As much as possible, we’d like to move control of all the Bus Pirate peripheral hardware to the FPGA. Then everything can be controlled through the state machine command pipeline. In Ultra v1c we moved analog voltage measurement to the FPGA by adding an SPI ADC. In a future revision it would make sense to move a few other things to the FPGA:
- Pull-up resistor control
- Programmable output power supply enable
- Programmable output power supply margining (using a DAC)
- v1d stuff not yet announced
For debugging and self-testing we need to keep some redundant connections to the MCU as well, but primary control should be through the FPGA.
We had a look at a few chips that could replace the Digital to Analog Converter in the MCU, here’s a few inexpensive options we considered:
MCP4902 8bit dual DAC TSSOP14
MCP4902 seems to be a classic Microchip part, available at Mouser for $0.99 in 100s. However, the smallest package size is TSSOP14 and a quick check of SZLCSC shows they only have the SOIC version with 17 pieces in stock. That’s not a great sign.
Each update of a DAC channel uses a 16bit command, maximum speed is 20MHz.
MCP48FVB02 8bit dual DAC MSOP10
MCP48FVB02 appears to be a part Microchip acquired when they bought Micrel. It comes in an MSOP10 package which is an improvement over the MCP4902. It’s a bit cheaper at $0.90 for 100pcs at Mouser. It’s not stocked at SZLCSZ, which is a huge warning sign. Microchip Direct is really good about delivering parts in China if need be, but they can only deliver 1200 today and new stock won’t be available until February (three and a half months away).
MCP48FVB02 uses a 24bit command to update each DAC, which is a full byte longer than the MCP4092. Both the MCP4092 and MCP48FVB02 operate at maximum write speeds of 20MHz, so the MCP48FVB02 will have a significantly slower maximum update rate.
DAC082S085CIMM/DAC084S085CIMM 8bit dual/quad DAC MSOP10
Here’s where it gets a bit interesting. DAC082S085CIMM is a dual 8 bit DAC from Texas Instruments, available for $1.24 in 100s at Mouser. SZLCSC only has 8 in stock for around $2 in 100s (13.20RMB). Low stock is bad news, and a higher RMB price than USD price that points to a limited stock or specialty chip to avoid (i.e. not something with high demand in China).
However, the DAC084S085CIMM is similar but has 4 DACs. We could use the extra DACs to add more programmable output power supplies, or add a simple analog signal generator on a few of the IO pins. It’s available at Mouser for $1.58 in 100s, and at SZLCSC for $1.15 (8.68RMB) with 1700 available and 1300 shipped in the last month. That’s several good signs: it’s cheaper than the 2 DAC version ($2 vs $1.15), RMB price is cheaper than USD price, and there is a fair amount of stock and turnover at SZLCSC which means it’s probably being used in production somewhere. This seems like a good candidate.
Just to further verify, there are 50K in stock at the TI store, and 20K in stock at Digikey for a slightly higher price.
Commands are 16bits, but where it really shines is the 40MHz maximum update speed. Twice as many DACs, and twice twice as fast as the Microchip DACs.
This is by no accounts an exhaustive list, but after looking at stock on Mouser, Digikey, and SZLCSC these were the best options close to $1. Did we miss your favorite DAC?