Many registers described below are common with other BM13xx chips. Here we focus mainly on BM1366 as this register map has been reverse engineered from the 'bmminer' of S19XP-Hydro original Firmware from Bitmain.
In order to get a higher hash rate, we need to increase the communication Baudrate because at 115200bps (default baudrate) a Send Job Command with 4 midstate would take 15.2ms which could be longer than the time the full chain would take to Hash the complete Nonce space of the previous job.
BaudRate = fBase / ((BT8D + 1) * 8)
At chip reset, in Fast UART Configuration Register :
So the default BaudRate = fCLKI / ((BT8D + 1) * 8) = 115740 bps with reset values (0.47% error with target 115200 bps).
This is possible up to 3.125Mbps BaudRate with BT8D = 0.
For higher baud rates, here are the necessary steps (numeric example below is for 6.25 Mbps BaudRate):
- enabling and configuring PLL1 using PLL1 Parameter for example writing 0xC0700111 will result of a PLL3 with frequency equal to
fPLL3 = fCLKI * FBDIV / (REFDIV * POSTDIV1 * POSTDIV2) = 25MHz*112/(1*1*1) = 2.8 GHz - setting PLL1_DIV4 in Fast UART Configuration for example writing 0xTBD give a
PLL1_DIV4 = 6 - setting BCLK_SEL to 1
fBase = fPLL1 / (PLL1_DIV4 + 1) = 2.8 GHz / (6+1) = 400 MHz - setting BT8D to 7
BaudRate = fBase / ((BT8D + 1)*8) = 400 MHz / ((7+1)*8) = 6.25 MBps