra “enable‑write” command. (github.com)

Detailed problem analysis

1) Hardware prerequisites and timing

  • Supply/clock: The BK4829 datasheet specifies VDD = 3.0…3.6 V and relies on a 26 MHz crystal that also clocks the digital domain; it must meet the crystal load requirements to oscillate correctly. Wait long enough after power‑up for the XO to settle before your first transaction. In practice 10–20 ms is conservative. (device.report)
  • Interface: The device’s 3‑wire SPI uses SCK (pin 25), SCN (pin 26), and bidirectional SDATA (pin 27). Data is latched on SCK rising edges and driven by the chip on falling edges during reads. The frame is R/W + A6..A0 + D15..D0 with a defined bus turn‑around between address and readback. Respect the tNXT/tCDV timing and switch your SDATA GPIO to input immediately after the A0 bit when performing reads. (device.report)

2) Minimal, robust “first‑contact” sequence

  • Power‑up guard time: after VDD reaches regulation and the XO is oscillating, delay a few ms (typ. 10–20 ms).
  • SPI sanity check: perform a benign read of a known, read‑only or status register to verify wiring and bit order. If you do not yet have the BK4829 register manual, pick a register that community code consistently reads at boot (see note below), or use a write‑then‑readback on a safe, non‑critical field.
  • Optional software reset: Many BK48xx parts expose a soft‑reset bit near REG_00; toggling it returns blocks to defaults and avoids indeterminate states after brown‑out. While BK4829’s public datasheet does not publish the register map, BK4819 documentation shows a REG_00 soft‑reset bit, and BK4829 is a close architectural relative; use this only if your register map confirms it. (es.scribd.com)
  • Configure basics: Once SPI is proven, proceed to your normal configuration (frequency synthesizer, RX/TX path enables, audio path, squelch/tones). Community firmware for BK4829 sets certain low‑numbered control registers very early (example: REG 0x19 to stock value) before the rest of RF/audio setup, confirming that no separate “write‑enable” step exists. (github.com)

3) Why you may see “writes don’t take”

  • Crystal not running: With the 26 MHz XO stalled or mis‑loaded, the digital domain may not function even though pins wiggle; verify XO amplitude and load caps per the datasheet. (device.report)
  • 3‑wire readback turn‑around: If SDATA is not released to input immediately after the address phase during a read, the first returned bit is often corrupted or the whole word reads as 0x0000/0xFFFF. Follow the datasheet’s read timing diagram. (device.report)
  • Wrong signal polarity/edge: BK4829 samples on SCK↑ and drives on SCK↓; mismatched host SPI mode or bit‑banged edges will cause shifted/garbage reads. (device.report)

Current information and trends

  • Public BK4829 documentation provides electricals, the 3‑wire protocol and timing, but not the full register map; vendors typically share the register manual under NDA. (device.report)
  • Open‑source radio projects (UV‑K1/UV‑K5 V3) carrying BK4829 have converged on simple boot inits (e.g., writing REG 0x19 to stock value) with no prior unlock/enabling step, reinforcing that standard SPI access is available immediately after power‑up. (github.com)
  • The 26 MHz reference is critical; the datasheet explicitly calls out that it clocks both the RF synthesizer and the digital core; frequency tolerance and correct load are required for reliable comms. (device.report)

Supporting explanations and details

  • 3‑wire read example (bit‑bang outline):
    • SCN=0; shift out R/W=1 + A6..A0 on SDATA while SCK toggles; after A0’s rising edge, tri‑state SDATA.
    • Observe tCDV; the chip begins driving D15..D0 on SCK falling edges; sample on SCK falling or rising per your host scheme, consistent with the datasheet diagram. (device.report)
  • 3‑wire write example:
    • SCN=0; shift out R/W=0 + A6..A0 + D15..D0; SCN=1. No special “write enable” opcode is defined/required in the public docs. (device.report)
  • Practical delays:
    • XO settle: few ms (conservative 10–20 ms) before first access.
    • PLL settle after frequency register changes: sub‑millisecond per datasheet (<0.3 ms), but many designs still insert ~1–2 ms guard time during bring‑up. (device.report)

Ethical and legal aspects

  • BK4829 is a wide‑band transceiver. In the United States, transmitting requires compliance with FCC rules (e.g., Part 95/97 as applicable). Avoid enabling TX or altering band limits unless you are authorized and operating within regulations.

Practical guidelines

  • Do this first:
    • Verify VDD and decoupling; confirm the 26 MHz XO oscillates and load capacitance matches the crystal. (device.report)
    • Confirm SPI timing (edges) and bus turn‑around on SDATA with a logic analyzer; match the datasheet’s read/write diagrams. (device.report)
    • Read a stable register to prove SPI, then optionally issue a soft reset if your register map indicates one, then proceed with configuration writes (RF path enables, synthesizer, audio, tones).
  • Common pitfalls and remedies:
    • All reads return 0x0000/0xFFFF → check SDATA direction change timing and SCN polarity. (device.report)
    • Writes have no effect → crystal not running or writing a shadowed/locked field; re‑check XO and try a soft reset (if available). (device.report)
    • Intermittent behavior → insufficient XO load or poor decoupling; revisit layout and capacitor placement. (device.report)

Possible disclaimers or additional notes

  • The BK4829 public datasheet omits the full register list. Where I referenced a soft‑reset bit, that is inferred from the closely related BK4819 register documentation and should be confirmed against your BK4829 register manual or proven community code. (es.scribd.com)
  • Early‑boot register choices (e.g., the REG 0x19 write seen in community firmware) are implementation details rather than mandatory “enable” sequences. (github.com)

Suggestions for further research

  • Obtain the BK4829 Register Manual from Beken (usually NDA’d) for authoritative reset/control addresses.
  • Review actively maintained open‑source firmware for BK4829‑based radios to see proven init orders and timing (e.g., UV‑K1/UV‑K5 V3 projects; see their changelogs for BK4829 init notes). (github.com)
  • Validate your own minimal init on hardware with a logic analyzer and, if available, RF test equipment (spectrum analyzer, SA+TG) to verify LO lock and RF path enables.

Brief summary

  • No special “initial command” or unlock is required for a new BK4829—standard 3‑wire SPI works immediately once VDD and the 26 MHz XO are stable. Use a short bring‑up routine: sanity‑check SPI reads, optionally issue a software reset if your register map provides one, then proceed with normal configuration writes. Be meticulous about 3‑wire read timing and SDATA direction during reads. (device.report)

If you share your target platform and (if you have it) the BK4829 register map, I can draft a concrete, line‑by‑line init sequence tailored to your application.

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